Astronomy Picture of the Day |
APOD: 2008 December 11 - At the Center of the Milky Way
Explanation:
At the center of our Milky Way Galaxy lies a
supermassive black hole.
Once a controversial claim, this conclusion is now solidly based
on 16 years of observations
that map the orbits of 28 stars very near
the galactic center.
Using European Southern Observatory
telescopes and sophisticated
near infrared cameras, astronomers patiently measured the positions
of the stars over time, following one star, designated S2, through
a complete orbit as it came within about 1 light-day of the
center of the Milky Way.
Their results
convincingly show that S2 is moving under the influence of the
enormous gravity of a compact, unseen object -- a black hole
with 4 million times the mass
of
the Sun.
Their ability to track stars so close to the
galactic center
accurately measures the black hole's mass and also determines the
distance to the center to be 27,000 light-years.
This deep, near-infrared image shows the crowded inner 3
light-years of the central Milky Way.
Spectacular time-lapse animations of the stars orbiting
within light-days of the galactic center
can be found here.
APOD: 2008 November 10 - Our Galaxy's Central Molecular Zone
Explanation:
The central region of our Milky Way Galaxy is a mysterious and complex
place.
Pictured here in
radio
and infrared light, the galaxy's central square degree
is highlighted in fine detail.
The region is known as the
Central Molecular Zone.
While much of the extended emission is due to dense gas laced
with molecules, also seen are
emission nebulas
lit up by massive young stars, glowing
supernova remnants, and the
curving Galactic Center Radio Arc
in purple.
The identity and root cause for many other features remains unknown.
Besides a massive
black hole
named Sgr A*,
the Galactic Center houses the galaxy's most active star forming region.
This image is not just
interesting
scientifically.
It's esthetic
beauty won first prize this year in the
AUI/NRAO Image Contest.
APOD: 2008 August 22 - Active Galaxy NGC 1275
Explanation:
Active galaxy NGC 1275
is the central, dominant member of the large and
relatively nearby
Perseus Cluster of Galaxies.
A prodigious source of
x-rays
and
radio
emission, NGC 1275 accretes
matter as entire galaxies fall into it, ultimately
feeding a supermassive black hole at the galaxy's core.
This stunning
visible
light image from the Hubble Space Telescope
shows galactic debris
and filaments of glowing gas,
some up to 20,000 light-years long.
The filaments persist in NGC 1275, even though
the turmoil of galactic collisions should destroy them.
What keeps the filaments together?
Recent work
indicates that the structures, pushed out
from the galaxy's center by the black hole's activity, are
held together by magnetic fields.
To
add x-ray data
from the Chandra Observatory and radio data from
the Very Large Array
to the Hubble image, just slide
your cursor over the picture.
In the resulting composite, x-rays highlight the
shells of hot gas
surrounding the center of the galaxy, with radio emission filling
giant bubble-shaped cavities.
Also known as Perseus A,
NGC 1275
spans over 100,000 light years and
lies about 230 million light years away.
APOD: 2008 August 11 - Black Hole Candidate Cygnus X-1
Explanation:
Is that a black hole?
Quite possibly.
The Cygnus X-1
binary star system contains one of the best candidates for a
black hole.
The system was discovered because it is one of the brightest
X-ray
sources on the sky, shining so bright it was detected by the
earliest rockets
carrying cameras capable of seeing the previously unknown
X-ray sky.
The star's very name indicates that it is the single brightest X-ray source in the
constellation
of the Swan Cygnus.
Data indicate that a
compact object
there contains about nine times the mass of the Sun and changes its
brightness continually on several time scales, at least down to milliseconds.
Such behavior is expected for a
black hole,
and difficult to explain with other models.
Pictured above is an artistic impression of the
Cygnus X-1 system.
On the left is the bright blue
supergiant star
designated HDE 226868, which is estimated as having about 30 times the mass of our Sun.
Cygnus X-1 is depicted on the right, connected to its
supergiant companion by a stream of gas, and surrounded by an impressive
accretion disk.
The bright star in the
Cygnus X-1 system is visible with a small telescope.
Strangely, the
Cygnus X-1 black hole candidate
appears to have formed without a bright
supernova explosion.
APOD: 2008 June 27 - M81: Feeding a Black Hole
Explanation:
This impressive
color composite
shows spiral galaxy M81 across the
electromagnetic spectrum.
It combines X-ray data (blue) from the
Chandra
Observatory,
infrared data (pink) from the
Spitzer Space Telescope, and
an ultraviolet image (purple) from the
GALEX satellite,
with a visible light (green)
Hubble image.
The inset highlights X-rays from some of M81's black holes,
including
black
holes in binary star systems with about
10 times the mass of the sun, as well as the central,
supermassive black hole of over 70 million
solar masses.
Comparing computer models of the giant black hole's energy
output to the
multiwavelength data
suggests that feeding that
monster
is relatively simple -- energy and radiation is generated
as material in the central region swirls inwards forming an
accretion disk.
In fact, the process otherwise appears to be just like the
accretion process feeding M81's stellar mass black holes,
even though the central black hole is millions of times more massive.
M81 itself is about
70,000 light-years across and only 12 million
light-years away in the northern constellation
Ursa Major.
APOD: 2008 June 22 - Barred Spiral Galaxy NGC 1300
Explanation:
Big, beautiful, barred spiral galaxy
NGC 1300 lies some 70 million
light-years away on the banks of the
constellation
Eridanus.
This Hubble Space Telescope
composite view of the gorgeous island universe is one of the
largest Hubble images ever made of a complete
galaxy.
NGC 1300
spans over 100,000 light-years and the
Hubble
image reveals striking details of the galaxy's dominant
central bar and majestic spiral arms.
In fact, on close inspection
the nucleus of this classic barred
spiral itself shows a remarkable region of
spiral structure
about 3,000 light-years across.
Unlike other
spiral galaxies,
including our own Milky Way, NGC 1300
is not presently known to have a massive
central black hole.
APOD: 2008 April 27 - The Galactic Center Radio Arc
Explanation:
What causes this unusual structure near the center of our Galaxy?
The long parallel rays slanting across the top of the
above radio image are known collectively as the
Galactic Center Radio Arc and jut straight out from the
Galactic plane.
The Radio Arc is connected to the
Galactic center
by strange curving filaments known as the
Arches.
The bright radio structure at the bottom right likely surrounds a
black hole at the
Galactic center and is known as
Sagittarius A*.
One origin hypothesis holds that the
Radio Arc and the Arches have their geometry
because they contain hot
plasma flowing along lines of constant
magnetic field.
Images from the
Chandra X-ray Observatory
appear to show this
plasma colliding with a nearby cloud of cold gas.
APOD: 2008 January 10 - Active Galaxy Centaurus A
Explanation:
A mere 11 million light-years away,
Centaurus A is a giant elliptical
galaxy - the closest active galaxy to Earth.
This remarkable
composite view of the galaxy
combines
image data
from the x-ray (
Chandra),
optical(ESO), and
radio(VLA)
regimes.
Centaurus A's central region
is a jumble of gas, dust, and stars
in optical light,
but both radio and x-ray telescopes trace a
remarkable jet of
high-energy particles streaming from the galaxy's core.
The cosmic
particle accelerator's
power source is a
black
hole with about 10 million times the mass of the
Sun coincident with the x-ray bright spot at the galaxy's center.
Blasting out from the active galactic nucleus toward the upper left,
the energetic jet extends about 13,000 light-years.
A shorter jet extends from the nucleus in the opposite direction.
Other x-ray bright spots
in
the field are binary star systems with neutron stars or stellar mass
black holes.
Active galaxy Centaurus A is likely the result of a
merger with
a spiral galaxy some 100 million years ago.
APOD: 2007 December 16 - The Holographic Principle
Explanation:
Is this picture worth a thousand words?
According to the Holographic Principle, the most
information you can get from this image is about
3 x 1065 bits for a normal sized computer monitor.
The Holographic Principle, yet unproven, states that
there is a maximum amount of information content
held by regions adjacent to any surface.
Therefore, counter-intuitively, the information content
inside a room depends not on the volume of the room but
on the area of the bounding walls.
The principle derives from the idea that the
Planck length, the length scale where
quantum mechanics begins to dominate
classical gravity, is one side of an area
that can hold only about one bit of information.
The limit was first postulated by physicist
Gerard 't Hooft in 1993.
It can arise from generalizations from seemingly
distant speculation that the information held by a
black hole is determined not by its
enclosed volume but by the surface area of its
event horizon.
The term "holographic" arises from a
hologram analogy where three-dimension images are
created by projecting light though a flat screen.
Beware, other people looking at the
above image may not claim to see 3 x 1065 bits --
they might claim to see a
teapot.
APOD: 2007 September 6 - Time Tunnel
Explanation:
Spiky stars are nearby, but
fuzzy galaxies are strewn far across
the Universe in this cosmic view.
Spanning about 1/2 degree on the sky,
the pretty picture is the result of astronomer Johannes
Schedler's project to look back in time,
toward a quasar 12.7
billion light-years away.
The quasar is just visible in the
full resolution image at the
position marked by short vertical lines (center).
The intrinsically bright nucleus of a young,
active galaxy powered by a supermassive black hole, the quasar
was recently identified
as one of the most distant
objects known.
Since
light
travels at a finite speed, the
galaxies receding into the distance are seen as they
were in the increasingly remote past.
The quasar appears as it did about 12.7 billion years ago,
when the Universe was just 7 percent of its
present age.
Of course,
the
expansion of the Universe
has redshifted the light.
Schedler added image data extending to the near-infrared,
acquired by collaborator Ken Crawford,
to detect the distant quasar, with a measured
redshift of 6.04.
APOD: 2007 August 27 - Huge Void Implicated in Distant Universe
Explanation:
What has created this huge empty volume in the universe?
No one is yet sure, and even the extent of the estimated billion-light year
void is being researched.
The void
is not a hole in space like a
black hole,
but rather a vast region of the universe that appears to be mostly devoid of
normal matter
and even dark matter.
The void is still thought to contain
dark energy,
though, and is clearly traversable by light.
The void's existence is
being postulated
following scientific curiosity about how unusually cold spots came to appear on
WMAP's map of
cosmic
microwave background (CMB) radiation.
One possibility was that this CMB region was not actually
very cold but light from the spot somehow became more
cosmologically redshifted than normal along the way.
Other voids
in the universe are known to exist,
but this void appears to have an unusually large gravitational effect,
and so might possibly be the largest in our entire visible universe.
Investigating this, a recent study found an unusually low number of
cosmic radio sources
between Earth and the CMB cold spot, which led to the inference of this giant void.
An artist's depiction
of the huge cosmic void is
shown above.
APOD: 2007 July 29 - The Center of Centaurus A
Explanation:
A fantastic jumble of young blue star clusters,
gigantic glowing gas clouds, and imposing dark
dust lanes
surrounds the central region of the active galaxy Centaurus A.
This mosaic of Hubble Space Telescope images
taken in blue, green, and red light
has been processed to present a natural color picture
of this cosmic maelstrom.
Infrared images from the Hubble have also shown that hidden
at the center of this activity are what seem to be disks of matter spiraling into
a black hole with a billion times the mass of the Sun!
Centaurus A itself is apparently the result of a
collision of two galaxies
and the left over debris is steadily being consumed by the
black hole.
Astronomers believe that such
black hole central engines generate the
radio, X-ray, and gamma-ray energy radiated by Centaurus A
and other active galaxies.
But for an active galaxy Centaurus A is close,
a mere 10 million
light-years away, and is a relatively convenient laboratory for
exploring these powerful sources of energy.
APOD: 2007 June 30 - Jumbled Galaxy Centaurus A
Explanation:
At the center of
this
sharp skyscape, Centaurus A seems to be
a fantastic jumble of old yellow stars, young blue star clusters,
and imposing dark dust lanes.
Spanning over 60,000 light-years,
the
peculiar elliptical galaxy is
apparently the result of a collision of two otherwise
normal galaxies.
The left over cosmic debris is steadily being consumed by a
black
hole with a billion times the mass of the Sun which lies
at the center of Centaurus A.
It's likely that such
black
hole central engines generate the
radio, X-ray, and gamma-ray energy radiated
by Centaurus A and other
active
galaxies.
For an active galaxy Centaurus A
is close, a mere 10 million
light-years away,
and is well-studied by
earthbound
astronomers.
APOD: 2007 May 28 - A Hole in Mars
Explanation:
Black spots have been discovered on Mars that are so dark that nothing inside can be seen.
Quite possibly,
the spots
are entrances to deep
underground caves capable of protecting
Martian life, were it to exist.
The unusual hole pictured above was found on the slopes of the giant Martian volcano
Arsia Mons.
The above image was captured three weeks ago by the
HiRISE instrument onboard the
Mars Reconnaissance Orbiter
currently circling Mars.
The holes were originally identified on lower resolution images from the
Mars Odyssey spacecraft,
The above hole is about the size of a
football field
and is so deep that it is completely
unilluminated by the Sun.
Such holes and
underground caves
might be prime targets for
future spacecraft,
robots, and even the next generation of
human interplanetary explorers.
APOD: 2007 May 24 - The Tulip in the Swan
Explanation:
This expansive (1-degree wide)
telescopic
view looks out along the
plane of our Milky Way Galaxy toward the
nebula rich constellation
Cygnus
the Swan.
It is centered on a bright hydrogen emission region
recorded in the
1959 catalog
by astronomer Stewart Sharpless
as Sh2-101.
About 8,000 light-years distant the nebula is popularly known
as the Tulip Nebula, understandably not the only
cosmic cloud to evoke the imagery
of flowers.
Complex and beautiful in visible light, the area also includes
one of the brightest, most famous sources in the
x-ray
sky, Cygnus X-1.
Discovered in the early 1970s,
Cygnus
X-1 is a bizarre
binary system consisting of a massive, hot, supergiant star
(seen here)
in close orbit with a stellar mass
black hole.
The Cygnus
X-1 system is also estimated to lie a
comfortable
8,000 light-years away.
APOD: 2007 May 10 - SN 2006GY: Brightest Supernova
Explanation:
The stellar explosion cataloged as
supernova
SN 2006gy shines
in this
wide-field image (left) of its
host galaxy, NGC 1260,
and expanded view (upper right panel) of the region surrounding
the galaxy's core.
In fact, given its estimated distance of 240 million light-years,
SN 2006gy was brighter than, and has stayed brighter
longer than,
any previously seen supernova.
The Chandra observations in the lower right panel establish
the supernova's x-ray brightness and lend strong
evidence to the theory that
SN
2006gy was the death explosion of
a star well over 100 times as massive as the Sun.
In such an exceptionally massive star,
astronomers
suspect an instability producing matter-antimatter
pairs
led to the cosmic blast and obliterated the stellar core.
Thus, unlike in other massive star supernovae, neither
neutron star,
or even
black hole,
would
remain.
Intriguingly, analogs in our own galaxy
for SN 2006gy's progenitor may include the
well-known,
extremely massive star Eta Carinae.
APOD: 2007 April 18 - Barred Spiral Galaxy NGC 1672
Explanation:
Many spiral galaxies have bars across their centers.
Even our own
Milky Way Galaxy is thought to have a
modest central bar.
Prominently barred spiral galaxy NGC 1672,
pictured above, was captured in spectacular detail in this recently
released image taken by the orbiting
Hubble Space Telescope.
Visible are dark filamentary
dust lanes, young
clusters of bright blue stars, red
emission nebulas of glowing hydrogen gas,
a long bright bar of stars across the center, and a bright
active nucleus
that likely houses a supermassive
black hole.
Light takes about 60 million years to reach us from
NGC 1672, which spans about 75,000
light years across.
NGC 1672, which appears toward the constellation of the Swordfish
(Dorado), is
being studied to find out how a spiral bar contributes to star formation in a
galaxy's central regions.
APOD: 2007 March 28 - NGC 1365: Majestic Island Universe
Explanation:
Barred spiral galaxy
NGC 1365 is truly a majestic
island universe some 200,000
light-years across.
Located a mere 60 million light-years away toward the
chemical constellation
Fornax,
NGC 1365 is a dominant
member of the well-studied
Fornax
galaxy cluster.
This impressively
sharp color image shows intense
star forming regions at the ends of the bar and along the
spiral arms, as well as details of dust lanes cutting
across the galaxy's bright core.
At the core lies a supermassive black hole.
Astronomers think NGC 1365's prominent bar plays a crucial
role in the galaxy's evolution, drawing gas and dust into a
star-forming maelstrom and ultimately feeding material into the
central black hole.
APOD: 2007 January 14 - Sgr A*: Fast Stars Near the Galactic Center
Explanation:
Why are these stars moving so fast?
Shown above is a time-lapse movie in
infrared light detailing how stars in the central light-year of
our Galaxy
have moved over the past eight years.
The yellow mark at the image center represents the location of a peculiar
radio source named
Sgr A*.
If these fast stars are held to the
Galactic Center by gravity,
then the central object exerting this
gravity must be both compact and massive.
Analysis of the
stellar motions indicates that over
one million times the mass of our
Sun is somehow confined to a region
less than a fifth of a light-year across.
Astronomers interpret
these observations as strong evidence that the
center of our Galaxy
is home to a very massive
black hole.
APOD: 2006 December 1 - In the Arms of NGC 1097
Explanation:
A smaller companion seems wrapped in the spiral arms of enigmatic
galaxy NGC 1097.
This amazingly deep image of the peculiar spiral
system, also known as
Arp
77, actually combines data from two telescopes,
one in the northern and one in the southern hemisphere of planet
Earth.
The faint details revealed include hints of a
mysterious
jet emerging toward the top of the view.
Seen to be about 42,000 light-years from the larger galaxy's
center, the companion galaxy is gravitationally
interacting
with the spiral and will
ultimately merge with it.
NGC 1097's center also harbors a
massive black hole.
NGC 1097 is located about 45 million light-years away in
the chemical constellation
Fornax.
APOD: 2006 August 27 - Barred Spiral Galaxy NGC 1300
Explanation:
Big, beautiful, barred spiral galaxy
NGC 1300 lies some 70 million
light-years away on the banks of the
constellation
Eridanus.
This Hubble Space Telescope
composite view of the gorgeous island
universe is one of the
largest Hubble images ever made of a complete
galaxy.
NGC 1300
spans over 100,000 light-years and the
Hubble
image reveals striking details of the galaxy's dominant
central bar and majestic spiral arms.
In fact, on close inspection
the nucleus of this classic barred
spiral itself shows a remarkable region of
spiral structure
about 3,000 light-years across.
Unlike other
spiral galaxies,
including our own
Milky Way, NGC 1300
is not presently known to have a massive
central black hole.
APOD: 2006 August 10 - Galactic Center Star Clusters
Explanation:
If you had
x-ray
vision, the central regions
of our Galaxy would not be hidden from
view by cosmic dust clouds.
Instead,
the Milky Way
toward Sagittarius might look something
like this.
Pleasing to look at, the gorgeous false-color representation of
x-ray data
from the Chandra Observatory shows
high energies in blue, medium in green,
and low energy x-rays in red.
The
mosaic spans about 130 light-years at the 26,000 light-year
distance of the Galactic Center.
It reveals massive, x-ray emitting star clusters
in a crowded environment.
In particular,
the Galactic Center cluster and the enormous
black hole
Sagittarius A* are within the bright region near the
bottom.
Two other star clusters, the Arches,
and the Quintuplet
lie near the top.
Cluster interactions with dense molecular clouds in the region
may produce some of the diffuse emission detected in
the Chandra
x-ray view.
APOD: 2006 July 29 - The Swarm
Explanation:
What do you
call a group of black
holes ... a flock, a brace, a swarm?
Monitoring a region around the
center of our Galaxy,
astronomers have indeed found
evidence for a surprisingly large number of
variable x-ray sources - likely black holes or neutron stars in
binary star systems - swarming
around the Milky Way's
own central supermassive black hole.
Chandra Observatory combined
x-ray image data from their monitoring
program
is shown above,
with four variable sources circled and labeled A-D.
While four sources may not make a swarm,
these all lie
within only three light-years of the central supermassive black
hole known as
Sgr A* (the bright source just above C).
Their detection
implies that a much larger concentration of
black hole systems is present.
Repeated gravitational interactions with other stars are
thought to cause the
black hole systems to spiral inward
toward the Galactic Center region.
APOD: 2006 July 16 - The Galactic Center in Infrared
Explanation:
The center of our Galaxy is a busy place. In visible light, much of the
Galactic Center
is obscured by opaque
dust.
In infrared light, however,
dust glows more and obscures less,
allowing nearly one million stars to be recorded in the
above photograph.
The
Galactic Center
itself appears
on the right and is located about 30,000
light years away towards the constellation of Sagittarius.
The
Galactic Plane of our
Milky Way Galaxy, the plane in which the
Sun orbits, is identifiable by the
dark diagonal dust lane.
The absorbing
dust grains are created in the atmospheres of cool
red-giant stars and grow in
molecular clouds.
The region directly surrounding the
Galactic Center
glows brightly in
radio and
high-energy radiation,
and is thought to house a large
black hole.
APOD: 2006 July 1 - Wind from a Black Hole
Explanation:
Binary star system
GRO J1655-40
consists of a relatively normal star about twice as massive as the Sun
co-orbiting with a black hole of about seven
solar masses.
This striking
artist's
vision of the exotic binary
system helps visualize matter drawn from the normal star by
gravity and swirling toward the black hole.
But it also includes a wind of material escaping from the black hole's
accretion disk.
In fact, astronomers
now argue
that Chandra Observatory x-ray data
indicate a high-speed wind is being driven from this system's
disk by magnetic forces.
Internal magnetic fields also help drive material in the swirling
disk into the
black hole itself.
If you had
x-ray
eyes as good as Chandra's, you could find
GRO J1655-40 about 11,000 light-years away in the constellation
Scorpius.
APOD: 2006 May 28 - GRO J1655 40: Evidence for a Spinning Black Hole
Explanation:
In the center of a swirling whirlpool of hot gas
is likely a beast that has never been seen directly: a
black hole.
Studies of the bright light emitted by the
swirling gas frequently indicate not only that a
black hole is present, but also likely attributes.
The gas surrounding GRO J1655-40, for example, has been found to display an unusual flickering
at a rate of 450 times a second.
Given a
previous mass estimate for the central object of seven times the mass of our
Sun, the rate of the
fast flickering can be
explained by a black hole
that is rotating very rapidly.
What physical mechanisms actually cause the flickering -- and a slower quasi-periodic oscillation (QPO) -- in
accretion disks surrounding
black holes and
neutron stars remains a topic of much research.
APOD: 2006 May 20 - Elliptical Galaxy M87
Explanation:
In spiral galaxies, majestic
winding arms of young stars and
interstellar gas and dust rotate in a flat disk around a
bulging galactic nucleus.
But elliptical galaxies seem to be simpler.
Lacking gas and dust to form new stars, their
randomly swarming older stars, give them an ellipsoidal
(egg-like) shape.
Still, elliptical galaxies can be very large.
Over 120,000 light-years in diameter (larger than our own
Milky Way), elliptical galaxy M87 is the dominant
galaxy at the center
of the Virgo Galaxy Cluster, some 50 million light-years away.
M87 is likely home to a supermassive
black hole responsible
for the high-energy jet of particles emerging from the giant
galaxy's central region.
APOD: 2006 April 27 - NGC 4696: Energy from a Black Hole
Explanation:
In many cosmic environments,
when material falls toward
a black hole energy is produced as some of the matter is
blasted back out in jets.
In fact, such black hole "engines" appear to be the most
efficient in the Universe, at least on a galactic scale.
This
composite image
illustrates one example of an
elliptical galaxy with an efficient
black hole engine, NGC 4696.
The large galaxy is the brightest member of the
Centaurus
galaxy cluster, some 150 million light-years away.
Exploring
NGC 4696
in x-rays (red) astronomers
can measure the rate at which infalling matter fuels the
supermassive black hole and compare
it to the energy output in the jets to
produce giant radio emitting bubbles.
The bubbles, shown here in blue, are about 10,000
light-years across.
The results confirm
that the process is much more efficient
than producing energy through
nuclear
reactions - not to mention
using fossil fuels.
Astronomers also suggest that as the black hole
pumps out energy and heats the surrounding gas, star formation
is ultimately shut off, limiting the size of large galaxies like
NGC 4696.
APOD: 2006 April 12 - Binary Black Hole in 3C 75
Explanation:
The two bright sources at the center of
this
composite x-ray (blue)/
radio
(pink) image are co-orbiting supermassive black holes powering
the giant radio source
3C 75.
Surrounded by multimillion degree x-ray emitting gas, and
blasting out jets of relativistic particles the
supermassive black holes
are separated by 25,000 light-years.
At the cores of
two merging galaxies in the
Abell 400
galaxy cluster they are some 300 million light-years away.
Astronomers
conclude that these two supermassive
black
holes are bound together by gravity in a binary system
in part because
the jets' consistent swept back appearance is most likely due to their
common motion as they speed through the hot cluster gas
at 1200 kilometers per second.
Such spectacular cosmic mergers are thought to be common in crowded
galaxy cluster environments
in the distant universe.
In their final stages the mergers are expected to be intense
sources of gravitational waves.
APOD: 2006 March 4 - The Galaxy Within Centaurus A
Explanation:
Peering
deep inside Centaurus A, the closest
active galaxy
to Earth, the Spitzer Space Telescope's
penetrating infrared
cameras recorded
this startling vista in February 2004.
About 1,000 light-years across, the twisted cosmic dust cloud
apparently shaped like a
parallelogram is likely the result
of a smaller spiral galaxy falling into the giant
Centaurus A.
The parallelogram lies along the active galaxy's central band
of dust and stars visible in more
familiar optical images.
Astronomers believe that the striking geometric shape
represents an approximately edge-on view of the infalling
spiral galaxy's disk in the process of being
twisted and warped
by the interaction.
Ultimately, debris from the ill-fated spiral galaxy should
provide fuel for the supermassive
black hole lurking
at the center of
Centaurus A.
APOD: 2006 January 15 - The Sombrero Galaxy from HST
Explanation:
Why does the Sombrero Galaxy look like a hat?
Reasons include the
Sombrero's unusually large and extended central bulge of stars,
and dark prominent dust lanes
that appear in a disk that we see nearly
edge-on.
Billions of old stars
cause the diffuse glow of the extended central bulge.
Close inspection of the bulge in the
above
photograph shows many points of light that are actually
globular clusters.
M104's spectacular
dust rings harbor many younger
and brighter stars, and show intricate details astronomers
don't yet fully understand.
The very center of the
Sombrero glows across the electromagnetic spectrum, and is thought to house a large
black hole.
Fifty million-year-old light from the
Sombrero Galaxy can be seen with a
small telescope towards the constellation of Virgo.
APOD: 2005 December 8 - X-Rays from the Perseus Cluster Core
Explanation:
The Perseus Cluster of thousands of galaxies,
250 million light-years distant, is
one of
the most massive objects
in
the Universe and the brightest galaxy cluster in the
x-ray sky.
At its core lies the giant
cannibal galaxy Perseus A
(NGC
1275), accreting matter as
gas and galaxies fall into it.
This deep
Chandra Observatory x-ray image spans about 300,000 light-years
across the galaxy cluster core.
It shows
remarkable details
of x-ray emission from the monster galaxy and
surrounding hot (30-70 million degrees C) cluster gas.
The bright central source is the supermassive
black
hole at the core of Perseus A itself.
Low density regions are seen as dark bubbles or voids,
believed to be generated by cyclic outbursts of activity
from the central black hole.
The activity creates pressure waves -
sound waves on a cosmic scale-
that ripple through the x-ray hot gas.
Dramatically, the blue-green wisps just above centre in the
false-color view are likely x-ray shadows of
the remains of a small galaxy falling into the burgeoning
Perseus A.
APOD: 2005 October 23 - At the Center of the Milky Way
Explanation:
At the center of our
Milky
Way Galaxy lies a black hole with
over 2 million times the mass of the Sun.
Once a controversial claim, this
astounding conclusion
is now virtually inescapable and based on observations of
stars orbiting
very near the galactic center.
Using one of the Paranal
Observatory's very large telescopes
and a sophisticated infrared camera,
astronomers
patiently followed the orbit of a particular star,
designated S2, as it came within about
17
light-hours of the center of the Milky Way
(about 3 times the radius of Pluto's orbit).
Their results
convincingly show that S2 is moving
under the influence of the enormous gravity of an
unseen object that must be extremely compact -- a
supermassive black hole.
This deep near-infrared
image shows the crowded inner 2
light-years of the Milky Way with the exact position of the
galactic center indicated by arrows.
The ability to track stars so close to the
galactic center
can accurately
measure the black hole's mass and perhaps
even provide an unprecedented test of Einstein's
theory of gravity
as astronomers watch a star orbit a
supermassive black hole.
APOD: 2005 October 17 - Short Gamma Ray Bursts Localized
Explanation:
What causes gamma-ray bursts?
The most energetic type of explosions known in the cosmos has been an enigma since
discovered over 30 years ago.
It now appears that there may not be one unique type of progenitor.
Long duration
gamma-ray bursts (GRBs) have been
localized, over the past few years, to
blue regions in the universe rich in star formation.
Massive young stars nearing the end of their short lives commonly explode in these regions.
Astronomers associate these
long duration GRBs, that can last from seconds to minutes,
with a type of stellar explosion common in young massive stars.
Over the past few months, short duration GRBs have finally been
localized and found to occur in different types of regions -- not only
blue regions rich in star formation.
Many astronomers therefore now theorize that
short GRBs, which typically last less than one second, are the result of a
different progenitor process
than long GRBs.
A leading model is that a short GRB will occur when a
neutron star
either impacts
another neutron star or a
black hole.
Such collisions may occur well after star-forming regions have otherwise burned out.
Pictured in the above illustration, two energized
neutrons stars finally approach
each other in their orbits, a death spiral that might end with a short GRB.
APOD: 2005 October 16 - Astronomy Quilt of the Week
Explanation:
Demonstrating her mastery of a
traditional
astronomical imaging technique
quilter and
astronomy enthusiast Judy Ross
has produced this spectacular composition of "Astronomy
Quilt Piece of the Week".
Her year-long effort resulted in an arrangement for a six by seven foot
quilt consisting
of 52 individual pieces
(11 inches by 8 inches), one for each week,
which she reports were inspired by her steady diet of
APOD's daily offerings.
Some of the pieces are based on actual pictures, such as the
Hubble Space Telescope's
view of planet forming
AB Aurigae or Bill Keel's image of the
nearby Pinwheel Galaxy.
Others, with titles like
the Blue
Carpet Nebula and
Duck
Contemplates Black Hole, are from her own creative imaginings.
APOD: 2005 October 1 - NGC 613: Spiral of Dust and Stars
Explanation:
When morning twilight came to the
Paranal Observatory in Chile,
astronomers Mark Neeser and Peter Barthel interrupted their
search for faint
quasars, billions of
light-years away.
And just for a moment, they used
Very Large Telescopes
at the European Southern Observatory to appreciate the beauty of
the nearby Universe.
One result was
this stunning view
of beautiful barred spiral galaxy
NGC 613, a mere 65 million light-years away
in the southern constellation
Sculptor.
Over 100 thousand light-years across,
NGC 613 seems to have more than its fair
share of spiral arms
laced with cosmic dust clouds and bright star forming regions
near the ends of a dominant central bar.
Radio emission indicates the presence of a massive
black hole
at the center of
NGC 613.
APOD: 2005 July 30 - M106 in Canes Venatici
Explanation:
Close to the Great Bear
(Ursa Major) and
surrounded by the stars
of the Hunting Dogs (Canes Venatici), this celestial nebula was
discovered
in 1781 by the
metric
French astronomer
Pierre Mechain.
Later, it was added to the catalog of his friend and colleague
Charles Messier as
M106.
Modern deep telescopic views reveal it to be an
island universe --
a spiral galaxy around 30 thousand light-years across located
only about 21 million light-years beyond the stars of the Milky Way.
Youthful blue star clusters and reddish stellar nurseries
trace the striking spiral arms of M106.
Seen so clearly in
this
beautiful image, the galaxy's bright
core is also visible
across
the spectrum from radio to x-rays,
making M106 a nearby example of the
Seyfert class of
active galaxies.
The bright core of a Seyfert galaxy is believed to be powered
by matter falling into a massive
central black hole.
APOD: 2005 July 25 - Unusual Gas Filaments Surround Galaxy NGC 1275
Explanation:
How were the unusual gas filaments surrounding galaxy NGC 1275 created?
No one is sure. Galaxy NGC 1275 is the
central dominant galaxy of the
Perseus Cluster of Galaxies,
a cluster with many member galaxies visible in the above image.
In visible light, NGC 1275 appears to show a
spectacular collision between two distinct galaxies.
The galaxy and cluster are also bright emitters of
X-rays.
The unusual gas filaments are shown above in a very
specific color of light emitted by
hydrogen, here artificially colored pink.
Possible origins for the filaments may involve details of the collision between the two galaxies, or alternatively, interactions between a
galactic center black hole and the surrounding
intracluster gas.
NGC 1275,
pictured above, spans about 100,000
light years and lies about 230 million light years distant toward the
constellation of Perseus.
APOD: 2005 July 19 - A Nearby Supernova in M51
Explanation:
One of the nearest supernovas of recent years was
discovered late last month in the bright nearby galaxy
M51.
It is visible on the right of the
above before and after images of the
picturesque spiral.
Can you spot it?
The supernova, discovered originally by
Wolfgang Kloehr and now dubbed 2005cs,
is still near its maximum brightness and visible
with a telescope toward the constellation of the Hunting Dogs (Canes Venatici).
The supernova has been identified as a
Type II but has an unusual brightness history,
creating speculation that is similar in nature to the
brightest supernova of modern times:
1987A.
The progenitor star has been
identified as a bright blue star.
Although hundreds of supernovas are discovered each year by automated searches,
nearby supernova are rare and important because they frequently become bright enough to be studied by many telescopes and are near enough for their (former) host star and immediate surroundings to be spatially resolved.
Supernova 2005cs may have left behind a core that has been
compressed into a
neutron star or
black hole.
APOD: 2005 July 17 - The Center of Centaurus A
Explanation:
A fantastic jumble of young blue star clusters,
gigantic glowing gas clouds, and imposing dark
dust lanes
surrounds the central region of the active galaxy Centaurus A.
This mosaic of Hubble Space Telescope images
taken in blue, green, and red light
has been processed to present a natural color picture
of this cosmic maelstrom.
Infrared images from the Hubble have also shown that
hidden at the center of this activity are what seem to
be disks of matter spiraling into
a black hole with a billion times the mass
of the Sun!
Centaurus A itself is apparently the result of a
collision of two galaxies
and the left over debris is steadily being consumed by the
black hole.
Astronomers believe that such
black hole central engines generate the
radio, X-ray, and gamma-ray energy radiated by Centaurus A
and other active galaxies.
But for an active galaxy Centaurus A is close,
a mere 10 million
light-years away,
and is a relatively convenient laboratory for exploring these
powerful sources of energy.
APOD: 2005 June 28 - The Giant Radio Lobes of Fornax A
Explanation:
Together, the radio lobes span over one million light years --
what caused them?
In the center is a large but peculiar elliptical galaxy dubbed
NGC 1316.
Detailed inspection of the
NGC 1316 system indicates that
it began absorbing a
small neighboring galaxy
about 100 million years ago.
Gas from the
galactic collision has fallen inward toward the massive central
black hole, with
friction
heating the gas to 10 million degrees.
For reasons not yet well understood, two oppositely pointed
fast moving jets of
particles then developed, eventually smashing
into the ambient material on either side of the giant
elliptical galaxy.
The result is a huge reservoir of hot gas that emits
radio waves, observed as the orange (false-color)
radio lobes in the
above image.
The radio image is superposed on an
optical survey image of the same part of the sky.
Strange patterns in the radio lobes likely indicate
slight changes in the directions of the jets.
APOD: 2005 April 3 - The Galactic Center Radio Arc
Explanation:
What causes this unusual structure near the center of our Galaxy?
The long parallel rays slanting across the top of the
above radio image are known collectively as the
Galactic Center Radio Arc and jut straight out from the
Galactic plane.
The Radio Arc is connected to the
Galactic center
by strange curving filaments known as the Arches.
The bright radio structure at the bottom right
likely surrounds a
black hole at the
Galactic center and is known as
Sagittarius A*.
One origin hypothesis holds that the
Radio Arc and the Arches have their geometry
because they contain hot
plasma flowing along lines of constant
magnetic field.
Recent images from the
Chandra X-ray Observatory
appear to show this
plasma colliding with a nearby cloud of cold gas.
APOD: 2005 April 2 - Cyg X-1: Can Black Holes Form in the Dark?
Explanation:
The formation of a
black hole from the collapsing
core of a massive star is thought to be heralded by a spectacular
supernova explosion.
Such an extremely energetic collapse is also a
leading explanation
for the mysterious cosmic gamma-ray bursts.
But researchers now suggest that the Milky Way's most
famous black hole,
Cygnus X-1, was born
when a massive star collapsed --
without
any supernova explosion at all.
Their dynamical evidence is summarized in this
color image of a gorgeous
region in Cygnus,
showing Cyg X-1 and a cluster of massive stars
(yellow circles) known as Cygnus OB3.
Arrows compare the measured direction and speed of Cyg X-1
and the average direction and speed of the massive stars
of Cyg OB3.
The similar motions indicate that
Cyg X-1's progenitor star was itself a cluster member
and that its path was not altered at all when it
became a black hole.
In contrast, if Cyg X-1 were born in a violent supernova
it would have likely received a
fierce
kick, changing its course.
If not a supernova, could
the formation of the Cyg X-1 black
hole have produced a
dark gamma-ray burst in
the
Milky Way?
APOD: 2005 March 30 - ULXs in M74
Explanation:
In visual appearance, M74 is a
nearly perfect face-on spiral
galaxy, about 30 million light-years away toward the
constellation Pisces.
The red blotches seen in
this
composite view are ultraluminous
x-ray sources (ULXs) mapped by the
Chandra X-ray Observatory.
The ULXs are so called because they actually do radiate 10 to
1,000 times more x-ray power than "ordinary"
x-ray binary stars,
which harbor a neutron star or
stellar mass
black hole.
In fact,
watching
these ULXs change their
x-ray brightness over periods of 2 hours or so, astronomers
conclude that ULXs could well be
intermediate mass black holes --
black holes with
masses 10,000 times or so greater than the Sun, but still much less
than the million solar mass black holes which
lurk in the centers
of large spiral galaxies.
How did these intermediate mass black holes get there?
One intriguing suggestion is that they are left over from
the cores of much smaller galaxies that are
merging with
spiral galaxy M74.
APOD: 2005 March 12 - Accretion Disk Simulation
Explanation:
Don't be fooled by the familiar pattern.
The graceful spiral structure seen
in this computer visualization does not portray winding spiral arms in
a distant galaxy of stars.
Instead, the graphic shows spiral shock waves in a three dimensional
simulation of an
accretion disk -- material swirling onto a compact
central object that could represent a
white dwarf star,
neutron star,
or black hole.
Such accretion disks power bright
x-ray sources
within our own galaxy.
They form in binary star systems which consist of a donor star (not
shown above), supplying the accreting material, and a compact object
whose strong
gravity ultimately draws the material towards its surface.
For known
x-ray
binary systems the size of the accretion disk
itself might fall somewhere between the diameter of the Sun
(about 1,400,000 kilometers) and the diameter of the
Moon's orbit (800,000 kilometers).
One interesting result of the
virtual reality astrophysics
illustrated here is that the simulated
disk develops instabilities
which tend to smear out the pronounced spiral shocks.
APOD: 2005 January 30 - The Holographic Principle
Explanation:
Is this image worth a thousand words?
According to the Holographic Principle, the most
information you can get from this image is about
3 x 1065 bits for a normal sized computer monitor.
The Holographic Principle, yet unproven, states that
there is a maximum amount of information content
held by regions adjacent to any surface.
Therefore, counter-intuitively, the information content
inside a room depends not on the volume of the room but
on the area of the bounding walls.
The principle derives from the idea that the
Planck length, the length scale where
quantum mechanics begins to dominate
classical gravity, is one side of an area
that can hold only about one bit of information.
The limit was first postulated by physicist
Gerard 't Hooft in 1993.
It can arise from generalizations from seemingly
distant speculation that the information held by a
black hole is determined not by its
enclosed volume but by the
surface area of its
event horizon.
The term "holographic" arises from a
hologram analogy where three-dimension images are
created by projecting light through a flat screen.
Beware, other people looking at the
above image may not claim to see 3 x 1065 bits --
they might claim to see a
teapot.
APOD: 2005 January 28 - The Swarm
Explanation:
What do you
call a group of black
holes ... a flock, a brace, a swarm?
Monitoring a region around the
center of our Galaxy,
astronomers have indeed found
evidence for a surprisingly large number of
variable x-ray sources - likely black holes or neutron stars in
binary star systems - swarming
around the Milky Way's own
central supermassive black hole.
Chandra Observatory combined
x-ray image data from their monitoring
program
is shown above,
with four variable sources circled and labeled A-D.
While four sources may not make a swarm,
these all lie
within only three light-years of the central supermassive black
hole known as
Sgr A* (the bright source just above C).
Their detection
implies that a much larger concentration of
black hole systems is present.
Repeated gravitational interactions with other stars are
thought to cause the
black hole systems to spiral inward
toward the Galactic Center region.
APOD: 2005 January 12 - Barred Spiral Galaxy NGC 1300
Explanation:
Big, beautiful, barred spiral galaxy
NGC 1300 lies some 70 million
light-years away on the banks of the
constellation
Eridanus.
This Hubble Space Telescope
composite view of the gorgeous island
universe was released at this week's meeting of the American Astronomical
Society as one of the
largest Hubble images ever made of a complete
galaxy.
NGC 1300
spans over 100,000 light-years and the Hubble
image reveals striking details of the galaxy's dominant
central bar and majestic spiral arms.
In fact, on close inspection
the nucleus of this classic barred
spiral itself shows a remarkable region of
spiral structure
about 3,000 light-years across.
Unlike other spiral galaxies, including our own Milky Way, NGC 1300
is not presently known to have a massive
central black hole.
APOD: 2005 January 4 - Milky Way Illustrated
Explanation:
What does our Milky Way Galaxy look like from afar?
Since we are stuck inside, and since opaque
dust truncates our view in
visible light, nobody knows for sure.
Drawn above, however, is a good guess based on many different types of observations.
In the Milky Way's center
is a very bright core region centered on a
large black hole.
The Milky Way's bright
central bulge
is now thought to be an
asymmetrical bar
of relatively old and red stars.
The outer regions are where the
spiral arms
are found, dominated in appearance by
open clusters
of young, bright, blue stars, by red
emission nebula, and by dark dust.
The spiral arms reside in a disk dominated in mass by relatively dim stars and
loose gas composed mostly of
hydrogen.
What is not depicted is a huge spherical halo of invisible
dark matter that dominates the mass of the
Milky Way
as well as the motions of stars away from the center.
APOD: 2004 December 27 - Andromeda's Core
Explanation:
The center of the Andromeda galaxy is beautiful but strange.
Andromeda,
indexed as M31, is so close to our own
Milky Way Galaxy that it
gives a unique perspective into galaxy composition by
allowing us to see into its core.
Billions of stars swarm around a center that has
two nuclei and likely houses a
supermassive black hole
over 5 million times the mass of our
Sun.
M31 is about two million light years away and
visible with the unaided eye towards the constellation of Andromeda, the princess.
Pictured above, dark
knots of
dust are seen superposed on the inner 10,000
light years of M31's core.
The brighter stars are foreground stars located in our
Milky Way Galaxy.
APOD: 2004 December 26 - GRO J165540: Evidence for a Spinning Black Hole
Explanation:
In the center of a swirling whirlpool of hot gas
is likely a beast that has never been seen directly: a
black hole.
Studies of the bright light emitted by the
swirling gas frequently indicate not only that a
black hole is present, but also likely attributes.
The gas surrounding GRO J1655-40, for example, has been found to display an unusual flickering
at a rate of 450 times a second.
Given a
previous mass estimate for the central object of seven times the mass of our
Sun, the rate of the
fast flickering can be
explained by a black hole
that is rotating very rapidly.
What physical mechanisms actually cause the flickering -- and a slower quasi-periodic oscillation (QPO) -- in
accretion disks surrounding
black holes and
neutron stars remains a topic of much research.
APOD: 2004 December 11 - M87's Energetic Jet
Explanation:
An
energetic jet from the core of
giant elliptical galaxy M87 stretches outward for 5,000 light-years.
This monstrous jet
appears
in the panels above
to be a knotted and irregular structure, detected
across the spectrum, from
x-ray to optical to
radio wavelengths.
In all these bands,
the observed emission is likely created as high energy
electrons spiral along magnetic field lines, so called
synchrotron radiation.
But what powers this cosmic blowtorch?
Ultimately, the jet is thought to be produced
as matter near the center of M87
swirls toward a spinning, supermassive black hole.
Strong electromagnetic forces are generated and eject material
away from
the black hole along
the axis of rotation in a narrow jet.
Galaxy M87
is about 50 million light-years away and reigns as
the large central elliptical galaxy in the
Virgo
cluster.
APOD: 2004 November 7 - The Galactic Center in Infrared
Explanation:
The center of our Galaxy is a busy place. In visible light, much of the
Galactic Center
is obscured by opaque
dust.
In infrared light, however,
dust glows more and obscures less,
allowing nearly one million stars to be recorded in the
above photograph.
The
Galactic Center
itself appears
on the right and is located about 30,000
light years away towards the constellation of Sagittarius.
The
Galactic Plane of our
Milky Way Galaxy, the plane in which the
Sun orbits, is identifiable by the
dark diagonal dust lane.
The absorbing
dust grains are created in the atmospheres of cool
red-giant stars and grow in
molecular clouds.
The region directly surrounding the
Galactic Center
glows brightly in
radio and
high-energy radiation,
and is thought to house a large
black hole.
APOD: 2004 November 6 - X-Rays from the Galactic Core
Explanation:
Using the orbiting
Chandra
X-ray Observatory, astronomers have taken
this long look at the core of our
Milky Way galaxy, some 26,000 light-years away.
The spectacular false-color view spans about 130 light-years.
It reveals an energetic region
rich in x-ray sources and high-lighted by the
central source, Sagittarius A*, known
to be a supermassive black hole
with 3 million times the
mass of
the Sun.
Given its tremendous mass, Sagittarius A* is amazingly faint in x-rays
in comparison to central black holes observed in
distant galaxies,
even during its frequent x-ray flares.
This suggests that this supermassive black hole has been
starved
by a lack of infalling material.
In fact, the sharp Chandra image shows clouds of multi-million
degree gas dozens of light-years across flanking
(upper right and lower left) the
central
region -- evidence that violent events have
cleared much material from
the vicinity of the black hole.
APOD: 2004 September 16 - Microquasar in Motion
Explanation:
Microquasars, bizarre binary star systems
generating high-energy radiation and blasting out jets of particles
at nearly the speed of light, live in our
Milky Way galaxy.
The energetic microquasar systems seem to consist of a very compact
object, either a neutron star or a black hole, formed in a supernova
explosion but still co-orbiting with
an otherwise normal star.
Using a very
long array of radio telescopes,
astronomers
are reporting
that at least one microquasar, LSI +61 303, can be
traced back
to its probable birthplace -- within a cluster of young stars
in the constellation Cassiopeia.
About 7,500 light-years from Earth, the star cluster and surrounding
nebulosity, IC 1805, are shown in the
deep sky image above.
The cluster stars are identified by yellow boxes and circles.
A yellow arrow indicates the common apparent motion of the
cluster stars, the green arrow shows the deduced sky motion of the
microquasar system, and the red arrow depicts the microquasar's
motion relative to the star cluster itself.
Seen nearly 130 light-years from the cluster it once called home,
a powerful kick from the original
supernova explosion likely set this
microquasar in motion.
APOD: 2004 September 8 - Molecular Torus Surrounds Black Hole
Explanation:
Why do some black hole surroundings appear brighter than others?
In the centers of
active galaxies, supermassive
black holes at least
thousands of times the mass of our
Sun dominate.
Many, called
Seyfert Type I, are very bright in visible light.
Others, called Seyfert Type II, are rather dim.
The difference might be caused by some
black holes accreting
much more matter than others.
Alternatively, the black holes in the center of
Seyfert
Type II galaxies might be obscured by a surrounding
torus.
To help choose between these competing hypotheses,
the nearby Seyfert II galaxy
NGC 4388 has been observed in
X-ray light recently by many recent Earth-orbiting
X-ray observatories, including
CGRO,
SIGMA,
BeppoSAX,
INTEGRAL,
Chandra, and
XMM-Newton.
Recent data
from INTEGRAL and XMM-Newton have
found that the X-ray flux in some X-ray colors varies rapidly,
while flux in other X-ray colors is quite steady.
The constant flux and apparent absorption of very
specific X-ray colors by cool
iron together
give evidence that the central black hole in NGC 4388 is seen through a
thick torus composed of
molecular gas and dust.
APOD: 2004 July 1 - NGC 7331: A Galaxy So Inclined
Explanation:
If our own Milky Way
galaxy
were 50 million light-years
away with its disk inclined slightly to our line of sight, it
would look a lot like large
spiral
galaxy NGC 7331.
In fact, seen here in a
false-color infrared image from
the Spitzer Space Telescope,
NGC 7331 is interesting in
part because it is thought to be so similar to the
Milky Way.
Light from older, cooler stars, shown in blue, dominates the
central bulge of
NGC 7331, while Spitzer data also indicates the
presence of a black hole within this galaxy's central regions -
about the same size as the black hole at
our own galactic core.
Shown in red and brown, radiation from complex molecules associated
with dust traces NGC 7331's star forming spiral arms.
The arms span around 100,000 light-years,
about
the size of the Milky Way.
Curiously, a further
star forming ring is visible in yellowish hues,
20,000 light-years or so from the center of NGC 7331,
but it is not known if such a
structure
exists within our own galaxy.
APOD: 2004 June 24 - The Galaxy Within Centaurus A
Explanation:
Peering
deep inside Centaurus A, the closest
active galaxy
to Earth, the Spitzer Space Telescope's
penetrating infrared
cameras recorded
this startling vista.
About 1,000 light-years across, the twisted cosmic dust cloud
apparently shaped like a
parallelogram is likely the result
of a smaller spiral galaxy falling into the giant
Centaurus A.
The parallelogram lies along the active galaxy's central band
of dust and stars visible in more
familiar optical images.
Astronomers believe that the striking geometric shape
represents an approximately edge-on view of the infalling
spiral galaxy's disk in the process of being
twisted and warped
by the interaction.
Ultimately, debris from the ill-fated spiral galaxy should
provide fuel for the supermassive
black hole lurking
at the center of
Centaurus A.
APOD: 2004 April 11 - Journey to the Center of the Galaxy
Explanation:
In Jules Verne's
science fiction classic
A Journey
to the Center of the Earth, Professor Liedenbrock
and his fellow explorers encounter many strange and exciting wonders.
What wonders lie at the center of
our Galaxy?
Astronomers know of some of the bizarre objects that exist there,
like vast cosmic dust clouds,
bright star clusters,
swirling rings of gas, and even a
supermassive black hole.
Much of the Galactic Center is shielded from our view in visible light
by the intervening dust and gas, but it can be explored using
other forms of
electromagnetic radiation.
This haunting wide angle image of the Galactic Center region in
infrared light was constructed using data from the
Midcourse Space Experiment (MSX) satellite.
The image maps three
mid-infrared bands, otherwise invisible to human eyes,
into visible blue, green, and red colors revealing the thermal emission from
dust clouds near the galactic center that have been heated by starlight.
The galactic plane
runs along
the middle of this image while the galactic center itself
is the bright spot at picture center.
The field of view of this cropped picture is about 1.5 by 2.5 degrees.
APOD: 2004 February 24 - X-Rays Indicate Star Ripped Up by Black Hole
Explanation:
What could rip a star apart?
A black hole.
Giant black holes in just the right mass range would
pull on the front of a closely passing star much more
strongly than on the back.
Such a strong tidal force would stretch out a star and
likely cause some of the star's gasses to fall into the black hole.
The infalling gas has been predicted
to emit just the same blast of
X-rays that have
recently been seen in the center of galaxy
RX J1242-11.
Above, an artist's illustration depicts the
sequence of destruction (assuming that
image-distorting gravitational-lens effects of the
black hole are somehow turned off).
Most of the stellar remains would be flung out into the galaxy.
Such events are rare, occurring perhaps only
one in 10,000 years for typical
black holes at the center of
typical galaxies.
APOD: 2004 February 13 - NGC 613: Spiral of Dust and Stars
Explanation:
When morning twilight came to the
Paranal Observatory in Chile,
astronomers Mark Neeser and Peter Barthel interrupted their
search for faint
quasars, billions of
light-years away.
And just for a moment, they used
Very Large Telescopes
at the European Southern Observatory to appreciate the beauty of
the nearby Universe.
One result was
this stunning view
of beautiful barred spiral galaxy
NGC 613, a mere 65 million light-years away
in the southern constellation
Sculptor.
Over 100 thousand light-years across,
NGC 613 seems to have more than its fair
share of spiral arms
laced with cosmic dust clouds and bright star forming regions
near the ends of a dominant central bar.
Radio emission indicates the presence of a massive
black hole
at the center of
NGC 613.
APOD: 2004 February 3 - X-Rays From Antennae Galaxies
Explanation:
A bevy of
black holes and
neutron stars
shine as bright, point-like
sources against bubbles of
million degree gas in this
false-color
x-ray image from the
orbiting Chandra Observatory.
The striking picture spans about 80 thousand light-years across the
central regions of two
galaxies, NGC 4038 and NGC 4039, locked in a titanic collision
some 60 million light-years away in the
constellation Corvus.
In visible light images, long, luminous,
tendril-like structures emanating
from the wreckage lend the pair their
popular moniker, the Antennae Galaxies.
Galactic collisions are now thought to be
fairly common, but when
they happen individual stars rarely collide.
Instead gas and dust clouds merge and compress, triggering furious
bursts of massive star formation with
thousands of resulting supernovae.
The exploding stars litter the scene with bubbles of shocked gas
enriched
in heavy elements, and collapsed stellar cores.
Transfixed by this cosmic accident
astronomers watch and are beginning
to appreciate the
collision-driven evolution
of galaxies, not unlike our own.
APOD: 2003 November 28 - The Most Distant X-Ray Jet
Explanation:
A false-color
x-ray
image inset at upper left reveals emission
from a cosmic jet of high-energy particles, 100,000 light-years
in length, emerging
from
quasar GB1508+5714.
An estimated 12 billion (12,000,000,000) light-years away,
this appears to be the most distant energetic jet in the
known Universe.
Astrophysical jets of many sizes seem to be produced in a range of
environments where significant accretion, or infalling matter is
thought to arrange itself
in a disk, from contracting
star-forming
clouds to supermassive black holes in active
galactic nuclei.
Here, as depicted in the illustration, the accretion disk
is thought to surround a
supermassive black hole, accelerating
particles to near the
speed of light in two jets at right
angles to the disk itself.
In the case of
this quasar,
the jet tilted towards us is visible in
x-rays as the particles collide with low energy photons from the
cosmic
background radiation.
The collisions boost the photons to higher x-ray energies and
scatter some of them in our direction.
APOD: 2003 October 17 - Astronomy Quilt of the Week
Explanation:
Demonstrating her mastery of a
traditional
astronomical imaging technique
quilter and
astronomy enthusiast Judy Ross
has produced this spectacular composition of "Astronomy
Quilt Piece of the Week".
Her year-long effort resulted in an arrangement for a
six by seven foot quilt consisting
of 52 individual pieces
(11 inches by 8 inches), one for each week,
which she reports were inspired by her steady diet of
APOD's daily offerings.
Some of the pieces are based on actual pictures, such as the
Hubble Space Telescope's view of planet forming
AB Aurigae or Bill Keel's image of the
nearby Pinwheel Galaxy.
Others, with titles like
the Blue
Carpet Nebula and
Duck
Contemplates Black Hole, are from her own creative imaginings.
APOD: 2003 October 8 - The Sombrero Galaxy from HST
Explanation:
Why does the Sombrero Galaxy look like a hat?
Reasons include the
Sombrero's unusually large and extended central bulge of stars,
and dark prominent dust lanes
that appear in a disk that we see nearly
edge-on.
Billions of old stars
cause the diffuse glow of the extended central bulge.
Close inspection of the bulge in the
above photograph shows many points of light that are actually
globular clusters.
M104's spectacular dust rings harbor many younger
and brighter stars, and show intricate details astronomers
don't yet fully understand.
The very center of the
Sombrero glows across the electromagnetic spectrum, and is thought to house a large
black hole.
Fifty million-year-old light from the
Sombrero Galaxy can be seen with a
small telescope towards the constellation of Virgo.
APOD: 2003 September 12 - A Note on the Perseus Cluster
Explanation:
A truly enormous collection of thousands of galaxies, the
Perseus Cluster - like other
large galaxy clusters - is
filled with hot, x-ray emitting gas.
The x-ray hot gas
(not the individual galaxies) appears
in the left panel above, a false color
image
from the Chandra Observatory.
The bright central source flanked by two
dark cavities is
the cluster's supermassive black hole.
At right, the panel shows the
x-ray image
data specially processed
to enhance contrasts and reveals a strikingly regular
pattern of pressure waves
rippling through
the hot gas.
In other words,
sound
waves, likely generated by bursts of
activity from the black hole, are ringing through the
Perseus Galaxy Cluster.
Astronomers infer that these previously unknown sound waves are a
source of energy which keeps the cluster gas so hot.
So what note is the Perseus Cluster playing?
Estimates of the distance between the wave peaks and sound speed
in the cluster gas suggests
the cosmic note is about 57 octaves below B-flat above middle C.
APOD: 2003 September 7 - The Galactic Center in Infrared
Explanation:
The center of our Galaxy is a busy place. In visible light, much of the
Galactic Center
is obscured by opaque
dust.
In infrared light, however,
dust glows more and obscures less,
allowing nearly one million stars to be recorded in the
above photograph.
The
Galactic Center
itself appears
on the right and is located about 30,000
light years away towards the constellation of Sagittarius.
The
Galactic Plane of our
Milky Way Galaxy, the plane in which the
Sun orbits, is identifiable by the
dark diagonal dust lane.
The absorbing
dust grains are created in the atmospheres of cool
red-giant stars and grow in
molecular clouds.
The region directly surrounding the
Galactic Center
glows brightly in
radio and
high-energy radiation,
and is thought to house a large
black hole.
APOD: 2003 July 12 - X-Ray Milky Way
Explanation:
If you had x-ray vision,
the center regions
of our Galaxy would not be hidden from
view by the immense cosmic dust clouds
opaque to visible light.
Instead,
the Milky Way
toward Sagittarius might look something
like this stunning mosaic
of images from the orbiting
Chandra Observatory.
Pleasing to look at, the gorgeous false-color
representation of
the x-ray data shows
high energy x-rays in blue, medium energies in green,
and low energies in red.
Hundreds of white dwarf stars,
neutron stars, and black holes immersed in a
fog of multimillion-degree gas are included in the
x-ray vista.
Within the white patch at the image center lies
the Galaxy's central supermassive black hole.
Chandra's sharp
x-ray vision will likely lead to a new
appreciation of our Milky Way's most active neighborhood
and has already indicated that the hot gas itself may
have a temperature of a mere 10 million degrees Celsius
instead of 100 million degrees as previously thought.
The full mosaic is composed of 30 separate images and covers
a 900 by 400 light-year swath
at
the galactic center.
APOD: 2003 July 11 - NGC 1068 and the X-Ray Flashlight
Explanation:
At night,
tilting a flashlight up under your chin hides the
glowing bulb from the direct view of your friends.
Light from the bulb still reflects from your face though, and can
give you a startling appearance.
Spiral
Galaxy NGC 1068
may be playing a similar trick on a
cosmic scale,
hiding a central powerful source of x-rays -- likely a
supermassive black hole -- from direct view.
X-rays are
still scattered into our line-of-sight
though, by a dense torus of material surrounding the black hole.
The scenario is
supported by x-ray data from the
Chandra Observatory combined with a Hubble Space
Telescope optical image in
this
false-color composite picture.
Optical data in red shows spiral structure across NGC 1068's
inner 7 thousand light-years with the x-ray data overlaid in blue
and green.
A hot wind of gas streaming from the galaxy's core
is seen as the broad swath of x-ray emission while material
lit up
by the hidden black hole source is within the central
cloud of more intense x-rays.
Also well known
as M77, NGC 1068 lies a mere 50 million
light-years away toward the constellation Cetus.
APOD: 2003 July 5 - Centaurus A: X-Rays from an Active Galaxy
Explanation:
Its core hidden
from optical view by a thick lane of dust, the giant elliptical
galaxy
Centaurus A was among the first objects
observed by the orbiting Chandra X-ray Observatory.
Astronomers were not disappointed, as Centaurus A's
appearance in x-rays makes its classification as an
active galaxy easy to appreciate.
Perhaps the most striking feature of
this
Chandra false-color x-ray view
is the jet, 30,000 light-years long.
Blasting toward the upper left corner of the picture,
the jet
seems to arise from the galaxy's bright central x-ray source --
suspected of harboring a black hole with a million or so times
the mass of the Sun.
Centaurus A
is also seen to be teeming with other
individual x-ray sources and a pervasive, diffuse
x-ray glow.
Most of these individual sources are likely to be neutron stars
or solar mass black holes accreting material from their less
exotic binary companion stars.
The diffuse high-energy glow
represents gas throughout the galaxy
heated to temperatures of millions
of degrees C.
At 11 million light-years distant in the constellation
Centaurus,
Centaurus A (NGC 5128) is the closest
active galaxy.
APOD: 2003 June 13 - Neptune: Still Springtime After All These Years
Explanation:
In the 1960s spring came to the southern hemisphere of
Neptune,
the Solar System's outermost gas giant planet.
Of course, since Neptune orbits the Sun once every 165
earth-years,
it's still springtime for southern Neptune, where
each
season lasts over four decades.
Astronomers
have found that in recent years
Neptune has been
getting
brighter, as illustrated in
this Hubble Space Telescope image made in 2002.
Compared
to Hubble
pictures taken as early as 1996,
the 2002 image shows a dramatic increase in reflective
white cloud bands in Neptune's southern hemisphere.
Neptune's
equator is tilted 29 degrees from the plane of its orbit,
about the same as Earth's 23.5 degree tilt, and
Neptune's weather
seems to be dramatically responding to the
similar relative seasonal increase in sunlight -- even though
sunlight is 900 times
less intense for the distant gas giant than for planet Earth.
Meanwhile, summer is really just around the
corner, coming to
Neptune's southern hemisphere in 2005.
APOD: 2003 June 12 - Cyg X-1: Can Black Holes Form in the Dark?
Explanation:
The formation of a
black hole from the collapsing
core of a massive star is thought to be heralded by a spectacular
supernova explosion.
Such an extremely energetic collapse is also a
leading explanation
for the mysterious cosmic gamma-ray bursts.
But researchers now suggest that the Milky Way's most
famous black hole,
Cygnus X-1, was born when a massive
star collapsed --
without
any supernova explosion at all.
Their dynamical evidence is summarized in this
color image of a gorgeous
region in Cygnus,
showing Cyg X-1 and a cluster of massive stars
(yellow circles) known as Cygnus OB3.
Arrows compare the measured direction and speed of Cyg X-1
and the average direction and speed of the massive stars
of Cyg OB3.
The similar motions indicate that
Cyg X-1's progenitor star was itself a cluster member
and that its path was not altered at all when it
became a black hole.
In contrast, if Cyg X-1 were born in a violent supernova
it would have likely received a
fierce
kick, changing its course.
If not a supernova, could
the formation
of the Cyg X-1 black
hole have produced a dark
gamma-ray burst in
the
Milky Way?
APOD: 2003 June 1 - GRO J1655 40: Evidence for a Spinning Black Hole
Explanation:
In the center of a swirling whirlpool of hot gas
is likely a beast that has never been seen directly: a
black hole.
Studies of the bright light emitted by the
swirling gas frequently indicate not only that a
black hole is present, but also likely attributes.
The gas surrounding GRO J1655-40, for example, has been found to display an unusual flickering
at a rate of 450 times a second.
Given a
previous mass estimate for the central object of seven times the mass of our Sun,
the rate of the
fast flickering can be
explained by a black hole
that is rotating very rapidly.
What physical mechanisms actually cause the flickering --
and a slower quasi-periodic oscillation (QPO) -- in
accretion disks
surrounding
black holes and
neutron stars remains a topic of much research.
APOD: 2003 May 18 - The Holographic Principle
Explanation:
Is this image worth a thousand words?
According to the Holographic Principle, the most
information you can get from this image is about
3 x 1065 bits for a normal sized computer monitor.
The Holographic Principle, yet unproven, states that
there is a maximum amount of information content
held by regions adjacent to any surface.
Therefore, counter-intuitively, the information content
inside a room depends not on the volume of the room but
on the area of the bounding walls.
The principle derives from the idea that the
Planck length, the length scale where
quantum mechanics begins to dominate
classical gravity, is one side of an area
that can hold only about one bit of information.
The limit was first postulated by physicist
Gerard 't Hooft in 1993.
It can arise from generalizations from seemingly
distant speculation that the information held by a
black hole is determined not by its
enclosed volume but by the
surface area of its
event horizon.
The term "holographic" arises from a
hologram analogy where three-dimension images are
created by projecting light though a flat screen.
Beware, other people looking at the
above image may not claim to see 3 x 1065 bits --
they might claim to see a
teapot.
APOD: 2003 May 1 - The Energetic Jet from Centaurus A
Explanation:
The center of well-studied active galaxy
Centaurus A
is hidden from the view of optical
telescopes by a cosmic jumble of stars, gas, and dust.
But both radio and
x-ray
telescopes can trace the
remarkable jet
of high-energy particles streaming from the galaxy's core.
With Cen A's central region at the lower right,
this composite false-color image shows the
radio emission in red and x-rays in blue over
the inner 4,000 light-years of the jet.
One of the most detailed images of its kind,
the
picture shows how the x-ray
and radio emitting sites are related along the
jet, providing
a road map to understanding the energetic stream.
Extracting
its energy from a supermassive black hole at the
galaxy's center, the jet is confined to a relatively narrow angle
and seems to produce most of its x-rays (bluer colors) at the upper left,
farther from the core, where the jet begins to collide with
Centaurus A's
denser gas.
APOD: 2003 April 17 - M106 in Canes Venatici
Explanation:
Close to the Great Bear
(Ursa Major) and
surrounded by the stars
of the Hunting Dogs (Canes Venatici), this celestial nebula was
discovered
in 1781 by the
metric
French astronomer
Pierre Mechain
and later added to the catalog of his friend and colleague
Charles Messier as
M106.
Modern deep telescopic views reveal it to be an
island universe --
a spiral galaxy around 30 thousand light-years across located
only about 21 million light-years beyond the stars of the Milky Way.
Youthful blue star clusters and reddish stellar nurseries
trace the striking spiral arms of M106.
Seen so clearly in
this
beautiful image, the galaxy's bright
core is also visible
across
the spectrum from radio to x-rays,
making M106 a nearby example of the
Seyfert class of
active galaxies.
The bright core of a Seyfert galaxy is believed to be powered
by matter falling into a massive
central black hole.
APOD: 2003 February 6 - X-Rays from M83
Explanation:
Bright and beautiful spiral galaxy
M83 lies a mere
twelve million light-years from Earth, toward the
headstrong constellation
Hydra.
Sweeping spiral arms, prominent in visible light images,
lend this galaxy its popular moniker --
the Southern Pinwheel.
In fact, the spiral arms are still apparent in this
Chandra Observatory false-color
x-ray image of M83,
traced by diffuse, hot,
x-ray emitting gas.
But more striking in the
x-ray
image is the galaxy's bright central
region.
The central emission likely represents even hotter gas
created by a sudden burst
of massive star formation.
Point-like neutron star and black hole x-ray
sources,
final stages in the life cycles of massive stars,
also show a
concentration near
the center of M83 and offer
further evidence for a burst of star formation
at this galaxy's core.
Light from this burst of star formation
would have first reached Earth some 20 million years ago.
APOD: 2003 January 8 - X-Rays from the Galactic Core
Explanation:
Using the orbiting
Chandra
X-ray Observatory, astronomers have taken
this long look at the core of our
Milky Way galaxy, some 26,000 light-years away.
The spectacular false-color view spans about 130 light-years.
It reveals an energetic region
rich in x-ray sources and
high-lighted by the
central source, Sagittarius A*, known
to be a supermassive black hole
with 3 million times the
mass of
the Sun.
Given its tremendous mass, Sagittarius A* is amazingly faint in x-rays
in comparison to central black holes observed in
distant galaxies,
even during its frequent x-ray flares.
This suggests that this supermassive black hole has been
starved
by a lack of infalling material.
In fact, the sharp Chandra image shows clouds of multi-million
degree gas dozens of light-years across flanking
(upper right and lower left) the
central
region -- evidence that violent events have
cleared much material from
the vicinity of the black hole.
APOD: 2002 November 28 - The Supermassive Black Holes of NGC 6240
Explanation:
The
Hubble optical image on the left shows
NGC 6240 in
the throes of a
titanic galaxy - galaxy collision 400 million light-years away.
As the cosmic catastrophe plays out, the merging galaxies spew forth
distorted tidal tails
of stars, gas, and dust and undergo
frantic bursts of star formation.
Using the orbiting
Chandra
Observatory's x-ray vision to peer within
the bright central regions of NGC 6240 astronomers
believe they have uncovered,
for
the first time, not one but
two enormous orbiting black holes, by
detecting the characteristic x-ray radiation from the interstellar debris
swirling toward them.
In the false-color close-up view at right,
the x-ray data clearly show
the black hole
sources (shaded blue) separated by about 3,000 light-years.
Einstein's theory of gravity predicts that such a pair of black holes
must spiral closer together, and
ultimately coalesce into a single,
even more massive black hole
after
several hundred million
years.
In the final moments the merging supermassive black holes will
produce an extremely powerful burst of
gravitational radiation.
APOD: 2002 October 25 - Journey to the Center of the Galaxy
Explanation:
In Jules
Verne's science fiction classic
A Journey
to the Center of the Earth,
Professor Hardwigg and his fellow explorers
encounter many strange and exciting wonders.
What wonders lie at the
center of our Galaxy?
Astronomers know of some of the bizarre objects which exist there,
like vast cosmic dust clouds,
bright star clusters,
swirling rings of gas,
and even a
supermassive
black hole.
Much of the Galactic Center
is shielded from our view in visible light by the
intervening dust and gas,
but it can be explored using other forms of
electromagnetic
radiation.
This haunting wide angle image of the
Galactic Center region in infrared light was constructed
using data from the
Midcourse
Space Experiment (MSX) satellite.
The image maps three
mid-infrared bands, otherwise invisible
to human eyes, into visible blue, green, and red colors
revealing the thermal emission from dust clouds near the galactic
center that have been heated by starlight.
The galactic plane
runs along
the middle of this image while
the galactic center itself is the bright spot at picture center.
The field of view of this cropped picture is about 1.5 by 2.5 degrees.
APOD: 2002 October 18 - At the Center of the Milk Way
Explanation:
At the center of our
Milky
Way Galaxy lies a black hole with
over 2 million times the mass of the Sun.
Once a controversial claim, this
astounding conclusion
is now virtually inescapable and based on observations of
stars orbiting
very near the galactic center.
Using one of the Paranal
Observatory's very large telescopes
and the sophisticated infrared camera
NACO,
astronomers
patiently followed the orbit of a particular star,
designated S2, as it came within about 17 light-hours of the
center of the Milky Way
(17 light-hours is only about 3 times the radius of Pluto's orbit).
Their
results convincingly show that
S2 is moving
under the influence of the enormous gravity of an
unseen object which must be extremely compact -- a
supermassive black hole.
This deep NACO
near-infrared image shows the crowded inner 2
light-years of the Milky Way with the exact position of the
galactic center indicated by arrows.
NACO's ability to track stars so close to the
galactic center
can accurately
measure the black hole's mass and perhaps
even provide an unprecedented test of Einstein's
theory of gravity
as astronomers watch a star orbit a
supermassive black hole.
APOD: 2002 October 12 - Chandra Deep Field
Explanation:
Officially the
Chandra
Deep Field - South, this picture represents the deepest ever
x-ray image of the Universe.
One million seconds of accumulated exposure time with the orbiting
Chandra X-ray Observatory went in to its making.
Concentrating on a single, otherwise unremarkable patch
of sky in the constellation
Fornax,
this x-ray image corresponds to the
visible light Hubble Deep Field - South
released in 1998.
Chandra's view, color coded with low energies in red, medium in green,
and high-energy x-rays in blue, shows many faint sources of relatively
high-energy x-rays.
These are likely active galaxies feeding supermassive central
black holes
and large clusters of galaxies
at distances of up to 12 billion light-years.
The stunning picture supports
astronomers' ideas
of a youthful
universe in which massive black holes
were much more dominant than at present.
APOD: 2002 October 8 - The X-Ray Jets of XTE J1550
Explanation:
The motion of ultra-fast
jets shooting out from a candidate
black hole star system have now been documented
by observations from the orbiting
Chandra X-ray Observatory.
In 1998, X-ray source
XTE J1550-564 underwent a tremendous outburst.
Jets of material sent streaming into space at
near light-speed impacted existing gas heating it so much
it glowed in
X-ray light.
The panels on the left of the
above image show in X-rays that the
hot spots have moved out by more than three
light years in the time since the explosion,
with the left jet recently fading below detectability.
The drawing of the right depicts the
binary star system
that likely produced the X-ray jets,
with a normal red star on the left dumping matter into an
accretion disk around the
black hole on the right.
The jets are thought to be emitted along the
spin axis of the
black hole.
APOD: 2002 October 5 - X-Ray Cygnus A
Explanation:
Amazingly detailed,
this
false-color x-ray image is centered on the galaxy Cygnus A.
Recorded by the orbiting
Chandra Observatory, Cygnus A is
seen here as a
spectacular high energy x-ray source.
But it is actually more
famous at the low energy end of
the electromagnetic spectrum as one of
the brightest celestial radio sources.
Merely 700 million light-years distant,
Cygnus A is
the closest powerful radio galaxy and the false-color
radio image (inset right)
shows remarkable similarity to Chandra's x-ray view.
Central in both pictures, the center of
Cygnus A shines brightly while emission
extends 300,000 light-years to either side along the same axis.
Near light speed jets of atomic particles
produced by a massive central
black hole are believed to cause
the emission.
In fact, the x-ray image reveals "hot spots" suggestive
of the locations where the particle jets are stopped in
surrounding cooler, denser gas.
The x-ray image also shows that the jets have
cleared out a huge cavity in the surrounding gas.
Bright swaths of emission within the cavity likely indicate
x-ray hot material ... swirling toward the
central black hole.
APOD: 2002 September 28 - X-Ray Rainbows
Explanation:
A drop of water
or prism of
glass can spread out visible sunlight into
a
rainbow of colors.
In order of increasing energy, the well known spectrum of colors in
a rainbow
runs red, orange, yellow, green, blue, indigo, violet.
X-ray
light too can be spread out into
a spectrum
ordered by energy ... but not by drops of water or glass.
Instead, the orbiting
Chandra
X-ray Observatory
uses a set of 540 finely ruled, gold gratings to spread out the
x-rays, recording the results with digital detectors.
The resulting x-ray spectrum reveals much about the compositions,
temperatures, and motions within
cosmic x-ray sources.
This false color
Chandra image shows
the x-ray spectrum of a
star system in Ursa Major cataloged
as XTE J1118+480 and thought to consist of a sun-like star orbiting a
black hole.
Unlike the familiar appearance of a
prism's visible light rainbow,
the energies here are ordered
along radial lines with the highest energy x-rays near the center
and lowest energies near the upper left and lower right edges of the image.
The central spiky region itself is created by x-rays from the
source which are not spread out by the array of gratings.
APOD: 2002 September 27 - Accretion Disk Simulation
Explanation:
Don't be fooled by the familiar symmetry.
The graceful spiral structure seen
in this computer visualization does not portray winding spiral arms in
a distant galaxy of stars.
Instead, the graphic shows spiral shock waves in a three dimensional
simulation of an
accretion disk -- material
swirling
onto a compact
central object that could represent a
white dwarf star,
neutron star,
or black hole.
Such accretion disks power bright
x-ray sources
within our own galaxy.
They form in binary star systems which consist of a donor star (not
shown above), supplying the accreting material, and a compact object
whose strong
gravity ultimately draws the material towards its surface.
For known
x-ray
binary systems the size of the accretion disk
itself might fall somewhere between the diameter of the Sun
(about 1,400,000 kilometers) and the diameter of the
Moon's orbit (800,000 kilometers).
One interesting result of the
virtual
reality astrophysics
illustrated here is that the simulated
disk develops instabilities
which tend to smear out the pronounced spiral shocks.
APOD: 2002 September 8 - Too Close to a Black Hole
Explanation:
What would you see if you went right up to a
black hole?
Above are two computer generated images
highlighting how strange things would look.
On the left is a normal star field containing the
constellation Orion.
Notice the three stars of nearly equal brightness that make up
Orion's Belt.
On the right is the same star field but this time with a
black hole superposed in the center of the frame.
The
black hole has such strong
gravity that light is noticeably bent towards it -
causing some very unusual
visual distortions.
In the distorted frame, every star in the normal frame
has at least two bright images - one on each side of the
black hole.
In fact, near the
black hole, you can see the whole sky -
light from every direction is
bent around and comes back to you.
Black holes are thought to be the densest state of matter,
and there is
indirect evidence for their presence in
stellar binary systems
and the centers of
globular clusters,
galaxies, and
quasars.
APOD: 2002 August 3 - The Galactic Center A Radio Mystery
Explanation:
Tuning in
to the center of our Milky Way galaxy,
radio
astronomers explore a complex, mysterious place.
A premier high resolution view,
this startlingly beautiful picture
covers a 4x4 degree region around the galactic center.
It was constructed from 1 meter wavelength radio data
obtained by telescopes of the
Very Large Array near
Socorro, New Mexico, USA.
The
galactic center
itself is at the edge of the extremely
bright object labeled Sagittarius (Sgr) A, suspected of harboring
a million solar mass black hole.
Along the galactic plane which runs diagonally
through the image are tortured clouds of gas energized by
hot stars and bubble-shaped supernova remnants (SNRs)
- hallmarks of
a violent and
energetic cosmic environment.
But perhaps most intriguing are
the arcs, threads, and filaments which
abound in the scene.
Their uncertain origins challenge
present theories of the dynamics of the galactic center.
APOD: 2002 July 21 - Nearby Spiral Galaxy NGC 4945
Explanation:
For such a close galaxy, NGC 4945 is easy to miss.
NGC 4945 is a
spiral galaxy in the
Centaurus Group of galaxies,
located only six times farther away than the prominent
Andromeda Galaxy. The
thin disk galaxy
is oriented nearly edge-on, however, and shrouded in dark
dust.
Therefore galaxy-gazers searching the
southern constellation of Centaurus need a telescope to see it.
The above picture was taken with a large telescope testing a new wide-angle, high-resolution CCD camera.
Most of the spots scattered about the
frame are
foreground stars in our own
Galaxy, but some spots are
globular clusters
orbiting the distant galaxy.
NGC 4945 is thought to be
quite similar to our own
Milky Way Galaxy.
X-ray observations reveal, however,
that NGC 4945 has an unusual, energetic,
Seyfert 2 nucleus
that might house a
large black hole.
APOD: 2002 July 11 - M51: X Rays from the Whirlpool
Explanation:
Fresh from yesterday's episode,
a popular pair of interacting galaxies
known as the
Whirlpool debut here beyond
the realm of visible
light -- imaged at high energies by the orbiting Chandra X-ray
Observatory.
Still turning in a remarkable performance, over 80 glittering
x-ray stars are present in
the
Chandra image data from the region.
The number of luminous x-ray
sources, likely neutron star and black hole
binary
systems within the confines of M51, is unusually high
for normal spiral or elliptical galaxies and suggests this cosmic
whirlpool has experienced
intense
bursts of massive star formation.
The bright cores of both galaxies, NGC 5194 and NGC 5195
(right and left respectively), also exhibit high-energy
activity in this false-color x-ray picture showing a diffuse
glow from multi-million degree gas.
An
expanded view of the region near the core of NGC 5194
reveals x-rays
from a supernova remnant, the debris from
a spectacular stellar explosion,
first detected by
earthbound astronomers in 1994.
APOD: 2002 June 17 - NGC 4697: X-Rays from an Elliptical Galaxy
Explanation:
The many bright, point-like sources in
this
Chandra Observatory x-ray image lie within NGC 4697, an
elliptical galaxy some 40 million
light-years away towards Virgo.
Like other normal
elliptical
galaxies, NGC 4697 is a spherical
ensemble of mainly older, fainter, low mass stars, with
little star forming gas and dust compared to spiral galaxies.
But the luminous x-ray
sources in the Chandra image indicate that
NGC 4697 had a wilder youth.
Powering the x-ray sources are neutron stars
and black holes in
binary
star systems, where x-rays are generated as matter from
a more ordinary companion star falls in to these bizarre,
compact objects.
Since neutron
stars and
black holes
are the endpoints in the lives of
massive stars, NGC 4697 must have had many bright, massive stars
in its past.
An exceptionally large number of NGC 4697's x-ray binaries are
found in the galaxy's globular star clusters, suggesting that
dense star clusters
are a good place for neutron stars and
black holes to capture a companion.
Stellar winds and
supernovae explosions of massive stars
could also have produced the hot gas responsible
for this galaxy's diffuse x-ray glow.
APOD: 2002 May 21 - The Galactic Center Radio Arc
Explanation:
What causes this unusual structure near the center of our Galaxy?
The long parallel rays slanting across the top of the
above radio image are known collectively as the
Galactic Center Radio Arc and jut straight out from the
Galactic plane.
The Radio Arc is connected to the
Galactic center
by strange curving filaments known as the Arches.
The bright radio structure at the bottom right
likely surrounds a
black hole at the
Galactic center and is known as
Sagittarius A*.
One origin hypothesis holds that the
Radio Arc and the Arches have their geometry
because they contain hot
plasma flowing along lines of constant
magnetic field.
Recent images from the
Chandra X-ray Observatory
appear to show this
plasma colliding with a nearby cloud of cold gas.
APOD: 2002 April 30 - The Holographic Principle
Explanation:
Is this image worth a thousand words?
According to the Holographic Principle, the most
information you can get from this image is about
3 x 1065 bits for a normal sized computer monitor.
The Holographic Principle, yet unproven, states that
there is a maximum amount of information content
held by regions adjacent to any surface.
Therefore, counter-intuitively, the information content
inside a room depends not on the volume of the room but
on the area of the bounding walls.
The principle derives from the idea that the
Planck length, the length scale where
quantum mechanics begins to dominate
classical gravity, is one side of an area
that can hold only about one bit of information.
The limit was first postulated by physicist
Gerard 't Hooft in 1993.
It can arise from generalizations from seemingly
distant speculation that the information held by a
black hole is determined not by its
enclosed volume but by the surface area of its
event horizon.
The term "holographic" arises from a
hologram analogy where three-dimension images are
created by projecting light though a flat screen.
Beware, other people looking at the
above image may not claim to see 3 x 1065 bits --
they might claim to see a
teapot.
APOD: 2002 April 21 - The Center of Centaurus A
Explanation:
A fantastic jumble of young blue star clusters,
gigantic glowing gas clouds, and imposing dark
dust lanes
surrounds the central region of the active galaxy Centaurus A.
This mosaic of Hubble Space Telescope images
taken in blue, green, and red light
has been processed to present a natural color picture
of this cosmic maelstrom.
Infrared images from the Hubble have also shown that
hidden at the center of this activity are what seem to
be disks of matter spiraling into
a black hole with a billion times the mass
of the Sun!
Centaurus A itself is apparently the result of a
collision of two galaxies
and the left over debris is steadily being consumed by the
black hole.
Astronomers believe that such
black hole central engines generate the
radio, X-ray, and gamma-ray energy radiated
by Centaurus A and other active galaxies.
But for an active galaxy Centaurus A is close,
a mere 10 million
light-years away,
and is a relatively convenient laboratory for exploring these
powerful sources of energy.
APOD: 2002 March 9 - A Quasar Portrait Gallery
Explanation:
Quasars
(QUASi-stellAR objects) lie near the edge of the observable
Universe.
Discovered in 1963,
astronomers
were astounded that such objects could be
visible across billions of light-years, as this implies
they must emit prodigious
amounts of energy.
Where does
the
energy come from?
Many believe the quasar's central engine is a giant black hole
fueled by tremendous amounts of infalling gas, dust, and stars.
This
gallery of quasar portraits from the Hubble Space
Telescope offers a look at their local neighborhoods: the quasars themselves
appear as the bright star-like objects with
diffraction spikes.
The
images in the center and right hand columns reveal quasars
associated with disrupted colliding and merging galaxies
which should provide
plenty
of debris to feed a hungry
black
hole.
APOD: 2002 February 26 - Jets from Radio Galaxy 3C296
Explanation:
Jets of streaming
plasma expelled by the central
black hole of a massive
elliptical galaxy
likely light up this composite image of
3C296.
The jets emanating from NGC 5532 and are nearly a million light years long.
Exactly how the
central black hole expels the
infalling matter is still unknown.
After clearing the galaxy, however, the
jets inflate large
radio bubbles that could glow for millions of years.
If excited by a passing front,
radio bubbles can even light up again after a billion years.
Visible light is depicted in the
above image in blue, while
radio waves are shown in red.
The radio map was created with the
Very Large Array of
radio telescopes.
APOD: 2002 February 17 - The Local Bubble and the Galactic Neighborhood
Explanation:
What surrounds the Sun in this neck of the
Milky Way Galaxy?
Our current best guess is depicted in the
above map of the surrounding 1500
light years constructed from various observations and deductions.
Currently, the Sun is passing through a
Local Interstellar Cloud
(LIC), shown in violet, which is
flowing away from the
Scorpius-Centaurus Association of young stars.
The LIC resides in a low-density hole in the
interstellar medium (ISM) called the
Local Bubble, shown in black.
Nearby, high-density
molecular clouds including the
Aquila Rift surround star forming regions,
each shown in orange.
The Gum Nebula, shown in green,
is a region of hot
ionized
hydrogen gas.
Inside the Gum Nebula is the
Vela Supernova Remnant,
shown in pink, which is expanding to create
fragmented shells of material like the
LIC.
Future observations should help astronomers
discern more about the local Galactic Neighborhood and how it might have affected
Earth's past climate.
APOD: 2002 February 8 - PKS 1127-145: Quasar View
Explanation:
The quasar known as
PKS 1127-145 lies
ten billion
light-years from our fair planet.
A Hubble Space Telescope
view in the left panel shows this quasar
along with other galaxies as they appear in optical light.
The
quasar itself is the brightest object in the lower right corner.
In the right panel is a
Chandra Observatory x-ray picture, exactly
corresponding to the Hubble field.
While the more ordinary galaxies are
not seen in the Chandra image,
a striking jet, nearly a million light-years long, emerges
from the quasar to dominate the x-ray view.
Bright in both optical and x-ray light, the quasar is thought to
harbor a supermassive black hole
which powers the jet and makes
PKS 1127-145
visible across the
spectrum -- a beacon from the
distant
cosmos.
APOD: 2002 February 2 - Centaurus A: The Galaxy Deep Inside
Explanation:
Deep inside Centaurus A, the closest
active galaxy to Earth, lies ... another galaxy!
Cen A is a giant elliptical galaxy a mere 10 million light-years
distant
with a central jumble
of stars, dust, and gas that probably hides
a massive black hole.
This composite combines an optical picture of Cen A with dark
lines tracing lobes of radio emission and an
infrared image from the
ISO
satellite (in red).
The
ISO data maps out the dust in what appears to be
a barred spiral galaxy
about the size of the prominent
nearby spiral M33.
The discoverers believe that the giant elliptical's gravity helps this
barred spiral galaxy maintain its shape.
In turn, material funneled along the spiral's bar fuels the central black hole
which powers the elliptical's radio lobes.
This apparently intimate association between two distinct
and dissimilar galaxies suggests a truly
cosmic symbiotic relationship.
APOD: 2002 January 16 - Abell 2597 s Cosmic Cavities
Explanation:
Typical of large galaxy clusters
billions of
light-years away,
Abell 2597 features hundreds of galaxies
embedded in a cloud
of multimillion degree gas which glows in x-rays.
This Chandra Observatory
x-ray
image shows the hot gas in this
cluster's central regions and also reveals two large dark cavities
within the x-ray glow; one below and right of center, the other
above and left.
Not a comment on dental health,
Abell 2597's cavities
are about 60,000 light-years across.
They are thought to be
remnants of a 100 million year old explosion originating
from a supermassive black hole at the cluster's core.
But the dim ghost
cavities are not completely
empty or they would have collapsed long ago.
Instead they are likely filled with hotter gas, high energy particles,
and magnetic fields and are moving away from the cluster center,
like bubbles rising in champagne.
Over the life of
a galaxy cluster such explosions may happen
over and over, creating a series of cavities which
transport
magnetic fields away from the cluster center.
In fact, radio observations
suggest another explosion has
since occurred in the center of Abell 2597.
APOD: 2002 January 10 - X-Ray Milky Way
Explanation:
If you had x-ray vision,
the center regions
of our Galaxy would not be hidden from
view by immense cosmic dust clouds
opaque to visible light.
Instead,
the Milky Way
toward Sagittarius might look something
like this stunning mosaic
of images from the orbiting
Chandra Observatory.
Pleasing to look at, the gorgeous false-color
representation of
the x-ray data shows
high energy x-rays in blue, medium energies in green,
and low energies in red.
Hundreds of white dwarf stars,
neutron stars, and black holes immersed in a
fog of multimillion-degree gas are included in the
x-ray vista.
Within the white patch at the image center lies
the Galaxy's central supermassive black hole.
Chandra's sharp
x-ray vision will likely lead to a new
appreciation of our Milky Way's most active neighborhood
and has already indicated that the hot gas itself may
have a temperature of a mere 10 million degrees Celsius
instead of 100 million degrees as previously thought.
The full mosaic is composed of 30 separate images and covers
a 900 by 400 light-year swath
at
the galactic center.
APOD: 2001 December 10 - Globular Cluster M15
Explanation:
Stars, like bees, swarm around the center of bright
globular cluster M15.
This ball of over 100,000 stars is a relic from the
early years of
our Galaxy,
and continues to orbit the
Milky Way's center.
M15, one of about 150
globular clusters
remaining, is noted for being easily visible with only
binoculars, having at its center one of the
densest concentrations of stars known,
and containing a high abundance of unusual
variable stars and
pulsars.
The above image, taken in
ultraviolet light with the
WIYN Telescope,
spans about 120 light years and shows the gradual
increase in stars toward the cluster's center.
M15 lies about 35,000
light years away toward the
constellation of Pegasus.
Recent evidence indicates that a massive
black hole might reside as the
center of M15.
APOD: 2001 November 1 - M87's Energetic Jet
Explanation:
An
energetic jet from the core of
giant elliptical galaxy M87 stretches outward for 5,000 light-years.
This monstrous jet
appears
in the panels above
to be a knotted and irregular structure, dectected
across the spectrum, from
x-ray to optical to
radio wavelengths.
In all these bands,
the observed emission is likely created as high energy
electrons spiral along magnetic field lines, so called
synchrotron radiation.
But what powers this cosmic blowtorch?
Ultimately, the jet is thought to be produced
as matter near the center of M87
swirls toward a spinning, supermassive black hole.
Strong electromagnetic forces are generated and eject material
away from
the black hole along
the axis of rotation in a narrow jet.
Galaxy M87
is about 50 million light-years away and reigns as
the large central elliptical galaxy in the
Virgo
cluster.
APOD: 2001 October 29 - Spinning Black Holes and MCG 6 30 15
Explanation:
What makes the core of galaxy MCG-6-30-15 so bright?
Some
astronomers believe the answer is a massive
spinning black hole.
If so, this would be the first
observational indication that it is possible to make a
black hole act like a
battery -- and tap into its rotational energy.
MCG-6-30-15 is a distant
galaxy that has
recently been observed with the orbiting
XMM-Newton satellite in
X-ray light.
These observations show the
galaxy's nucleus not only to be
very bright but also to show evidence that
much of the light is climbing out of a deep gravitational well.
A spinning black hole could explain both effects.
A strong
magnetic field could be the
mediator transferring rotational energy from the
black hole to the surrounding gas.
Pictured above is an artist's illustration of a
black hole surrounded by an
accretion disk.
For clarity, the illustration does not include
distorting gravitational lens effects.
APOD: 2001 October 21 - The Sombrero Galaxy from VLT
Explanation:
Why does the Sombrero Galaxy look like a
hat?
Reasons include the
Sombrero's unusually large and
extended central bulge of stars,
and dark prominent
dust lanes
that appear in a disk that we see nearly
edge-on.
Billions of
old stars
cause the diffuse glow of the extended
central bulge.
Close inspection of the bulge in the above photograph shows many points of light that are actually
globular clusters.
M104's spectacular dust rings harbor many younger and brighter stars,
and show intricate details astronomers
don't yet fully understand.
The very center of the
Sombrero glows across the electromagnetic spectrum, and is thought to house a large
black hole.
Fifty million-year-old light from the
Sombrero Galaxy can be seen with a
small telescope towards the constellation of Virgo.
APOD: 2001 September 21 - Where a Black Hole Roams
Explanation:
Black hole candidate XTE J1118+480 is known to roam the halo
of our Milky Way Galaxy.
This exotic
system - thought to be a
stellar mass black hole consuming
matter from a companion star - was discovered only last year as a
flaring celestial x-ray source.
Suggestively termed a
microquasar,
recent radio and archival optical observations of
its
motion through the sky have now allowed its
orbit to be calculated.
Illustrated above, the
black hole's present galactic location is
indicated by the purple dot, with the
Sun's position in yellow.
A mere 6,000 light-years from the Sun now, XTE J1118+480's orbit is
traced by the orange line, backtracked for some 230 million years into
the past based on models of the Galaxy.
Astronomers note this black hole's orbit about the galactic center,
looping high above and below
the Galaxy's
plane of gas, dust,and stars,
is similar to orbits of globular
star clusters,
ancient denizens of our Galaxy.
It seems likely that
XTE J1118+480 too has its origins in the early
history and
halo of the Milky Way.
APOD: 2001 September 10 - Galactic Center Flicker Indicates Black Hole
Explanation:
Why would the center of our Galaxy flicker?
Many astronomers believe the only credible answer involves a
black hole.
During observations of
Sagittarius A* with the orbiting
Chandra X-ray Observatory, the bright
X-ray source at the very
center of our
Milky Way brightened dramatically for a few minutes.
Sagittarius A* is visible as the
bright dot near the center of the
above image.
Since large objects cannot vary quickly,
a small source is implicated in the variation.
Evidence including the
motions of central stars
indicates that the
center of our Galaxy
is a massive place, however, estimated to be over
a million times the mass of our
Sun.
Only one known type of object can fit
so much mass in so small a volume: a
black hole.
This short flicker therefore provides additional evidence that a
black hole does indeed reside at our
Galaxy's center.
If true,
the flicker might have been caused by an object
disrupting as it fell toward the
disruptive monster.
APOD: 2001 August 16 - Centaurus A: X-Rays from an Active Galaxy
Explanation:
Its core hidden
from optical view by a thick lane of dust, the giant elliptical
galaxy Centaurus A was among the first objects
observed by the orbiting Chandra X-ray Observatory.
Astronomers were not disappointed, as Centaurus A's
appearance in x-rays makes its classification as an
active galaxy easy to appreciate.
Perhaps the most striking feature of
this
Chandra false-color x-ray view
is the jet, 30,000 light-years long.
Blasting toward the upper left corner of the picture,
the jet
seems to arise from the galaxy's bright central x-ray source --
suspected of harboring a black hole with a million or so times
the mass of the Sun.
Centaurus A
is also seen to be teeming with other
individual x-ray sources and a pervasive, diffuse
x-ray glow.
Most of these individual sources are likely to be neutron stars
or solar mass black holes accreting material from their less
exotic binary companion stars.
The diffuse high-energy glow
represents gas throughout the galaxy
heated to temperatures of millions
of degrees C.
At 11 million light-years distant in the constellation
Centaurus,
Centaurus A (NGC 5128) is the closest
active galaxy.
APOD: 2001 July 8 - The Galactic Center in Infrared
Explanation:
The center of our Galaxy is a busy place. In visible light, much of the
Galactic Center
is obscured by opaque
dust.
In infrared light, however,
dust glows more and obscures less,
allowing nearly one million stars to be recorded in the
above photograph.
The
Galactic Center
itself appears
on the right and is located about 30,000
light years away towards the constellation of Sagittarius.
The
Galactic Plane of our
Milky Way Galaxy, the plane in which the
Sun orbits, is identifiable by the
dark diagonal dust lane.
The absorbing dust grains are
created in the atmospheres of cool
red-giant stars and grow in
molecular clouds.
The region directly surrounding the
Galactic Center
glows brightly in radio and
high-energy radiation,
and is thought to house a large
black hole.
APOD: 2001 June 14 - Around The Arches Cluster
Explanation:
The most compact cluster of stars known in our galaxy,
the Arches cluster, boasts 100 or so massive, young
stars contained within a diameter of one light-year.
Seen toward the
constellation Sagittarius, the
Arches cluster is
about 25,000 light-years from planet
Earth and lies within a scant 100 light-years of
the supermassive black hole believed to lurk
in our Milky Way Galaxy's center.
This
combination of images in
radio,
infrared, and
x-ray light
illustrates this star cluster's bizarre galactic neighborhood.
Shown
in red, radio emission traces the filamentary arching
structures near
the galactic center around the
Arches cluster location.
Within the zoomed inset box, infrared image data shows some of
the cluster's individual stars as bright point-like sources.
The diffuse emission in blue surrounding the cluster stars is a
false-color
x-ray image of an enveloping cloud of 60 million degree
gas -- the
first time such an energetic star cluster halo has
been detected.
Astronomers
consider the tightly packed and relatively nearby Arches cluster,
an analog of the furious star forming regions
in galaxies
millions of light-years away.
APOD: 2001 June 7 - NGC 253: X-Ray Zoom
Explanation:
Astronomers now report
that Chandra X-ray Observatory
observations of galaxies known to be
frantically forming stars
show that these galaxies also
contain luminous x-ray sources -- thought to be
intermediate mass
black holes and immense clouds of superheated gas.
Take the lovely
island universe NGC 253 for example.
At distance of a mere 8 million light-years, NGC 253's prodigious
starforming activity has been well studied
using high-resolution optical images like the
one seen here at lower left.
Zooming in on this energetic galaxy's central region,
Chandra's
x-ray detectors reveal
hidden details indicated in the inset at right.
In the false-color image,
x-ray hot gas clouds glow near the core
and at least four very powerful x-ray sources
lie within 3,000 light-years of the center of the galaxy.
Much more luminous than
black hole binary star systems in our own
galaxy, these extreme x-ray sources may be gravitating toward
NGC 253's center.
As a result,
NGC 253 and other similar starforming galaxies
could ultimately develop a single, central, supermassive black hole,
transforming their cores into quasars.
APOD: 2001 May 24 - X-Ray Stars of 47 Tucanae
Explanation:
A deep optical image (left)
of 47 Tucanae shows an ancient
globular star cluster so dense and crowded that individual stars
can not be distinguished in its closely packed core.
An
x-ray image of its central regions (inset right) from the
Chandra
Observatory reveals a wealth of x-ray stars hidden there.
Color-coded by energy, low energies are red, medium are green,
and high energy cosmic
x-ray sources are blue, while
whitish sources are bright across the x-ray energy bands.
The x-ray stars here are double stars or "compact"
binary star systems.
They are so called because one of the pair of stellar companions is
a normal star and the other a compact object --
a white dwarf,
neutron star,
or possibly a black hole.
Chandra's
x-ray vision detects the presence of
an unexpectedly large number of these exotic star systems
within 47 Tucanae, but it also indicates the apparent
absence of a large central black hole.
The finding suggests that compact binary star systems of
47 Tucanae
may be ejected from the cluster before coalescing
to form a large black hole at its core.
APOD: 2001 May 16 - The Center of the Circinus Galaxy in X-Rays
Explanation:
Are black holes the cause of X-rays that pour out
from the center of the
Circinus galaxy?
A new high-resolution image from the orbiting
Chandra X-ray Observatory has resolved the
inner regions of this nearby galaxy into
several smaller sources.
The image is shown above in representative-color.
Extended
X-ray emission from the center appears to
match optical light and appears consistent
with a model where hot gas is escaping from a
supermassive black hole at
Circinus' center.
At least one of the other sources varies
its X-ray brightness as expected from a
binary star system,
indicating that the system is small yet massive,
and giving credence to a model where a
black hole
is surrounded by doughnut-shaped ring.
The region shown spans about 5000
light-years across.
APOD: 2001 May 11 - X-Ray Rainbows
Explanation:
A drop of water
or prism of
glass can spread out visible sunlight into
a
rainbow of colors.
In order of increasing energy, the well known spectrum of colors in
a rainbow
runs red, orange, yellow, green, blue, indigo, violet.
X-ray
light too can be spread out into
a spectrum
ordered by energy ... but not by drops of water or glass.
Instead, the orbiting
Chandra
X-ray Observatory
uses a set of 540 finely ruled, gold gratings to spread out the
x-rays, recording the results with digital detectors.
The resulting x-ray spectrum reveals much about the compositions,
temperatures, and motions within
cosmic x-ray sources.
This false color
Chandra image shows
the x-ray spectrum of a
star system in Ursa Major cataloged
as XTE J1118+480 and thought to consist of a sun-like star orbiting a
black hole.
Unlike the familiar appearance of a
prism's visible light rainbow,
the energies here are ordered
along radial lines with the highest energy x-rays near the center
and lowest energies near the upper left and lower right edges of the image.
The central spiky region itself is created by x-rays from the
source which are not spread out by the array of gratings.
APOD: 2001 May 8 - GRO J1655 40: Evidence for a Spinning Black Hole
Explanation:
In the center of a swirling whirlpool of hot gas
is likely a beast that has never been seen directly: a
black hole.
Studies of the bright light emitted by the
swirling gas frequently indicate not only that a
black hole is present, but also likely attributes.
The gas surrounding GRO J1655-40, for example, has recently been found to display an unusual flickering
at a rate of 450 times a second.
Given a
previous mass estimate for the central object of seven times the mass of our Sun,
the rate of the
fast flickering can be
explained by a
black hole that is rotating very rapidly.
What physical mechanisms actually cause the flickering --
and a slower quasi-periodic oscillation (QPO) -- in
accretion disks
surrounding
black holes and
neutron stars remains a topic of much research.
APOD: 2001 March 28 - Chandra Deep Field
Explanation:
Officially the
Chandra
Deep Field - South, this picture represents the deepest ever
x-ray image of the Universe.
One million seconds of accumulated exposure time with the orbiting
Chandra X-ray Observatory went in to its making.
Concentrating on a single, otherwise unremarkable patch
of sky in the constellation
Fornax,
this x-ray image corresponds to the
visible light Hubble Deep Field - South
released in 1998.
Chandra's view, color coded with low energies in red, medium in green,
and high-energy x-rays in blue, shows many faint sources of relatively
high-energy x-rays.
These are likely active galaxies feeding supermassive central
black holes
and large clusters of galaxies
at distances of up to 12 billion light-years.
The stunning picture supports
astronomers' ideas
of a youthful
universe
in which massive black holes were much more dominant
than at present.
APOD: 2001 January 19 - Black Holes Are Black
Explanation:
Q: Why are
black holes black?
A: Because they have an
event horizon.
The event horizon is that one-way boundary predicted by
general
relativity beyond which nothing, not even light, can return.
X-ray
astronomers using the space-based Chandra Observatory now
believe they have direct evidence for event horizons - therefore
black holes - in binary star systems which can be
detected in x-ray light.
These binaries, sometimes called x-ray novae, are
known to consist of relatively normal stars dumping
material on to massive, compact companions.
As illustrated,
the material swirls toward the companion in an
accretion disk which itself glows in x-rays.
If the compact companion is a neutron star
(right), the material ultimately smashes into the solid surface
and glows even more brightly in high energy x-rays.
But if it is indeed a
black hole with a defining event
horizon, then the x-ray hot material approaches the speed of
light as it swirls past the surface of no
return and is lost from view.
Recent
work describes observations of two classes of
x-ray binaries,
one class 100 times fainter than the other.
The results imply the presence of an event horizon in the
fainter class which causes the extreme difference in x-ray
brightness.
APOD: 2000 December 20 - Sgr A: Fast Stars Near the Galactic Center
Explanation:
Why are these stars moving so fast?
Shown above is a time-lapse movie in
infrared light detailing how stars in the central light-year of
our Galaxy
have moved over the past eight years.
The yellow mark at the image center represents the
location of a peculiar
radio source named
Sgr A*.
If these fast stars are held to the
Galactic Center by gravity,
then the central object exerting this
gravity must be both compact and massive.
Analysis of the
stellar motions indicates that over
one million times the mass of our
Sun is somehow confined to a region
less than a fifth of a light-year across.
Astronomers interpret
these observations as strong evidence that the
center of our Galaxy
is home to a very massive
black hole.
APOD: 2000 December 10 - Too Close to a Black Hole
Explanation:
What would you see if you went right up to a
black hole?
Above are two computer generated images
highlighting how strange things would look.
On the left is a normal star field containing the
constellation Orion.
Notice the three stars of nearly equal brightness that make up
Orion's Belt.
On the right is the same star field but this time with a
black hole superposed in the center of the frame.
The
black hole has such strong
gravity that light is noticeably bent towards it -
causing some very unusual
visual distortions.
In the distorted frame, every star in the normal frame
has at least two bright images - one on each side of the
black hole.
In fact, near the
black hole, you can see the whole sky -
light from every direction is
bent around and comes back to you.
Black holes are thought to be the densest state of matter,
and there is indirect evidence for their presence in
stellar binary systems
and the centers of
globular clusters,
galaxies, and
quasars.
APOD: 2000 December 4 - The Circinus Galaxy
Explanation:
Powerful forces are at play in the nearby Circinus Galaxy.
Hot gas, colored pink, is being ejected out of the
spiral galaxy
from the central region.
Much of
Circinus' tumultuous gas, however,
is concentrated in two rings.
The outer ring, located about 700
light-years from the center, appears mostly red and
is home to tremendous bursts of
star formation.
A previously unseen inner ring,
inside the green disk above,
is visible only 130 light years from the center on
this recently released, representative color image
taken by the
Hubble Space Telescope.
At the very
center is an
active galactic nucleus,
where matter glows brightly before
likely spiraling into a massive
black hole.
Although only 15 million light years distant, the
Circinus Galaxy went unnoticed until 25 years ago
because it is so obscured by material in the
plane of our own Galaxy.
The galaxy can be seen with a small telescope, however, in the
constellation of
Circinus.
APOD: 2000 November 10 - X-Ray Cygnus A
Explanation:
Amazingly detailed,
this
false-color x-ray image is centered on the galaxy Cygnus A.
Recorded by the orbiting
Chandra Observatory, Cygnus A is
seen here as a
spectacular high energy x-ray source.
But it is actually more
famous at the low energy end of
the electromagnetic spectrum as one of
the brightest celestial radio sources.
Merely 700 million light-years distant,
Cygnus A is
the closest powerful radio galaxy and the false-color
radio image (inset right)
shows remarkable similarity to Chandra's x-ray view.
Central in both pictures, the center of
Cygnus A shines brightly while emission
extends 300,000 light-years to either side along the same axis.
Near light speed jets of atomic particles
produced by a massive central
black hole are believed to cause
the emission.
In fact, the x-ray image reveals "hot spots" suggestive
of the locations where the particle jets are stopped in
surrounding cooler, denser gas.
The x-ray image also shows that the jets have
cleared out a huge cavity in the surrounding gas.
Bright swaths of emission within the cavity likely indicate
x-ray hot material ... swirling toward the
central black hole.
APOD: 2000 November 9 - The Cosmic X-Ray Background
Explanation:
Early on,
x-ray satellites revealed a surprising
cosmic background glow
of x-rays and astronomers have struggled
to understand its origin.
Now, peering through
a hole in the obscuring gas and dust of
our own Milky Way Galaxy, the powerful orbiting
XMM-Newton telescope
has recorded this deep image of
the x-ray sky, resolving some of the
mysterious background
into many faint individual sources.
The tantalizing image
is color-coded, with red representing
relatively low energy x-rays, photons with 500 or so times the
energy of visible light.
Green and blue colors correspond to increasingly energetic
x-rays with up to about 10,000 times visible light energies.
Notably, the faint sources tend to be green and blue,
showing x-ray characteristics of huge amounts of material
falling into massive black holes in very distant galaxies.
Do massive black holes reside in the
hearts of all large galaxies?
The XMM-Newton
results add
to the growing consensus that they
do and that, from across the
universe,
x-rays produced as matter
feeds these black holes
account for
the cosmic x-ray background.
APOD: 2000 October 31 - The Perseus Cluster s X Ray Skull
Explanation:
This haunting image from the orbiting
Chandra Observatory
reveals the Perseus Cluster of Galaxies
in x-rays,
photons with a thousand or more times the energy of visible light.
Three hundred twenty million light-years distant, the
Perseus Cluster
contains thousands of galaxies, but none of them are
seen here.
Instead of mere galaxies, a fifty million degree cloud of
intracluster gas, itself more
massive than all the cluster's galaxies
combined, dominates the x-ray view.
From this angle, voids and bright knots in the
x-ray hot gas cloud lend it a very
suggestive appearance.
Like eyes in a skull, two dark bubbles flank a bright central source
of x-ray emission.
A third elongated bubble (at about 5 o'clock) forms a toothless mouth.
The bright x-ray source is likely a supermassive black hole at the
cluster center with the bubbles blown by explosions of
energetic particles ejected from the black hole and expanding into
the immense gas cloud.
Fittingly, the dark spot forming the skull's "nose" is an
x-ray shadow ... the shadow of a large galaxy inexorably falling into
the cluster center.
Over a hundred thousand light-years across, the Perseus Cluster's
x-ray skull is a bit larger than skulls you
may see tonight.
Have a safe and happy Halloween!
APOD: 2000 October 17 - Gemini North Images Bow Shock Near Galactic Center
Explanation:
What's going on near the center of our Galaxy?
Glowing across the
electromagnetic spectrum,
the center of our
Milky Way Galaxy
is thought to be home to massive stars,
rotating gas rings, and a
massive black hole.
Now the
central Galactic zoo just got larger.
The 8-meter
Gemini North telescope in
Hawaii in its
first scientific observation has just imaged the
Galactic Center
and revealed a star only three light years out
colliding with gas and
dust.
The bow shock, similar to that caused by a
boat moving through water,
appears arrow-shaped and is visible on the upper right of the
above photograph taken in representative
infrared colors.
Gemini's new
flexible-mirror technology has imaged
this structure, known as
IRS8,
in finer detail than ever before.
APOD: 2000 October 1 - The Center of Centaurus A
Explanation:
A fantastic jumble of young blue star clusters,
gigantic glowing gas clouds, and imposing dark
dust lanes
surrounds the central region of the active galaxy Centaurus A.
This mosaic of Hubble Space Telescope images
taken in blue, green, and red light
has been processed to present a natural color picture
of this cosmic maelstrom.
Infrared images from the Hubble have also shown that
hidden at the center of this activity are what seem to
be disks of matter spiraling into
a black hole with a billion times the mass
of the Sun!
Centaurus A itself is apparently the result of a
collision of two galaxies
and the left over debris is steadily being consumed by the
black hole.
Astronomers believe that such
black hole central engines generate the
radio, X-ray, and gamma-ray energy radiated
by Centaurus A and other active galaxies.
But for an active galaxy Centaurus A is close,
a mere 10 million
light-years away,
and is a relatively convenient laboratory for exploring these
powerful sources of energy.
APOD: 2000 September 14 - M82 s Middle Mass Black Hole
Explanation:
Black
holes are probably the most
bizarre
creatures in
the modern astronomical zoo.
And after years of pondering
black
holes as either stellar mass objects seen in
binary star systems or enormous
supermassive black holes at the
centers of galaxies,
astronomers now have strong evidence for another exotic species --
middle mass black holes.
The leading candidate for the ultradense middle ground is
indicated in this false-color detail of a sharp x-ray picture
from the space-based Chandra Observatory.
A close-up of x-ray sources near the center of
starburst galaxy M82,
the cropped
Chandra
image spans about 4,000 light-years.
M82 itself is around 11 million light-years distant.
The arrowed source has recently been convincingly demonstrated to
exhibit x-ray characteristics of an object whose
gravitational field holds more than 500 times the mass of the sun
within a volume the size of the moon!
Astronomers
also
note that unlike the supermassive variety
which are thought to lie at the centers of galaxies, this
middle mass black hole is about 600 light-years from the
center of M82.
Theories for the formation of a middle mass black hole
include the collapse of a "hyperstar"
formed by
the coalescence of many normal stars, or the
direct
merger of stellar mass black holes.
APOD: 2000 August 18 - X-Rays From Antennae Galaxies
Explanation:
A bevy of
black
holes and
neutron stars
shine as bright, point-like
sources against bubbles of
million degree gas in this
false-color x-ray image from the
orbiting Chandra Observatory.
The striking picture shows the central regions of two
galaxies, NGC 4038 and NGC 4039, locked in a titanic collision
some 60 million light-years distant in the
constellation Corvus.
In visible
light images, long, luminous, tendril-like structures emanating
from the wreckage lend the pair their
popular moniker, the Antennae Galaxies.
Galactic collisions are now thought to be fairly common, but when
they happen individual stars rarely collide.
Instead gas and dust clouds merge and compress, triggering furious
bursts of massive
star
formation with thousands of resulting supernovae.
The exploding stars litter the scene with
bubbles
of shocked hot gas and
collapsed stellar cores.
Transfixed by this cosmic accident
astronomers watch and are beginning
to
appreciate the collision-driven evolution
of galaxies, not unlike
our own.
APOD: 2000 July 6 - A Jet from Galaxy M87
Explanation:
What's causing a huge jet to emanate from the center of galaxy
M87?
Although the
unusual jet was first noticed early in the
twentieth century,
the exact cause is still debated.
The above recently released picture taken by the
Hubble Space Telescope
shows clear details, however.
The most
popular hypothesis holds that the
jet is created
by energetic gas swirling around a massive
black hole at the galaxy's center.
The result is a 5000
light-year long blowtorch where
electrons are ejected outward at near light-speed,
emitting eerily blue light during a magnetic spiral.
M87 is a giant
elliptical galaxy
residing only 50 million light-years away in the
Virgo Cluster of Galaxies.
The faint dots of light surrounding
M87's center are large ancient
globular clusters of stars.
APOD: 2000 June 19 - The Long Jet of Pictor A
Explanation:
A jet stretching nearly a million light years
has been imaged emanating from galaxy Pictor A.
The thin
jet of
electrons and
protons
shoots out at
nearly light-speed
likely from the vicinity of a large
black hole at the galaxy center.
At the left of the
above image in
X-rays is the radio galaxy
Pictor A, known as a
radio galaxy for its strong
radio emission.
At the far end of the
jet on the right a
hot spot glows as the intense particle beam bores
through a gas cloud in intergalactic space.
The jet and hot spot of
Pictor A had been seen previously in radio waves,
but only recently has the orbiting
Chandra X-ray Observatory confirmed its unusual power.
APOD: 2000 June 15 - X-Rays From The Perseus Cluster Core
Explanation:
The Perseus Cluster
of thousands of galaxies, 320 million
light-years distant, is
one of
the most massive objects
in
the Universe.
At its core lies the giant cannibal galaxy
Perseus A
(NGC 1275), accreting matter as
gas and galaxies fall into it.
Representing low, medium, and high energy
x-rays as red, green,
and blue colours respectively,
this Chandra X-ray Observatory image
shows remarkable details of x-ray emission from this monster galaxy and
surrounding hot (30-70 million degrees C)
cluster gas.
The bright central source is the supermassive
black
hole at the core of Perseus A itself.
Dark circular voids just above and below the galaxy center,
each about half the size of our own
Milky Way Galaxy,
are believed to be magnetic bubbles of
energetic particles
blown by the accreting black hole.
Settling
toward
Perseus A, the cluster's x-ray hot gas piles up
forming bright regions around the bubble rims.
Dramatically, the long greenish wisp just above the galaxy's centre
is likely the x-ray shadow produced by
a small galaxy falling into the burgeoning
Perseus A.
APOD: 2000 June 12 - A Bubbling Galaxy Center
Explanation:
What's happening in the center of this galaxy?
Close inspection of the center of
NGC 4438, as visible in
this recently released representative-color image by the
Hubble Space Telescope,
reveals an unusual bubble of hot gas, colored in red.
Astronomers
speculate that this strange bubble was
created by a massive
central black hole that resides there.
As gas swirls around the
black hole,
gravity and
friction pull it in and heat it up.
Some of the hot gas then falls into the
black hole, but not all -
some gas gets so hot it shoots
out the poles in fast
jets.
When these
jets impact nearby material,
they heat it up and cause the detected glow.
Galaxy
NGC 4438 resides about 50 million
light years from
Earth,
and the pictured central
bubble measures about 800 light-years across.
APOD: 2000 June 1 - X-Ray Wind From NGC 3783
Explanation:
A black
hole is supposed to inexorably attract matter.
But the intense radiation generated as material swirls and plunges into
its high gravity field also heats up surrounding gas and drives it away.
In fact, measurements made using
this recent Chandra Observatory X-ray
spectrum of active galaxy
NGC 3783
reveal a wind of highly ionized
atoms blowing away from the galaxy's suspected
central
black hole at a million miles per hour.
Displayed in false color, the bright central spot is the
X-ray image of NGC 3783 while the
lines radiating away represent
an X-ray
spectrum of this source
produced by Chandra's
High
Energy Transmission Grating (HETG).
An X-ray spectrum is the analog to
the rainbow spread of colors in a visible light spectrum.
It represents a detailed, spread out image of X-ray colors or
energies arising from the source.
Ionized atoms of iron, magnesium, oxygen, nitrogen and other
elements produce
patterns of absorption at known X-ray energies.
These patterns have been identified in
the spectrum of NGC 3783 at slightly shifted energies
and the measured shifts indicate the hot wind's velocity.
APOD: 2000 April 22 - Journey to the Center of the Galaxy
Explanation:
In Jules Verne's science fiction classic
A Journey
to the Center of the Earth,
Professor Hardwigg and his fellow explorers
encounter many strange and exciting wonders.
What wonders lie at the
center of our Galaxy?
Astronomers now know of some of the bizarre objects which exist there,
like vast dust clouds,
bright
young stars,
swirling rings of gas,
and even a large
black hole.
Much of the Galactic Center
is shielded from our view in visible light by the
intervening dust and gas,
but it can be explored using other forms of
electromagnetic radiation.
This
haunting image of the
Galactic Center region in infrared light was made in 1996 by
a telescope onboard the
Midcourse Space Experiment.
The center itself
appears as a bright spot near the middle of this roughly 1 degree field of
view, and the north galactic pole is towards the top.
The picture is in false color -
starlight appears blue while dust is greenish grey, tending to red in
the cooler areas.
APOD: 2000 April 19 - Redshift 5.8: A New Farthest Quasar
Explanation:
The distance record for a
quasar
has been broken yet again.
At the present time, no other object in the
universe
has been found to be more distant than the above speck.
The recently discovered quasar has been clocked
at redshift 5.82.
The exact relation between
redshift and distance remains
presently unknown, although surely higher
redshifts do mean greater distance.
The above quasar is likely billions of
light-years away and so is seen when the
universe was younger
than one billion years old,
less than a tenth of its present age.
Like all
quasars, this object is probably a
large black hole
in the center of a distant galaxy.
Don't close the
record book yet, though.
The redshifts to several
other SDSS-discovered quasars
are currently being measured,
some of which might have
redshifts greater than six.
APOD: 2000 April 12 - The Local Bubble and the Galactic Neighborhood
Explanation:
What surrounds the Sun in this neck of the
Milky Way Galaxy?
Our current best guess is depicted in the
above map of the surrounding 1500
light years constructed from various observations and deductions.
Currently, the Sun is passing through a
Local Interstellar Cloud
(LIC), shown in violet, which is
flowing away from the
Scorpius-Centaurus Association of young stars.
The LIC resides in a low-density hole in the
interstellar medium (ISM) called the
Local Bubble, shown in black.
Nearby, high-density
molecular clouds including the
Aquila Rift surround star forming regions,
each shown in orange.
The Gum Nebula, shown in green, is a region of
hot ionized hydrogen gas.
Inside the Gum Nebula is the
Vela Supernova Remnant,
shown in pink, which is expanding to create
fragmented shells of material like the LIC.
Future observations should help astronomers
discern more about the local Galactic Neighborhood and how it might have affected
Earth's past climate.
APOD: 2000 February 28 - The Sombrero Galaxy from VLT
Explanation:
Why does the Sombrero Galaxy look like a
hat?
Reasons include the
Sombrero's unusually large and
extended central bulge of stars,
and dark prominent
dust lanes
that appear in a disk that we see nearly edge-on.
Billions of old stars cause the diffuse glow of the extended
central bulge.
Close inspection of the bulge in the above photograph shows many points of light that are actually
globular clusters.
M104's spectacular dust rings harbor many younger and brighter stars,
and show intricate details astronomers
don't yet fully understand.
The very center of the
Sombrero glows across the electromagnetic spectrum, and is thought to house a large
black hole.
Fifty million-year-old light from the
Sombrero Galaxy can be seen with a
small telescope towards the constellation of Virgo.
APOD: 2000 February 15 - M106: A Spiral Galaxy with a Strange Core
Explanation:
What's happening at the center of spiral galaxy M106?
A swirling disk of stars and gas,
M106's appearance is dominated by
two bright spiral arms and dark
dust lanes near the nucleus.
Bright newly formed stars near their outer tips
distinguish the spiral arms in the
above photograph.
The core of M106 glows brightly in
radio waves and
X-rays where
twin jets have been found running the length of the galaxy.
An unusual central glow makes M106 one
of the closest examples of the
Seyfert class of galaxies,
where vast amounts of glowing gas are thought
to be falling into a central massive
black hole.
M106,
also designated
NGC 4258,
is a relatively close 25 million light years away,
spans 30 thousand
light years across, and can be seen with a small telescope
towards the constellation of Canes Venatici.
APOD: 2000 January 21 - X For Andromeda
Explanation:
A big beautiful spiral galaxy 2 million light-years away,
Andromeda (M31)
has long been touted as an analog to the Milky Way,
a distant mirror of our own galaxy.
The popular 1960s British sci-fi series,
A For Andromeda,
even postulated that it was home to another technological civilization
that communicated
with us.
Using the newly unleashed observing power of the orbiting
Chandra X-ray telescope,
astronomers have now imaged the
center of our near-twin
island universe, finding evidence
for an object so bizarre it would have impressed many
60s science fiction writers (and readers).
Like the Milky Way,
Andromeda's galactic center appears to
harbor an X-ray source characteristic of
a black hole of a million or more solar masses.
Seen above,
the false-color X-ray picture shows a number of
X-ray sources, likely
X-ray binary stars, within
Andromeda's central region as yellowish dots.
The blue source located right at the galaxy's center is coincident
with the position of the suspected massive black hole.
While
the X-rays are produced as material falls into the
black hole and heats up, estimates from the X-ray data show Andromeda's
central source to be surprisingly cool - only a million
degrees or so compared to the tens of millions of degrees
indicated for Andromeda's X-ray binaries.
APOD: 2000 January 20 - X-Rays From The Galactic Center
Explanation:
Exploring quasars
and active galaxies in the distant
universe, astronomers have come to believe that
most galaxies have massive black holes at their centers.
Swirling stars and a strong, variable
radio source offer convincing evidence that even our own Milky Way
galaxy's center harbors such a
bizarre object,
a mere 30,000 light-years away.
Still, it has long been realized that if a massive black hole
lurks there
it should produce X-rays
- which have not previously been identified.
Now, though relatively faint,
the missing X-ray source may have been found.
Taking advantage of the sensitive Chandra Observatory
astronomers have recorded this false-color
X-ray image of the Galactic Center.
Embedded in a diffuse cloud of
X-ray hot gas,
the white dot at the center corresponds to an X-ray
source at exactly the position of the strong radio source
and suspected black hole.
Other individual X-ray sources are also present in
the picture which spans about 10 light-years at the distance
of the galactic center.
With radio and X-ray emission generated by infalling material,
the Milky Way's central black hole is thought to have a mass of
over 2 million suns.
APOD: 2000 January 19 - A Big Black Hole Floats By
Explanation:
A black hole glides silently through space.
Is there any way to know it's there?
Until last week, all objects that might be
black holes in
our Galaxy were part of
binary star systems.
There, gas from the companion star was hypothesized to
swirl around the black hole,
heat up, and emit
X-rays before falling in.
Last week, however, analysis was released of a mysteriously dark object
that floated in front of a distant background star,
dramatically increasing this star's light by the
lens effect of its gravity, as
pictured above.
The high mass and low light of this
strange lens
have astronomers guessing they might just have
detected a lone
black hole,
weighing in at six times the mass of our Sun.
The existence of isolated black holes
is not in itself surprising,
as they should be the end result of the cores of massive stars.
APOD: 2000 January 17 - V4641 Sgr: The Closest Black Hole Candidate
Explanation:
An object many astronomers believe is a
black hole has been found only 1500
light-years from Earth,
making it the closest
black hole candidate.
Although dramatic explosions emanate from the object,
it is far enough away so that we are in no danger.
Pictured above, V4641 was imaged just after
emitting an outburst in the radio band.
Jets, which lasted only minutes, are visible.
V4641 is the fourth known
microquasar, a miniature version of the massive, matter spewing
black holes thought to exist in the
centers of galaxies.
The explosions are not thought to emanate from within the
black hole, a location where neither matter
nor information can escape, but from around the
black hole, where matter from its companion star may
be heating up as it falls in.
Astronomers are
working to understand why
V4641 acts strangely even for a
black hole, as the
explosions it creates fade within minutes,
and appear at different times in
different bands of light.
APOD: December 19, 1999 - Accretion Disk Binary System
Explanation:
Our Sun
is unusual in that it is alone - most
stars occur in multiple or
binary systems.
In a binary system, the
higher mass star will evolve faster and will eventually
become a compact object - either a
white dwarf star, a
neutron star, or
black hole.
When the lower mass star later
evolves into an expansion phase,
it may be so close to the compact star that its
outer atmosphere actually falls
onto the compact star.
Such is the case
diagrammed above. Here
gas from a blue
giant star is
shown being stripped away into an accretion disk around its compact binary
companion.
Gas in the
accretion disk swirls around, heats up, and
eventually falls onto the compact star.
Extreme conditions frequently occur on the surface
of the compact star as gas falls in, many times
causing detectable
X-rays,
gamma-rays, or even
cataclysmic novae explosions.
Studying the extreme conditions in these
systems tells us about the inner properties
of ordinary matter around us.
APOD: November 25, 1999 - 3C 295: X-rays From A Giant Galaxy
Explanation:
Did this galaxy eat too much?
Five billion light-years away,
the giant elliptical galaxy 3C295
is a prodigious source of
energy at radio wavelengths.
Bright knots of
X-ray emission are also seen at the center of
this false-color Chandra Observatory image of the region.
The X-ray and radio emission are believed to be the result of
an explosive event triggered when too much material flowed
into a supermassive
black hole at the heart of the giant galaxy.
Additionally, the
Chandra
picture beautifully reveals an extensive
cloud of 50 million degree gas surrounding
3C295.
Embedded in the cloud is a
cluster of about 100 galaxies,
too cool to be seen in the X-ray picture.
About two million light-years across, the
X-ray hot cloud
itself contains enough material to create another
1,000 galaxies or so making the cluster and cloud among
the most massive objects in the Universe.
However, X-ray data indicate that there is
still not enough observed mass to hold the cloud and cluster together
gravitationally,
suggesting the presence of large amounts of
dark matter.
APOD: November 7, 1999 - The Heart Of NGC 4261
Explanation:
Who knows what
evil lurks in the hearts of galaxies?
The Hubble knows.
This Hubble Space Telescope picture
of the center of the nearby
elliptical galaxy
NGC 4261 tells one
dramatic
tale.
The gas and
dust in this disk
are swirling into what is almost
certainly a massive black hole.
The disk is probably what remains of a
smaller galaxy that fell in hundreds of millions of years ago.
Collisions
like this may be a common way of creating such active galactic nuclei as
quasars.
Strangely, the center of
this fiery whirlpool is offset from the
exact center of the galaxy - for a reason that for now remains an
astronomical mystery.
APOD: October 28, 1999 - X Ray Jet From Centaurus A
Explanation:
Spanning over 25,000 light-years, comparable to the distance from
the Sun to the center of our own Milky Way galaxy, a
cosmic jet seen in X-rays blasts from
the center of Centaurus A.
Only 10 million light-years away,
Centaurus A is a giant
elliptical galaxy - the closest
active galaxy to Earth.
This composite image illustrates
the jumble of gas, dust, and stars visible
in an optical picture
of Cen A superposed on
a new image recorded by the orbiting
Chandra X-ray Observatory.
The X-ray data is shown in red.
Present theories hold that the X-ray bright jet
is caused by electrons driven to extremely high energies
over enormous distances.
The jet's
power source is likely to be a black hole with about 10 million
times the mass of the Sun
coincident with the X-ray bright spot at the galaxy's center.
Amazingly, while
some material in the vicinity of the black hole
falls in, some material is blasted outward in energetic jets.
Details of this
cosmic power
generator can be explored with the
Chandra X-ray data.
APOD: October 17, 1999 - Black Holes in Galactic Centers
Explanation:
Do all galaxies have black holes at their centers?
Although not even a single
galaxy
has yet been proven to have a central black hole,
the list of candidates continues to increase.
Results by astronomers using instruments like the
Hubble Space Telescope
now indicate that most - and possibly even all - large galaxies
may harbor one of these dense beasts.
In all the galaxies studied, star speeds continue to increase
closer the very center. This in
itself indicates a center millions of times more massive than
our Sun is needed to contain the stars.
This mass when combined with the limiting size make the
case for the central black holes.
Will we ever know for sure?
APOD: October 8, 1999 - NGC 1365: Barred Spiral Galaxy
Explanation:
NGC 1365
is a giant
barred spiral galaxy about 200,000 light-years
in diameter and 60 million light-years distant in the southern
constellation Fornax.
These
three recently released images offer views of
this majestic
island universe in visible and infrared light.
In the middle is an optical ground-based image showing NGC 1365's
dramatic spiral arms trailing away from its central
galactic bar.
Superposed colored rectangles define the corresponding
fields of the inset images.
At upper left, a Hubble Space Telescope
near visible light image
shows young blue star clusters and dark dust lanes
located near the center of
NGC 1365.
The bright yellow nucleus likely houses a
massive black hole.
At lower right, the
Hubble infrared view of the galaxy's
center also shows young star clusters as bright blue spots
but additionally reveals infrared-bright spots corresponding
to newborn clusters
still hidden from optical view by dust clouds.
Astronomers believe the gravity field of NGC 1365's bar plays
a crucial role in the galaxy's evolution,
funneling gas and dust into the central
star-forming maelstrom
and ultimately feeding material into its massive
black hole.
APOD: August 22, 1999 - The Center of Centaurus A
Explanation:
A fantastic jumble of young blue star clusters,
gigantic glowing gas clouds, and imposing dark
dust lanes
surrounds the central region of the active galaxy Centaurus A.
This mosaic of Hubble Space Telescope images
taken in blue, green, and red light
has been processed to present a natural color picture
of this cosmic maelstrom.
Infrared images from the Hubble have also shown that
hidden at the center of this activity are what seem to
be disks of matter spiraling into
a black hole with a billion times the mass
of the Sun!
Centaurus A itself is apparently the result of a
collision of two galaxies
and the left over debris is steadily being consumed by the
black hole.
Astronomers believe that such
black hole "central engines" generate the
radio, X-ray, and gamma-ray energy radiated
by Centaurus A and other active galaxies.
But for an active galaxy
Centaurus A is close,
a mere 10 million light-years away,
and is a relatively convenient laboratory for exploring these
powerful sources of energy.
APOD: August 15, 1999 - M104: The Sombrero Galaxy
Explanation:
What's going on in the center of this spiral galaxy? Named the Sombrero Galaxy for its hat-like resemblance,
M104 features a prominent
dust
lane and a bright halo of stars and
globular clusters.
Something truly energetic is going on in the
Sombrero's center, as it not only appears bright in visible light,
but glows prodigiously in
X-ray
light as well. This X-ray emission coupled with unusually
high central stellar speeds
cause many astronomers to speculate that a
black hole lies at the Sombrero's center - a black hole possibly a billion
times the mass of our Sun.
APOD: June 21, 1999 - The Galactic Center in Infrared
Explanation:
The center of our Galaxy is a busy place. In visible light, much of the
Galactic Center
is obscured by opaque
dust.
In infrared light, however,
dust glows more and obscures less,
allowing nearly one million stars to be recorded in the
above photograph.
The
Galactic Center
itself appears on the right and is located
about 30,000 light years away
towards the constellation of Sagittarius.
The
Galactic Plane of our
Milky Way Galaxy, the plane in which the
Sun orbits, is identifiable by the
dark diagonal dust lane.
The absorbing dust grains are
created in the atmospheres of cool
red-giant stars and grow in
molecular clouds.
The region directly surrounding the
Galactic Center
glows brightly in radio and
high-energy radiation,
and is thought to house a large
black hole.
APOD: April 12, 1999 - Nearby Spiral Galaxy NGC 4945
Explanation:
For such a close galaxy, NGC 4945 is easy to miss.
NGC 4945 is a
spiral galaxy in the
Centaurus Group of galaxies,
located only six times farther away than the prominent
Andromeda Galaxy. The
thin disk galaxy
is oriented nearly edge-on, however, and shrouded in dark
dust.
Therefore galaxy-gazers searching the
southern constellation of Centaurus need a telescope to see it.
The above picture was taken with a large telescope testing a new wide-angle, high-resolution CCD camera.
Most of the spots scattered about the frame are
foreground stars in our own Galaxy, but some spots are
globular clusters
orbiting the distant galaxy.
NGC 4945 is thought to be
quite similar to our own
Milky Way Galaxy.
X-ray observations reveal, however,
that NGC 4945 has an unusual, energetic,
Seyfert 2 nucleus
that might house a
large black hole.
APOD: February 16, 1999 - The Large and Small of M87
Explanation:
The small core of
elliptical galaxy M87
appears to be energizing its whole galactic neighborhood.
Recent images from the
Very Large Array (VLA) of
radio telescopes indicate that huge bubbles of
hot gas not only exist but are still being created.
These bubbles measure 200,000 light-years across
and surround the
entire galaxy.
The source creating and feeding the bubbles
has been traced to jets pointing back to
M87's center, where a
supermassive black hole
is thought to live. The smallest scale on the
above radio-map is 0.2 light-years and imaged by
many radio telescopes working together (VLBI).
The labeled numbers
refer to the wavelength of the radio waves observed.
The exact composition of these jets is not known, but thought to contain various
subatomic particles.
APOD: January 28, 1999 - The Galactic Center A Radio Mystery
Explanation:
Tuning in to the center of our Milky Way galaxy, radio astronomers
explore a complex, mysterious place.
A premier high resolution view,
this startlingly beautiful picture
covers a 4x4 degree region around the galactic center.
It was constructed from 1 meter wavelength radio data
obtained by telescopes of the
Very Large Array near
Socorro, New Mexico, USA.
The galactic center
itself is at the edge of the extremely
bright object labeled Sagittarius (Sgr) A, suspected of harboring
a million solar mass black hole.
Along the galactic plane which runs diagonally
through the image are tortured clouds of gas energized by
hot stars and round-shaped supernova remnants (SNRs)
- hallmarks of
a violent and
energetic cosmic environment.
But perhaps most intriguing are
the arcs, threads, and filaments which
abound in the scene.
Their uncertain origins challenge
present theories of the dynamics of the galactic center.
APOD: December 4, 1998 - Centaurus A: The Galaxy Deep Inside
Explanation:
Deep inside Centaurus A, the closest
active galaxy to Earth, lies ... another galaxy!
Cen A is a giant elliptical galaxy a mere 10 million light-years
distant
with a central jumble
of stars, dust, and gas that probably hides
a massive black hole.
This composite combines an optical picture of Cen A with dark
lines tracing lobes of radio emission and an
infrared image from the
ISO satellite (in red).
The ISO data maps out the dust in what appears to be
a barred spiral galaxy
about the size of the prominent
nearby spiral M33.
The discoverers believe that the giant elliptical's gravity helps this
barred spiral galaxy maintain its shape.
In turn, material funneled along the spiral's bar fuels the central black hole
which powers the elliptical's radio lobes.
This apparently intimate association between two distinct
and dissimilar galaxies suggests a truly
cosmic symbiotic relationship.
APOD: November 28, 1998 - A Lonely Neutron Star
Explanation:
How massive can a star get without imploding into a black hole?
These limits are being tested by the discovery of a lone
neutron star in space.
Observations by the Hubble Space Telescope
have been combined with previous observations by the
X-ray
ROSAT observatory and
ultraviolet
EUVE
observatory for the isolated star at the location of the arrow.
Astronomers are able to directly infer the star's size from
measurements of its unblended brightness, temperature, and an upper
limit on the distance.
Assuming that the object is a
neutron star of typical mass,
some previous theories of neutron star structure would have predicted
an implosion that would have
created a black hole.
That this neutron star
even exists therefore allows a window to the extreme
conditions that exist in the interiors of neutron stars.
APOD: July 29, 1998 - The High Energy Heart Of The Milky Way
Explanation:
These high resolution false color pictures of the Galactic center
region in high energy
X-ray and gamma-ray light result from a very long
exposure of roughly 3,000 hours performed from 1990 to 1997 by the
French SIGMA telescope onboard the
Russian GRANAT spacecraft.
Each image covers a 14x14 degree field which includes most of the
central bulge of
our Milky Way Galaxy.
The X-ray picture (left) reveals a cluster of sources
releasing enormous amounts of energy.
They are probably
binary star systems where matter accretes
onto a collapsed object, either
a neutron star or
a black hole.
But
according to recent theories, only those
binary systems with black holes
can radiate above X-ray energies -- in the gamma-ray regime.
In that case, the SIGMA sources also shining in the gamma-ray picture
(right) betray the presence of accreting
stellar black holes!
Surprisingly, no high energy source seems to coincide exactly
with the Galactic center itself,
located near the brightest source at the bottom of both
pictures.
This indicates that
the large black hole
thought
to be lurking there
is unexpectedly quiet at these energies.
APOD: June 22, 1998 - The Doomed Dust Disk of NGC 7052
Explanation:
What created the dust disk in the center of NGC 7052,
and what keeps it spinning?
Although the disk might appear as a relatively tame
"hubcap in space",
the unusual center of
elliptical galaxy
NGC 7052 is probably the remnant of a
titanic collision between galaxies.
What's more, the disk's spin indicates the
tremendous gravity of a massive central black hole. Analysis of this recently released photo by the
Hubble Space Telescope
indicates that the disk is thousands of light-years across,
rotates faster than 100 kilometers per second,
at a distance of 150 light-years from the center,
and contains more mass than a million Suns.
The theorized
central black hole is
thought to be yet 100 times more massive,
and may swallow the entire disk in the next few million years.
APOD: May 22, 1998 - The Center of Centaurus A
Explanation:
A fantastic jumble of young blue star clusters, gigantic glowing gas clouds,
and imposing dark dust lanes
surrounds the central region of the
active galaxy Centaurus A.
This mosaic of Hubble Space Telescope images
taken in blue, green, and red light
has been processed to present a natural color picture
of this cosmic maelstrom.
Infrared images from the Hubble have also shown that
hidden at the center of this activity are what seem to
be disks of matter spiraling into
a black hole with a billion times the mass
of the Sun!
Centaurus A itself is apparently
the result of a collision of two galaxies and the left over debris
is steadily being consumed by the black hole.
Astronomers believe that such black hole "central engines"
generate the
radio, X-ray, and gamma-ray energy
radiated
by Centaurus A and other
active galaxies.
But for an active galaxy
Centaurus A is close,
a mere 10 million light-years away, and is a relatively convenient
laboratory for exploring these
powerful sources of energy.
APOD: February 23, 1998 - M104: The Sombrero Galaxy
Explanation:
What's going on in the center of this spiral galaxy? Named the Sombrero Galaxy for its hat-like resemblance,
M104 features a prominent
dust
lane and a bright halo of stars and
globular clusters.
Something truly energetic is going on in the
Sombrero's center, as it not only appears bright in visible light,
but glows prodigiously in
X-ray
light as well. This X-ray emission coupled with unusually
high central stellar speeds
cause many astronomers to speculate that a
black hole lies at the Sombrero's center - a black hole possibly a billion
times the mass of our Sun.
APOD: January 17, 1998 - At The Core Of M15
Explanation:
Densely packed stars in the core of the
globular cluster
M15 are shown
in this Hubble Space Telescope (HST) image.
The star colors
roughly indicate their temperatures - hot stars
appear blue, cooler stars look reddish-orange.
The region visible here is only about 1.6 light-years across,
compared to the 4.3 light-year distance to
our own Sun's nearest neighbor.
Imagine the night
sky viewed from a planet orbiting a star near this cluster's
center!
M15 has long been
recognized as one of the densest clusters of stars in our galaxy outside of
the galactic center itself.
Even the unprecedented resolving
power of the HST cameras could not separate the individual stars in its
innermost regions.
However,
this HST image reveals that the density of stars continues
to rise toward the cluster's core, suggesting that a sudden,
runaway collapse due to the gravitational attraction of many closely
packed stars or a single central massive object, perhaps a
black hole,
could account for the core's extreme density.
APOD: December 6, 1997 - A Quasar Portrait Gallery
Explanation:
QUASARs (QUASi-stellAR objects)
lie near
the edge of the observable Universe.
Discovered in 1963,
astronomers were astounded that such objects could be
visible across billions of light-years, as this implies
they must emit prodigious
amounts of energy. Where does the energy come from?
Many believe
the quasar's central engine is a giant black hole
fueled by tremendous amounts of infalling gas, dust, and stars.
This gallery of quasar portraits from the Hubble Space
Telescope offers a look at their local neighborhoods: the quasars themselves
appear as the bright star-like objects with
diffraction spikes.
The images in the center and right hand columns reveal quasars
associated with disrupted colliding and merging galaxies
which should provide
plenty of debris to feed a hungry
black hole.
APOD: December 2, 1997 - Micro-Quasar GRS1915 Puffs
Explanation:
On the far side of
our Galaxy,
gas clouds explode away from a small
black hole.
This might seem peculiar, as
black holes are supposed to attract matter.
But material falling toward a
black hole collides and heats up,
creating an environment similar to a
quasar that is far from stable.
In the
above time-lapse sequence,
micro-quasar GRS1915 expels bubbles of hot gas in spectacular
jets.
These computer enhanced radio images show one
plasma bubble coming almost directly toward us at 90 percent the
speed of light,
and another moving away. Each of the four frames
marks the passage of one day.
Originally detected on October 29th, these bubbles have now faded from view.
APOD: November 24, 1997 - Jet Near Light Speed
Explanation:
Einstein's Special Theory of Relativity says that nothing can
travel faster than the speed of light. Jets of protons and electrons that shoot away from
objects such as
quasars and
black holes appear to travel at speeds
approaching this maximum speed, though.
Such jets carry tremendous energy and can ram
straight through interstellar material. In the above frame from a computer simulation,
a jet traveling only 98 percent of light speed
rams and mixes with interstellar material.
Even higher energy jets might well explain the structure seen around
Cygnus A.
APOD: November 7, 1997 - Evidence for Frame Dragging Black Holes
Explanation:
Gravity can do more than floor you.
According to
recent measurements of a
star system thought to contain a black hole, it can spin you too.
This effect, called frame-dragging, is most prominent near massive, fast spinning objects.
Now, a team led by
W. Cui
(MIT)
has used the orbiting
Rossi X-ray Timing Explorer to search for it near a system
thought to contain a
black hole.
Cui's team claim that matter in this system gets caught up
and spun around the
black hole at just the rate expected from frame-dragging.
Such discoveries help scientists better understand
gravity itself.
APOD: October 19, 1997 - The Heart Of NGC 4261
Explanation:
What evil lurks in the hearts of galaxies?
This Hubble Space Telescope picture
of the center of the nearby
elliptical galaxy
NGC 4261 tells one
dramatic
tale.
The gas and dust in this disk are swirling into what is almost
certainly a massive black hole.
The disk is probably what remains of a
smaller galaxy that fell in hundreds of millions of years ago.
Collisions
like this may be a common way of creating such active galactic nuclei as
quasars.
Strangely, the center of this fiery whirlpool is offset from the
exact center of the galaxy - for a reason that for now remains an
astronomical mystery.
APOD: September 26, 1997 - A Lonely Neutron Star
Explanation:
How massive can a star get without imploding into a black hole?
These limits are being tested by the discovery of a lone neutron star in space. Observations by the
Hubble Space Telescope
released Wednesday,
have been combined with previous observations by the
X-ray ROSAT observatory and
ultraviolet
EUVE
observatory for the isolated star at the location of the arrow.
Astronomers are able to directly infer the star's size from measurements of its
unblended brightness, temperature, and an upper limit on the distance.
Assuming that the object is a
neutron star of typical mass,
some previous theories of neutron star structure would have predicted
an implosion that would have created a
black hole. That this
neutron star
even exists therefore allows a window to the extreme
conditions that exist in the interiors of neutron stars.
APOD: September 12, 1997 - The Center of NGC 6251 is Glowing
Explanation:
Something is lighting up the center of galaxy NGC 6251. Leading speculation holds that it is a large
black hole
not shrouded by gas and
dust typically found near the
center of a galaxy.
Observations taken with the
Hubble Space Telescope
and released earlier this week indicate a new perspective on the
strange beasts
that rule the centers of galaxies:
a bright central object that is illuminating a
surrounding material disk, shown in blue.
The lack of reflection from the upper part of the disk indicates
that this disk is warped in shape. Although not visible in the
above composite image, a
huge plasma jet streams out
from the central object,
perpendicular to the warped disk.
APOD: June 17, 1997 - Arp 220: Spirals in Collision
Explanation:
Arp 220 is the
brightest object in the local universe. But why does it shine so brightly?
Arp 220
was cataloged as a peculiar galaxy in the 1960s. In the late 1980s, it was discovered to be an
ultraluminous
infrared galaxy and headed a list compiled from observations with the now-defunct
IRAS satellite.
New observations with the
Hubble Space Telescope are quite revealing.
Photos by
NICMOS in the
infrared taken in April and released just last week now better resolve the two colliding
spiral galaxies at the center of
Arp 220.
A result of this
spiral collision are fantastic knots of new star formation visible as the bright spots on the above photograph. Below the "half-moon" shaped knot on the right is a massive disk of dust possibly hiding a dying spiral's
central black hole. The bright knot to the left is the center of the other broken spiral galaxy. The galaxy cores are about 1200 light years apart and are orbiting each other.
APOD: June 13, 1997 - Streaming From A Black Hole
Explanation:
Glowing gas clouds are
streaming from the core of galaxy NGC4151 at
hundreds of thousands of miles per hour.
A powerful tool,
the Hubble Space Telescope's
new STIS instrument,
makes it possible to map out the cloud velocities - producing
this false color "velocity map" for the central regions of NGC4151.
The horizontal line is light from the intensely bright region near
the galaxy nucleus.
Emission at two wavelengths characteristic of Oxygen atoms
in the gas clouds
is visible along this line.
Below the line the emission is displaced to the left, indicating motion
toward us
(blue shift); above the displacement is to the right indicating
a receding motion
(red shift).
Where do the clouds come from?
As evidence mounts,
the widely accepted explanation for energetic
nuclear activity in galaxies is based on material spiraling
into a central black hole with over a million
times the mass of our sun.
The rotating disk of interstellar debris which develops is thought
to blast out high velocity jets along the axis of the disk.
Do all galaxies contain supermassive black holes?
APOD: May 16, 1997 - Signed, "A Black Hole"
Explanation:
This artistic image
is actually the signature of
a supermassive black hole in the center of distant galaxy M84
- based on data recently recorded by
Hubble's new Space Telescope Imaging Spectrograph
(STIS).
Very near black holes
the force of gravity is so strong that even light
can not escape ...
but the presence of a black hole can also be
revealed by watching matter fall into it.
In fact, material spiraling into a black hole would find its speed increasing
at a drastic rate.
These extreme velocity increases provide a "signature" of the
black hole's presence.
STIS relies on
the Doppler effect to measure gas velocity rapidly increasing to
nearly 240 miles per second within 26 light years of
the center of M84, a galaxy in the Virgo Cluster
about 50 million light years away.
The STIS data show
that radiation from approaching gas, shifted to blue wavelengths
left of the centerline, is suddenly redshifted
to the right of center indicating a rapidly rotating disk of material
near the galactic nucleus.
The resulting sharp S-shape is effectively the signature of a
black hole estimated to contain at least 300 million solar masses.
Do all galaxies have central black holes?
APOD: April 13, 1997 - Jets from SS433
Explanation: SS433
is one of the most exotic star systems known. Its unremarkable
name stems from its inclusion in a catalog of stars which emit
radiation characteristic of atomic hydrogen.
Its very remarkable behavior stems from a compact object, a black hole
or neutron star,
which has produced an accretion disk with jets. As illustrated
in this artist's vision
of the SS433 system
based on observational data,
a massive, hot star (left) is locked in a mutual orbit with a
compact object. Material transfers from the massive star into
an accretion disk surrounding the
compact object blasting out two jets of ionized gas in opposite
directions - at about 1/4 the speed of light!
Radiation from the jet tilted toward the observer is blueshifted,
while radiation from the jet tilted away is redshifted.
The binary system itself completes an orbit in about 13 days while
the jets precess (wobble like a top) with a period of about 164
days. Are the jets from SS433 related to those from black holes at the centers of galaxies?
APOD: April 5, 1997 - A Black Hole in M87?
Explanation:
The center of nearby giant galaxy
M87 is a dense and violent place. In
this
1994 photograph by the
Hubble Space Telescope, a disk of hot
gas was found to be orbiting at the center of this massive
elliptical galaxy. The
disk is evident at the lower left of the picture. The rotation
speed of gas in this disk indicates the mass of the object the gas is
orbiting, while the size of the disk indicates an approximate volume of the
central object.
These observations
yield a central density so high that the
only hypothesized object that could live there is a
black hole. The
picture also shows a highly
energetic jet emanating from the central
object like a cosmic blowtorch.
The jet is composed of fast moving charged particles and has
broken into knots as small as 10 light years across.
APOD: February 8, 1997 - M104: The Sombrero Galaxy
Explanation: The famous Sombrero galaxy (M104) is a bright
nearby spiral galaxy. The prominent
dust lane and halo of stars and globular clusters
give this galaxy its name. Something very energetic is going on
in the Sombrero's
center, as much X-ray light
has been detected from it. This X-ray
emission coupled with unusually high central stellar velocities
cause many astronomers to speculate that a black hole
lies at the Sombrero's center
- a black hole
a billion times the mass of our Sun.
APOD: January 21, 1997 - Journey to the Center of the Galaxy
Explanation:
In Jules Verne's science fiction classic
A Journey to the Center of the Earth,
Professor Hardwigg and his fellow explorers
encounter many strange and exciting wonders.
What wonders lie at the center of our Galaxy?
Astronomers now know of some of the bizarre objects which exist there,
like vast dust clouds,
bright young stars,
swirling rings of gas, and
possibly even a
large black hole.
Much of the Galactic center region
is shielded from our view in visible light by the
intervening dust and gas.
But it can be explored using other forms of electromagnetic
radiation, like
radio, infrared, X-rays, and gamma rays.
This beautiful high resolution image of the
Galactic center region in infrared light was made by the
SPIRIT III telescope onboard the
Midcourse Space Experiment.
The center itself
appears as a bright spot near the middle of the roughly 1x3 degree field of
view, the plane of the Galaxy is vertical, and the
north galactic pole is towards the right. The picture is in false color -
starlight appears blue while dust is greenish grey, tending to red in
the cooler areas.
APOD: January 15, 1997 - Black Hole Signature From Advective Disks
Explanation: What does a black hole look like? If alone,
a black hole
would indeed appear
quite black, but many black hole candidates are part of binary star systems.
So how does a black hole binary system
look different from a neutron star binary system?
The above drawings indicate it
is difficult to tell! Recent theoretical work,
however, has provided a new way to tell them apart: advective accretion flows (ADAFs).
A black hole system so equipped
would appear much darker than a similar neutron star
system. The difference is caused by the hot gas from the ADAF disk
falling through the event horizon
of the black hole and disappearing - gas that would have emitted
much light were the central object only a neutron star. Recent observations
of the soft X-ray transient
V404 Cyg
has yielded a spectrum
much like an ADAF onto a black hole
- and perhaps brighter than allowable from an ADAF onto a neutron
star.
APOD: January 14, 1997 - Black Holes and Galactic Centers
Explanation: Do all galaxies have black holes at their
centers? Although not even a single galaxy
has yet been proven to have a central black hole,
the list of candidates has increased yet again. Recent results by astronomers using the Hubble Space Telescope
now indicate that most - and possibly even all - large galaxies
may harbor one of these dense beasts.
In all the galaxies studied, star speeds continue to increase
closer the very center. This in
itself indicates a center millions of times more massive than
our Sun is needed to contain the
stars. This mass when combined with the limiting size make the
case for the central black holes.
Will we ever know for sure?
APOD: January 5, 1997 - Too Close to a Black Hole
Explanation: What would you see if you went right up to
a black hole?
Above are two computer generated pictures highlighting how strange
things would look. On the left is a normal star field containing
the constellation Orion.
Notice the three stars of nearly equal brightness that make up
Orion's Belt. On the right is the
same star field but this time with a black hole superposed in
the center of the frame. The black hole
has such strong gravity that light is noticeably bent towards
it - causing some very unusual visual distortion. In the distorted
frame, every star in the normal frame has at least two bright
images - one on each side of the black hole.
In fact, near the black hole, you can see the whole sky - light
from every direction is bent around and comes back to you. Black
holes are thought to be the densest state of matter, and there
is indirect evidence for their presence in stellar binary systems
and the centers of globular clusters,
galaxies, and quasars.
APOD: January 4, 1997 - A Star Where Photons Orbit
Explanation: The above computer animated picture
depicts how a very compact star
would look to a nearby observer. The star pictured
is actually more compact that any known except a black hole,
so it is only hypothetical. The observer is situated at the photon sphere,
where photons can orbit in a circle. To help the viewer better
visualize the great distortions created by gravity, a map of the
Earth was projected onto the star,
and a map of the familiar night sky was projected above. From
here one can either look down and see several duplicate images
of the entire surface of the star, look up and see several duplicate
images of the entire night sky, or look along the photon sphere
and see the back of one's own head.
APOD: November 25, 1996 - A Quasar Portrait Gallery
Explanation:
QUASARs (QUASi-stellAR objects)
lie near
the edge of the observable Universe.
Discovered in 1963,
astronomers were astounded - to be
visible at such extreme
distances of billions of light-years they must emit prodigious
amounts of energy. Where does the energy come from?
Many believe
the quasar's central engine is a giant black hole
fueled by tremendous amounts of infalling gas, dust, and stars.
This recently released gallery of quasar portraits from the Hubble Space
Telescope offers a look at their local neighborhoods: the quasars themselves
appear as the bright star-like objects with diffraction spikes.
The images in the center and right hand columns reveal quasars
associated with disrupted colliding and merging galaxies
which should provide
plenty of debris to feed a hungry
black hole.
Yet, in the left hand column a quasar is seen at the
center of an otherwise normal looking spiral (above) and
elliptical galaxy.
Whatever the secret of the quasar's energy,
all these sites must provide fuel for its central engine.
APOD: November 1, 1996 - Spiral Galaxy NGC 3628 Edge On
Explanation: This is what a spiral galaxy looks like sideways.
This view of NGC 3628 nearly resembles our own Milky Way Galaxy,
which is also known to be a spiral.
The dark band across the center is absorbed starlight caused
by the galaxy's own interstellar dust.
NGC 3628 is the faintest member of the Leo Triplet,
a group of galaxies dominated by M65
and M66. The Leo Triplet
lies about 35 million light years distant. The center of NGC 3628 emits variable X-ray radiation
perhaps indicating the presence of a massive black hole.
APOD: September 11, 1996 - In the Center of Spiral M77
Explanation:
What is happening in the center of nearby spiral galaxy M77? To find out, astronomers used the
Hubble Space Telescope to peer deep into the
dusty chaos of this
active galactic nucleus
in 1994.
They found a network of filamentary gas and opaque dust that provides only clues as to what central monster had left this mess. Due to the presence of hot ionized gas clouds near the core, changes in brightness that can take less than a week, and the
ultraviolet halo surrounding the whole galaxy, the leading hypothesis is that a supermassive
black hole lies at the center of this
Seyfert Type 2 galaxy. Also known as
NGC 1068, this galaxy lies only about 50 million light years distant and is visible with only a small telescope.
APOD: September 10, 1996 - M77: Spiral with a Strange Glow
Explanation:
Why is M77 surrounded by an ultraviolet glow?
M77, also called NGC 1068, appears at first sight to be a relatively normal
barred spiral galaxy. But when photographed in the
ultraviolet (UV), as
shown
above in false color, the galaxy sports an
ultraviolet halo - shown as violet in the photograph.
The blue spiral structure closer to the picture's center indicates normal ultraviolet emission from bright young stars that have recently formed there.
Astronomers now hypothesize that the outer glow arises from UV light emitted from the galaxy's active center and reflected to us from clouds of
gas and
dust. These same gas and dust clouds obscure the active center of this
Seyfert galaxy - where an ultramassive
black hole is thought to live.
APOD: August 24, 1996 - Why is QSO 1229+204 so Bright?
Explanation:
What causes the center of this barred spiral galaxy to light up brighter
than almost anything in the universe? The
quasar
there is a good fraction of the way across our observable
universe but appears so bright that
astronomers had to use the high resolving power of the
Hubble Space Telescope (HST) just to see the
host galaxy.
HST then
resolved something very interesting. Not only was QSO 1229+204 at the
core of an unusual barred
spiral galaxy,
but this galaxy was in the process of
colliding with a
dwarf galaxy.
Gas from this collision
quite possibly fuels
a supermassive black hole causing
QSO 1229+204 to shine so brightly.
APOD: May 29, 1996 - The COMPTEL Gamma-Ray Sky
Explanation:
This premier gamma-ray view of the sky was produced by
the COMPTEL instrument
onboard NASA's orbiting
Compton Gamma Ray Observatory.
The entire sky is seen projected on a coordinate system
centered on our Milky Way Galaxy with the
plane of the Galaxy
running across the middle of the picture.
Gamma-ray intensity is represented by a false color map -
low (blue) to high (white).
COMPTEL's sensitivity to gamma-rays which have
over 1 million times the energy of visible light photons
reveals the locations of some of the Galaxy's most exotic objects.
The brightest source, the Crab pulsar,
is located near the plane of the Galaxy on the far right.
Moving along the plane from the Crab, more than halfway toward
the galactic center, another bright gamma-ray source,
the Vela pulsar, appears.
The galactic center itself, along with the
famous black hole candidate Cygnus X-1 (near the plane, halfway from the
center to the left edge) are also seen as bright sources.
Both above and below the plane, spots of gamma-ray emission due to
distant active galaxies are also visible.
APOD: May 15, 1996 - The Milky Way Near the Northern Cross
Explanation:
This beautiful image of the sky near the bright star
Deneb
(just above center) reveals the stars, nebulae, and dark clouds
along the plane of our Milky Way Galaxy as seen from the northern
hemisphere (near Columbia Missouri, USA).
Just below Deneb lies the suggestively shaped North America emission nebula.
Deneb is
the brightest star in the constellation
Cygnus,
located in the tail of this
celestial swan.
Cygnus
contains the asterism known as the Northern Cross and
marks one side of the "Great Rift" in the Milky Way, a series of dark
obscuring dust clouds which stretches on through the constellation
Sagittarius.
Deneb defines the top of the Northern
Cross while the body of the cross extends past the
upper right corner of the picture.
Cygnus also harbors the most famous
candidate for a black hole in our galaxy,
Cygnus X-1.
APOD: May 5, 1996 - Planet Near a Galaxy Core
Explanation:
What would the night sky look like if you lived on a planet near the center
of a galaxy? Now imagine that this galaxy houses a
black hole billions of
times more massive than a star. From this spectacular vantage
point, the sky
might look like the above illustration. This drawing is based on recent
observations of the center of NGC 4261,
made by the Hubble Space Telescope.
Results indicate that a disk of
dust 800-light years wide
surrounds the black hole. The hypothetical planet depicted above lies
within this disk. The black hole itself heats
gas to white-hot
temperatures, generating light that is reddened when reflected off the dust.
Jets
shoot off from the poles of the black hole, perpendicular to the disk.
However, friction with the dust and gas would cause planets near the black
hole to spiral in and disappear forever.
NASA has recently announced a
new
initiative to search for Earth-like planets in
our Galaxy.
APOD: March 6, 1996 - Jets From SS433
Explanation:
SS433 is one of the most exotic star systems known to astronomers.
Its unremarkable name stems from its inclusion in a catalog of
stars which emit radiation characteristic of
atomic hydrogen.
Its very remarkable behavior stems from a compact
object, a
black hole or
neutron star, which has produced an
accretion disk with jets.
As illustrated in this
artist's vision of
the SS433 system based on
observational data,
a massive, hot star (left) is locked in
a mutual orbit with a compact object. Material transfers from the
massive star into
an accretion disk surrounding the
compact object
blasting out two jets of ionized gas in opposite directions -
at about 1/4 the speed of light! Radiation from the jet tilted toward
the observer is blueshifted, while radiation from the jet tilted away
is redshifted.
The binary system itself completes an orbit in about 13 days while the jets
precess (wobble like a top) with a period of about 164 days.
Are the jets from SS433 related to those from
black holes at the centers of galaxies?
APOD: March 5, 1996 - A Black Hole in M87's Center?
Explanation:
The center of nearby giant galaxy
M87 is a dense and violent place. In
this
1994 photograph by the
Hubble Space Telescope, a disk of hot
gas was
found to be orbiting at the center of this massive
elliptical galaxy. The
disk is evident on the lower left of the above photograph. The rotation
speed of gas in this disk indicates the mass of the object the gas is
orbiting, while the size of the disk indicates an approximate volume of the
central object.
These observations yield a central density so high that the
only hypothesized object that could live there is a
black hole. The
picture also shows a highly energetic
jet emanating from the central
object. The jet is composed of fast moving charged particles and has
broken into knots as small as 10 light years across.
APOD: December 30, 1995 - LMC X-1: A Black Hole Candidate
Explanation:
The strongest source of X-rays in the
Large Magellanic Cloud originates
from an unusually energetic
binary star system. This strong source, dubbed
LMC X-1, is thought to be a normal and compact star orbiting each other.
Gas stripped of the normal star
falls onto the compact star, heats up, and
emits X-rays. The X-rays shining from the system knock electrons off atoms
for light years around, causing some atoms to glow noticeably in X-rays
when the electrons re-combine. Motion in the binary system indicates the
compact star is probably a
black hole, since its high mass -
roughly five times that of our
Sun -
should be enough to cause even a
neutron star to implode.
APOD: December 26, 1995 - Accretion Disk Binary System
Explanation:
Our
Sun is unusual in that it is alone - most
stars occur in multiple or
binary systems. In a binary system, the
higher mass star will evolve faster and will eventually
become a compact object - either a
white dwarf star, a
neutron star, or
black hole. When the lower mass star later
evolves into an expansion phase, it may be so close to the compact star
that its outer atmosphere actually falls onto the compact star. Such is
the case
diagrammed
above. Here
gas from a blue giant star is
shown being stripped away into an accretion disk around its compact binary
companion. Gas in the accretion disk swirls around, heats up, and
eventually falls onto the compact star. Extreme conditions frequently
occur on the surface of the compact star as gas falls in, many times
causing detectable
X-rays,
gamma-rays, or even
cataclysmic novae explosions. Studying the extreme conditions in these
systems tells us about the inner properties of ordinary matter around us.
APOD: December 5, 1995 - The Swirling Center of NGC 4261
Explanation:
What evil lurks in the hearts of galaxies? The
above picture
by the
Hubble Space Telescope of the center of the
nearby galaxy NGC 4261 tells us one
dramatic
tale. Here
gas and
dust are
seen swirling near this
elliptical galaxy's center into what is almost
certainly a massive
black hole. The disk is probably what remains of a
smaller galaxy that fell in hundreds of millions of years ago. Collisions
like this may be a common way of creating such active galactic nuclei as
quasars.
Strangely, the center of this fiery whirlpool is offset from the
exact center of the galaxy - for a reason that for now remains an
astronomical mystery.
APOD: November 27, 1995 - Too Close to a Black Hole
Explanation:
What would you see if you went right up to a
black hole? Above are two computer generated
pictures highlighting how strange things would look.
On the left is a normal star field containing the constellation
Orion.
Notice the three stars of nearly equal brightness that
make up Orion's Belt. On the right is the same star field but this time with a black hole superposed
in the center of the frame. The
black hole has such strong gravity
that light is noticeably bent towards it -
causing some very unusual visual distortion.
In the distorted frame, every star in the normal frame
has at least two bright
images - one on each side of the black hole. In fact, near the black hole, you can see the whole sky - light from every
direction is bent around and comes back to you.
Black holes are thought to be the densest state of matter, and
there is indirect evidence for their presence in
stellar binary systems and the centers of
globular clusters,
galaxies, and
quasars.
APOD: November 26, 1995 - A Star Where Photons Orbit
Explanation:
The above
computer animated picture depicts how a
very compact star would
look to a nearby observer. The star pictured is actually more compact that
any known except a
black hole, so it is only hypothetical. The observer is
situated at the photon sphere, where photons can orbit in a circle. To
help the viewer better visualize the great distortions created by gravity,
a map of the Earth was projected onto the star, and a map of the familiar
night sky was projected above. From here one can either look down and see
several duplicate images of the entire surface of the star, look up
and see several duplicate images of the entire night sky, or look along the
photon sphere and see the back of one's own head.
APOD: November 20, 1995 - At the Core of M15
Explanation:
Densely packed stars in the core of the globular cluster
M15 are shown
in this Hubble Space Telescope (HST)
image taken in April of 1994. The
stars revealed are contained in an area 1.6 light years across and
their colors roughly indicate their temperatures - hot stars
appear blue, cooler stars look reddish-orange. M15 has long been
recognized as one of the densest cluster of stars in our galaxy outside of
the galactic center itself.
Even the unprecedented resolving
power of the HST cameras could not separate the individual stars in its
innermost regions. However,
this HST image reveals that the density of stars continues
to rise toward the cluster's core, suggesting that a sudden,
runaway collapse due to the gravitational attraction of many closely
packed stars or a single central massive object, perhaps a
black hole,
could account for the core's extreme density.
APOD: November 9, 1995 - M104: The Sombrero Galaxy
Explanation:
The famous Sombrero galaxy (M104) is a bright nearby
spiral galaxy. The
prominent
dust lane and halo of
stars and
globular clusters give this
galaxy its name. Something
very energetic is going on in the Sombrero's center, as much
X-ray
light has been detected from it. This X-ray emission
coupled with unusually high central stellar velocities cause many
astronomers to speculate that a
black hole
lies at the Sombrero's center - a
black hole a billion times the mass of our
Sun. This image was taken
in blue light by the 0.9 meter telescope at Kitt Peak National Observatory.
APOD: October 22, 1995 - A Quasar-Galaxy Collision?
Explanation:
In 1963 astronomers were astounded to discover that certain faint,
star-like objects have
very large redshifts.
The large redshifts imply that these objects, now known as
quasars (QUASi-stellAR objects),
lie near the edge of the observable Universe.
To be visible at such extreme distances of billions of light years,
they must
emit tremendous amounts of energy. Where does the energy come from?
In the most widely accepted model, a quasar is the bright nucleus of
an active galaxy powered by a central, supermassive
black hole.
This Hubble Space Telescope image shows
a quasar known as PKS 2349 (the star-like object
near the center) and a galaxy (surrounding fuzzy patch), but
the quasar is not at the galaxy's center! In fact, the
galaxy
and quasar seem to be colliding or merging.
This and other recent HST observations
suggest that astronomers' standard ideas about quasars may be wrong.