Astronomy Picture of the Day |
APOD: 2009 March 13 - Hickson Compact Group 90
Explanation:
Scanning the skies for galaxies, Canadian astronomer
Paul Hickson and colleagues identified some 100 compact
groups
of galaxies, now appropriately called
Hickson Compact Groups (HCGs).
This sharp Hubble image
shows one such galaxy group, HCG 90,
in startling detail.
Three galaxies are revealed to be strongly interacting:
a dusty spiral galaxy stretched and distorted between a pair of
large elliptical galaxies.
The close encounter will trigger
furious star formation.
On a cosmic timescale, the gravitational tug of war will eventually
result
in the merger of the trio into a large single galaxy.
The merger process is
now understood to be a normal part of the evolution of
galaxies, including
our own Milky Way.
HCG 90 lies about 100 million light-years away in
the constellation
Piscis
Austrinus.
This Hubble view
spans about 80,000 light-years at that estimated
distance.
Of course, Hickson Compact Groups also make for
rewarding viewing for Earth-bound astronomers
with more modest sized telescopes.
APOD: 2009 January 9 - NGC 4945 in Centaurus
Explanation:
Large, dusty, spiral galaxy NGC 4945 is seen
edge-on
near the center of this
rich
telescopic image.
The field of view spans nearly 2 degrees, or about 4 times the width
of the Full Moon, toward the
expansive southern
constellation Centaurus.
About 13 million light-years distant, NGC 4945 is almost
the size of our own
Milky Way Galaxy.
But X-ray and infrared observations
reveal
even more high energy emission and star formation in the core
of NGC 4945.
The other prominent galaxy in the field, NGC 4976, is an
elliptical galaxy.
Left of center,
NGC 4976
is much farther away, at a distance of about 35 million light-years,
and not physically associated with NGC 4945.
APOD: 2008 September 9 - M110: Satellite of the Andromeda Galaxy
Explanation:
Our Milky Way Galaxy is not alone.
It is part of a gathering of about 25 galaxies known as the
Local Group.
Members include the
Great Andromeda Galaxy (M31),
M32,
M33, the
Large Magellanic Cloud, the
Small Magellanic Cloud,
Dwingeloo 1, several small
irregular galaxies,
and many
dwarf elliptical and
dwarf spheroidal galaxies.
Pictured
on the lower right is one of the
dwarf
ellipticals:
NGC 205.
Like
M32,
NGC 205
is a companion to the large M31,
and can sometimes be seen to the south of
M31's center in photographs.
The image shows
NGC 205 to be unusual
for an elliptical galaxy
in that it contains at least two
dust clouds
(at 9 and 2 o'clock - they are visible but hard to spot)
and signs of recent star formation.
This galaxy is sometimes known as M110,
although it was actually not part of
Messier's original
catalog.
APOD: 2008 September 2 - NGC 1316: After Galaxies Collide
Explanation:
Astronomers turn detectives when trying to
figure out the cause of startling sights like
NGC 1316.
Their investigation indicates that
NGC 1316
is an enormous
elliptical galaxy
that started, about 100 million years ago, to devour a smaller
spiral galaxy neighbor,
NGC 1317, just above it.
Supporting evidence includes the dark
dust lanes
characteristic of a
spiral galaxy,
and faint swirls of stars and gas visible in this
wide and deep image.
What remains unexplained are the unusually small
globular star clusters,
seen as faint dots on
the image.
Most elliptical
galaxies have
more and brighter globular
clusters than
NGC 1316.
Yet the observed
globulars are too old to have been
created by the recent
spiral collision. One
hypothesis is that these
globulars
survive from an even earlier galaxy
that was subsumed into
NGC 1316.
APOD: 2008 February 13 - Elliptical Galaxy NGC 1132
Explanation:
NGC 1132 is one smooth galaxy -- but how did it form?
As an
elliptical galaxy,
NGC 1132 has little dust and gas, and few stars have formed in it recently.
Although many elliptical galaxies are in clusters of galaxies, NGC 1132
appears as a large, isolated galaxy toward the constellation of
the River (Eridanus).
To probe the history of this intriguing trillion-star ball, astronomers imaged
NGC 1132 in both
visible light with the
Hubble Space Telescope and
X-ray light with the
Chandra X-ray Observatory.
In this composite false-color image, visible light is white,
while the X-ray light is blue and indicates the unusual
presence of very hot gas.
The X-ray light also likely traces out the location of
dark
matter.
One progenitor hypothesis is that NGC 1132 is the result of a series of
galaxy mergers in what once was a small
group of galaxies.
NGC 1132 is over 300 million light years away, so the light we see
from it today left before
dinosaurs
roamed the Earth.
Many
fascinating background galaxies
can be seen far in the distance.
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 October 12 - The Whale and the Hockey Stick
Explanation:
NGC
4631 is a big beautiful spiral galaxy
seen edge-on (top right)
only 25 million light-years away towards the small northern
constellation
Canes
Venatici.
This galaxy's slightly distorted wedge shape suggests to
some a cosmic herring and to others the popular moniker of
The Whale Galaxy.
Either way, it is similar in size to our
own Milky Way.
In this gorgeous color image,
the Whale's dark interstellar dust clouds, yellowish core, and young
blue star clusters are easy to spot.
A companion galaxy, the small elliptical NGC 4627, appears above the
Whale Galaxy.
At the lower left is another distorted galaxy,
the hockey stick-shaped
NGC
4656.
The distortions and mingling
trails of gas detected at other
wavelengths suggest that all three galaxies have had
close
encounters with each other in their past.
The Whale Galaxy is also known to have spouted a halo of hot gas glowing
in x-rays.
APOD: 2007 October 8 - Galaxy NGC 474: Cosmic Blender
Explanation:
What's happening to galaxy NGC 474?
The multiple layers of emission appear strangely complex and unexpected given the relatively featureless appearance of the elliptical galaxy in less deep images.
The cause of the shells is currently unknown, but possibly
tidal tails related to debris left over from absorbing
numerous small galaxies in the past billion years.
Alternatively the shells may be like ripples in a pond,
where the ongoing collision with the spiral galaxy to the right of
NGC 474
is causing density
waves to
ripple though the galactic giant.
Regardless of the actual cause, the
above image dramatically highlights the increasing consensus that the
outer halos of most
large galaxies are not really smooth
but have complexities induced by frequent interactions with -- and accretions of --
smaller nearby galaxies.
The halo of our own
Milky Way Galaxy
is one example of such
unexpected complexity.
NGC 474 spans about 250,000
light years
and lies about 100 million light years distant toward the constellation of the Fish
Pisces.
APOD: 2007 July 19 - The Hercules Cluster of Galaxies
Explanation:
These are galaxies of the
Hercules
Cluster, an archipelago of
island universes a mere
500 million light-years away.
Also known as
Abell 2151,
this cluster is loaded with gas and dust rich,
star-forming spiral galaxies
but has relatively few elliptical galaxies,
which lack gas and dust and the associated newborn stars.
The colors in
this remarkably
deep composite image
clearly show the star forming galaxies with a blue tint and
galaxies with older stellar populations with a yellowish cast.
The sharp picture spans about 1/2 degree across the
cluster center, corresponding to over 4 million light-years at the
cluster's estimated distance.
In the cosmic vista many galaxies seem to be
colliding or
merging
while others seem
distorted - clear evidence that
cluster galaxies
commonly interact.
In fact, the Hercules Cluster itself may be seen as the result of
ongoing mergers of smaller galaxy clusters and is thought to be
similar to young galaxy clusters in
the much more distant,
early Universe.
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 31 - Dwarf Galaxies in the Coma Cluster
Explanation:
In visible
light images, over a thousand galaxies are seen to
lie within a volume about 20 million light-years across in
the rich Coma Galaxy Cluster.
But infrared images of the Coma Cluster have now been used
to add thousands more to the Coma's galaxy count in the form
of previously
undiscovered dwarf galaxies.
This composite combines infrared
Spitzer Space Telescope image data
(red and green) with visible light Sloan Sky Survey data (blue)
for the central part of the cluster.
Over 1 degree wide, the field is
dominated by two giant
elliptical galaxies in blue.
Still, many of the small green smudges (see magnified inset)
are identified as dwarf galaxies,
roughly comparable to the
Small Magellanic Cloud.
Dwarf galaxies are thought to form
first, providing building blocks for larger galaxies.
The well-studied,
friendly,
Coma Cluster
is 320 million light-years away.
APOD: 2007 March 19 - Galaxy Group Hickson 44
Explanation:
Galaxies, like stars, frequently form groups.
A group of galaxies is a system containing more than
two galaxies but less than the tens or hundreds typically found in a
cluster of galaxies.
A most notable example is the Local Group of Galaxies, which
houses over 30 galaxies including our
Milky Way,
Andromeda, and the
Magellanic Clouds.
Pictured above is nearby compact group Hickson 44.
This
group is located about 60 million
light-years away toward the constellation of Leo.
Also known as the NGC 3190 Group,
Hickson 44
contains several bright spiral galaxies and one bright
elliptical galaxy on the upper left.
The bright source on the upper right is a foreground star.
Many galaxies in
Hickson 44 and other compact
groups are either slowly merging or
gravitationally pulling
each other apart.
APOD: 2007 February 8 - Galaxies Away
Explanation:
This stunning group of galaxies is far, far away -
about 450 million light-years from
planet Earth -
cataloged as galaxy cluster Abell S0740.
Dominated by the cluster's large central elliptical galaxy
(ESO 325-G004), this sharp Hubble view
takes in a remarkable assortment of
galaxy shapes and sizes with
only a few spiky foreground stars
scattered through the field.
The giant elliptical galaxy
spans over 100,000 light years and
contains about 100 billion stars, comparable in
size to our own spiral Milky Way.
The Hubble data reveal a
wealth
of detail in even these distant
galaxies, including magnificent
arms and dust lanes,
star clusters, ring structures,
and gravitational lensing arcs.
APOD: 2006 September 8 - Messier 110
Explanation:
This very sharp
telescopic vista features
the last object in the modern version of Charles
Messier's catalog
of bright clusters and nebulae -
Messier 110.
A dwarf elliptical galaxy,
M110 (aka NGC 205) is actually a
bright satellite of the
large spiral galaxy
Andromeda,
making M110 a fellow member
of the local
group of galaxies.
Seen through a foreground of nearby stars,
M110 is about 15,000 light-years across.
That makes it comparable
in size to satellite galaxies of our own Milky Way,
the Large and
Small Magellanic Clouds.
Though elliptical galaxies
are normally thought to be lacking in gas and dust to form new stars,
M110 is known to contain
young stars, and faint dust clouds
can easily be seen in this detailed image at about the
7 and 11 o'clock positions relative to the galaxy center.
APOD: 2006 July 4 - Elliptical Galaxy Centaurus A from CFHT
Explanation:
Why is peculiar galaxy Centaurus A so dusty?
Dramatic dust lanes that run
across the galaxy's center mark
Cen A.
These dust lanes are so thick they almost completely obscure the
galaxy's center in
visible light.
This is particularly unusual as
Cen A's
red stars and round shape are characteristic of a giant
elliptical galaxy,
a galaxy type usually low in dark dust.
Cen A,
also known as NGC 5128, is also unusual compared to an average
elliptical galaxy because it contains a
higher proportion of young blue stars and is a
very strong source of
radio emission.
Evidence indicates that
Cen A is likely the result of the
collision of two
normal galaxies.
During the collision, many
young stars were formed, but
details of the creation of
Cen A's unusual dust belts are still being researched.
Cen A lies only 13 million
light years away, making it the closest
active galaxy.
Cen A,
pictured above, spans 60,000 light years and can be
seen with binoculars toward the constellation of
Centaurus.
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 May 6 - Three Galaxies in Draco
Explanation:
This intriguing trio of galaxies is sometimes
called the NGC 5985/Draco Group
and so (quite reasonably) is located in the northern
constellation
Draco.
From left to right are
face-on spiral NGC 5985,
elliptical galaxy NGC 5982, and
edge-on spiral NGC 5981 --
all within this single telescopic
field of view spanning a little more than
half the width of the full moon.
While this grouping is far too small to be a
galaxy cluster
and has not been
cataloged
as a compact group,
these galaxies all do lie roughly
100 million light-years from planet Earth.
On close examination with spectrographs, the bright core of the
striking face-on spiral NGC 5985 shows
prominent emission in specific wavelengths of light, prompting
astronomers to classify it as a
Seyfert, a type of active galaxy.
Not as well known as other tight
groupings of galaxies,
the contrast in visual appearance
makes this triplet an attractive subject for
astrophotographers.
This impressively deep exposure of region also reveals faint
and even more distant
background galaxies.
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: 2005 November 22 - A Galactic Collision in Cluster Abell 1185
Explanation:
What is a guitar doing in a cluster of galaxies? Colliding.
Clusters of galaxies are sometimes packed so tight that the
galaxies that compose them
collide.
A prominent example occurs on the left of the
above image of the rich
cluster of galaxies Abell 1185.
There at least two galaxies, cataloged as
Arp 105 and dubbed
The Guitar
for their familiar appearance, are pulling each other apart gravitationally.
Most of Abell 1185's hundreds of galaxies are
elliptical galaxies, although
spiral,
lenticular, and
irregular galaxies are all clearly evident.
Many of the spots on the above image are fully galaxies themselves containing
billions of stars, but some spots are foreground stars in our own
Milky Way Galaxy.
Recent observations of
Abell 1185 have found unusual globular clusters of stars that appear to belong
only to the galaxy cluster and not to any individual galaxy.
Abell 1185 spans about one million
light years and lies 400 million light years distant.
APOD: 2005 July 16 - Galaxy Group HCG 87
Explanation:
Posing for
this cosmic family photo
are the galaxies of HCG
(Hickson Compact Group)
87, about four hundred million
light-years distant toward the amphibious constellation
Capricornus.
The large edge-on
spiral near picture center, the fuzzy
elliptical galaxy immediately to its right, and the
spiral near the top of the image are identified members of the
group, while the small spiral
galaxy in the middle is likely a
more distant background galaxy.
In any event, a careful examination of the deep image reveals other
galaxies which certainly lie far beyond
HCG 87.
While not exactly locked in
a
group hug, the HCG 87 galaxies
are interacting
gravitationally, influencing their
fellow group members' structure and evolution.
This image is from the commissioning phase of an instrument on the
Gemini Observatory's
South Telescope at Cerro Pachon, Chile.
It compares favorably with views of this photogenic galaxy group
recorded by the
Hubble
Space Telescope.
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 May 12 - Stars, Galaxies, and Comet Tempel 1
Explanation:
Faint
comet
Tempel 1 sports a fuzzy blue-tinted tail,
just right of center in this
lovely field of stars.
Recorded on May 3rd slowly sweeping through the
constellation Virgo,
periodic comet Tempel 1
orbits the Sun once every 5.5 years.
Also caught in the skyview are two galaxies
at the upper left -
NGC 4762 and NGC 4754 -
both members of the large
Virgo
Cluster of galaxies.
Classified as a
lenticular
galaxy, NGC 4762
presents an edge-on disk as a narrow gash of light
while NGC 4754 is a football-shaped
elliptical galaxy.
Similar in apparent size,
the galaxies and comet make for an intriguing
visual comparison,
but Tempel 1 is only about 3 light-minutes from planet Earth.
The two Virgo cluster galaxies are 50 million
light-years away.
NASA's
Deep Impact
spacecraft is scheduled to encounter
Tempel 1 on July 4th, launching a probe to impact
the comet's nucleus.
APOD: 2005 April 27 - The Hercules Cluster of Galaxies
Explanation:
These are
galaxies of the Hercules Cluster, an archipelago of
"island universes" a mere 650 million light-years distant.
This cluster is loaded with gas and dust rich, star forming,
spiral galaxies
but has relatively few
elliptical galaxies, which lack gas and dust and
the associated newborn stars.
Colors in the composite image show the star forming galaxies
with a blue tint and ellipticals with a slightly yellowish cast.
In this cosmic vista many galaxies seem to be
colliding or
merging
while others seem distorted - clear evidence that
cluster galaxies commonly interact.
Over time, the galaxy interactions are likely to affect the
the content of the cluster itself.
Researchers believe that the
Hercules
Cluster is significantly similar
to young galaxy clusters
in the distant, early Universe
and that exploring galaxy types and their interactions in nearby
Hercules will help unravel the threads
of galaxy andcluster evolution.
APOD: 2005 April 4 - NGC 1316: After Galaxies Collide
Explanation:
How did this strange-looking galaxy form?
Astronomers turn detectives when trying to
figure out the cause of unusual jumbles of stars, gas, and
dust like
NGC 1316.
A preliminary inspection indicates that
NGC 1316 is an enormous
elliptical galaxy
that includes dark dust lanes usually found in a spiral.
The above image taken by the
Hubble Space Telescope
shows details, however, that help in
reconstructing the history of this gigantic jumble.
Close inspection finds fewer low mass
globular clusters
of stars toward NGC 1316's center.
Such an effect is expected in galaxies that have undergone
collisions or
merging with other galaxies in the past few billion years.
After such collisions, many
star clusters would be
destroyed in the dense galactic center.
The dark knots and lanes of dust
indicate that one or more of the devoured galaxies were
spiral galaxies.
NGC 1316
spans about 60,000 light years and lies about 75 million
light years away toward the constellation of the Furnace.
APOD: 2005 March 16 - Markarian's Chain of Galaxies
Explanation:
Across the heart of the
Virgo Cluster of Galaxies
lies a striking string of galaxies known as Markarian's Chain.
The chain,
pictured above, is highlighted on the upper right with
two large but featureless
lenticular galaxies,
M84 and
M86,
and connects to the large
spiral on the lower left,
M88.
Prominent on the lower right but not part of
Markarian's Chain is the giant
elliptical galaxy
M87.
The home Virgo Cluster is the nearest
cluster of galaxies, contains over 2000 galaxies,
and has a noticeable gravitational pull on the galaxies of the
Local Group of Galaxies surrounding our
Milky Way Galaxy.
The center of the
Virgo Cluster is located about 70 million
light years away toward the constellation of Virgo.
At least seven galaxies in
the chain
appear to move coherently,
although others appear to be superposed by chance.
APOD: 2005 February 13 - In the Center of the Virgo Cluster
Explanation:
The Virgo Cluster of Galaxies
is the closest cluster of galaxies to our
Milky Way Galaxy.
The Virgo Cluster is so close that it spans more than 5 degrees on the sky - about 10 times the angle made by a
full Moon.
It contains over 100
galaxies of many types - including
spiral,
elliptical, and
irregular galaxies.
The Virgo Cluster is so massive that it is noticeably
pulling our Galaxy toward it.
The cluster contains not only galaxies filled with stars but also
gas so hot it glows in
X-rays.
Motions of galaxies in and around clusters
indicate that they contain more
dark matter than any visible matter we can see.
Pictured above, the center of the
Virgo cluster
might appear to some as a human face, and includes bright
Messier galaxies
M86 at the top,
M84 on the far right,
NGC 4388 at the bottom, and
NGC 4387 in the middle.
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 June 16 - Elliptical Galaxy M87
Explanation:
Elliptical galaxy
M87 is a type of
galaxy that looks much different than our own
Milky Way Galaxy.
Even for an
elliptical galaxy,
though, M87 is peculiar.
M87 is much bigger than an average galaxy, appears near the
center of a whole
cluster of galaxies known as the
Virgo Cluster,
and shows an unusually high number of globular clusters.
These globular clusters
are visible as faint spots surrounding the bright center of
M87.
In general,
elliptical galaxies contain similar numbers of stars as
spiral galaxies, but are ellipsoidal in shape (spirals are mostly flat), have no spiral structure, and little
gas and
dust.
The
above image of M87 was taken recently by the
Canada-France-Hawaii
Telescope
on top of the dormant volcano
Mauna Kea in
Hawaii,
USA.
APOD: 2004 January 23 - NGC 4631: The Whale Galaxy
Explanation:
NGC
4631 is a big beautiful spiral galaxy seen
edge-on
only 25 million light-years away towards the small northern
constellation
Canes
Venatici.
This galaxy's slightly distorted wedge shape suggests to
some a cosmic herring and to others the popular moniker of
The Whale Galaxy.
Either way, it is similar in size to our own
Milky Way.
In this gorgeous color image,
the Whale's dark interstellar dust clouds, young bright
blue star clusters, and purplish
star forming regions are easy
to spot.
A companion
galaxy, the small elliptical NGC 4627 appears above the
Whale Galaxy.
Out of view off the lower left corner of the
picture lies another distorted galaxy, the hockey stick-shaped
NGC 4656.
The distortions and mingling
trails
of gas and
dust detected at other
wavelengths suggest that all three galaxies have had close encounters
with each other in their past.
The Whale Galaxy is also known to have spouted a halo of hot gas glowing
in
x-rays.
APOD: 2003 August 6 - Dusty Galaxy Centaurus A
Explanation:
Why is peculiar galaxy Centaurus A so dusty?
Dramatic dust lanes that run
across the galaxy's center mark
Cen A.
These dust lanes are so thick they almost completely obscure the
galaxy's center in
visible light.
This is particularly unusual as Cen A's red stars and round shape are characteristic of a giant
elliptical galaxy,
a galaxy type usually low in dark dust.
Cen A,
also known as NGC 5128, is also unusual compared to an average
elliptical galaxy because it contains a
higher proportion of young blue stars and is a
very strong source of
radio emission.
Evidence indicates that
Cen A is likely the result of the
collision of two
normal galaxies.
During the collision, many
young stars were formed, but
details of the creation of
Cen A's unusual dust belts are still being researched.
Cen A lies only 13 million
light years away, making it the closest
active galaxy.
Cen A spans 60,000 light years and can be seen with binoculars
toward the constellation of
Centaurus.
APOD: 2003 August 4 - In the Center of the Virgo Cluster
Explanation:
The Virgo Cluster of Galaxies
is the closest cluster of galaxies to our
Milky Way Galaxy.
The Virgo Cluster is so close that it spans more than 5
degrees on the sky - about 10 times the angle made by a
full Moon.
It contains over 100
galaxies of many types - including
spiral,
elliptical, and
irregular galaxies.
The Virgo Cluster is so massive that it is noticeably
pulling our Galaxy toward it.
The cluster contains not only galaxies filled with stars but also
gas so hot it glows in
X-rays.
Motions of galaxies in and around clusters
indicate that they contain more
dark matter than any visible matter we can see.
Pictured above, the center of the
Virgo cluster
might appear to some as a human face, and includes bright
Messier galaxies
M86 at the top,
M84 on the far right,
NGC 4388 at the bottom, and
NGC 4387 in the middle.
APOD: 2003 July 31 - Galaxy Group HCG 87
Explanation:
Posing for
this cosmic family photo are the galaxies of HCG
(Hickson Compact Group)
87, about four hundred million
light-years distant toward the amphibious constellation
Capricornus.
The large edge-on
spiral near picture center, the fuzzy
elliptical galaxy immediately to its right, and the
spiral near the top of the image are identified members of the
group, while the small spiral
galaxy in the middle is likely a
more distant background galaxy.
In any event, a careful examination of the deep image reveals other
galaxies which certainly lie far beyond
HCG 87.
While not exactly locked in
a
group hug, the HCG 87 galaxies
are interacting
gravitationally, influencing their
fellow group members' structure and evolution.
This new image
is from an instrument undergoing commissioning
on the
Gemini Observatory's
South Telescope at Cerro Pachon, Chile.
It compares favorably with views of this photogenic galaxy group
recorded by the
Hubble
Space Telescope.
APOD: 2003 July 27 -The Aquarius Dwarf
Explanation:
Our Milky Way Galaxy is not alone.
It is part of a gathering of about 50 galaxies known as the
Local Group.
Members include the
Great Andromeda Galaxy (M31),
M32,
M33, the
Large Magellanic Cloud, the
Small Magellanic Cloud,
Dwingeloo 1, several small
irregular galaxies,
and many
dwarf elliptical and
dwarf spheroidal galaxies.
Pictured above is the
Aquarius Dwarf, a faint dwarf irregular galaxy over 3 million
light years away.
An earlier
APOD
erroneously identified the above image as the
Sagittarius Dwarf.
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 May 5 - NGC 1275: A Galactic Collision
Explanation:
In NGC 1275, one galaxy is slicing through another.
The disk of the
dusty
spiral galaxy
near the image center is cutting through a large
elliptical galaxy,
visible predominantly on the lower left.
Galaxies can change significantly during a
collision like this, with
gravitational tides
distorting each galaxy and
gas clouds being
compressed and lighting up with new
star formation.
Galaxy collisions
occur in slow motion to the
human eye, with a single pass taking as much as 100 million years.
NGC 1275 is a member of the
Perseus cluster of galaxies
that lies about 230 million light years away toward the constellation of
Perseus.
Each galaxy spans about 50,000 light years across.
The above picture is a composite of
images taken by the
Hubble Space Telescope in 1995 and 2001.
APOD: 2003 January 16 - NGC 1700: Elliptical Galaxy and Rotating Disk
Explanation:
In spiral galaxies, majestic winding arms
of young stars and interstellar
gas and dust rotate
in a disk around a bulging galactic nucleus.
Elliptical
galaxies
seem to be simpler, randomly
swarming with old stars and lacking gas and dust.
So astronomers were
excited
to find that NGC 1700, a young elliptical
galaxy about 160 million light-years away, shows evidence for a
90,000 light-year wide rotating disk of multi-million
degree hot gas.
The evidence comes from data recorded by the
orbiting
Chandra Observatory, whose sharp
x-ray image of NGC 1700 is seen above.
Balancing gravity, the rotation of the
x-ray hot disk,
the largest of its type yet discovered, gives the galaxy
a pronounced boxy profile in this false-color picture.
Theories about
the origin of the disk suggest that NGC 1700 may be the result
of a cosmic scale galactic merger,
perhaps between a spiral and
elliptical galaxy.
NGC 1700 is just visible with small telescopes toward the
flowing constellation Eridanus.
APOD: 2003 January 14 - 0313-192: The Wrong Galaxy
Explanation:
Centered
above is distant galaxy 0313-192,
some one billion light-years away.
Radio emission from the galaxy has been mapped by the National
Radio Astronomy Observatory's Very
Large Array and
is shown in red, composited with a visible light image from
the Hubble Space Telescope's new Advanced Camera for Surveys.
Dust lanes and other features
in the Hubble image as well as
infrared
Gemini telescope data
demonstrate clearly that 0313-192 is a spiral galaxy
seen edge-on.
(Note the unrelated spiral galaxy seen face-on
above and to the right.)
For years, double cosmic clouds
of radio emission such as those
flanking this spiral galaxy's core have been
studied and cataloged.
But, at least until now, such radio
sources were only known to arise
from the cores of giant elliptical galaxies or in
violent merging galaxy systems, making 0313-192 the wrong kind
of galaxy
to be found in this scenario.
Astronomers are
searching for clues to why this spiral galaxy,
potentially similar to our own Milky Way, shows such powerful
activity.
APOD: 2002 October 26 - Dark Matter, X-rays, and NGC 720
Explanation:
Elliptical galaxy NGC 720 is enveloped in a
cosmic cloud of x-ray emitting gas.
Seen in
this
false color image from the
Chandra
X-ray Observatory,
the extreme temperature of the gas - about 7 million degrees Celsius -
makes it impossible to confine the cloud to the vicinity of NGC 720 based
on the gravity of this galaxy's visible stars alone.
In fact, the x-ray cloud is taken as solid evidence for the
presence
of dark matter surrounding NGC 720 -- unseen material which has
gravitational influence that can keep the x-ray hot gas cloud
from escaping.
Chandra's remarkable vision clearly distinguishes the bright
point-like x-ray
sources from the diffuse cloud.
Astronomers can then use
the detailed shape
of the cloud to infer
the distribution of dark matter in NGC 720 and even test theories
about the fundamental nature of dark matter.
According to modern understanding, the mysterious
dark matter, whatever
it is, is by far the most common
form of matter in the Universe.
Galaxy NGC 720 lies about 80 million light-years distant
toward the constellation Cetus.
APOD: 2002 October 17 - Centaurus A: Young Blue Star Stream
Explanation:
Almost lost in this cosmic jumble of stars, gas and dust is a faint but
definite blue arc -- a
stream of young stars whose formation was
probably triggered as a small dwarf galaxy was
torn apart
approaching the giant elliptical galaxy
Centaurus A.
The 2,000 light-year long arc is revealed in the upper right corner of
this
processed color
digital image, while the dense central region
of Centaurus A is near the bottom.
Star clusters that make up the blue arc are likely strung out along the
incoming trajectory of the small galaxy and are estimated to be only
200-400 million years old.
The remarkable result
suggests that astronomers have identified
a spectacular example of a kind of galactic cannibalism in progress,
a process which is believed to contribute to the formation and
evolution
of large galaxies, including our own
Milky Way.
Over time, stars and star clusters in this stream should
eventually disperse and merge
with tumultuous Centaurus A.
The image data was recorded with the four meter
Blanco
telescope at Cerro Tololo Inter-American Observatory.
APOD: 2002 July 4 - Young Star Clusters in an Old Galaxy
Explanation:
Elliptical galaxy NGC 4365 is old, probably about 12 billion years old.
Like most
elliptical galaxies, this galaxy was
thought to be full of old stars too, its burst of
star forming activity
having long since ended.
But combining data from the
Hubble Space Telescope
and the European
Southern Observatory's ground-based
Antu Telescope,
a team of European and
US
astronomers discovered
NGC 4365's surprising secret -- some of its star
clusters are young.
In this composite image,
the galaxy's bright nucleus is at the upper left.
NGC 4365's
star clusters
themselves appear as bright dots against
a diffuse glow of unresolved starlight and fuzzy, distant
background galaxies.
The notched border outlines
Hubble's WFPC2 camera field.
Moving the cursor over the image identifies individual star
clusters, with the relatively young (few billion year-old) clusters
circled in blue, and the anticipated 12 billion year-old clusters
circled in red.
NGC 4365
is 60 million light-years away in the
Virgo galaxy cluster.
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 April 8 - NGC 2787: A Barred Lenticular Galaxy
Explanation:
Lenticular galaxies aren't supposed to be photogenic.
Like
spiral galaxies, they contain a
disk, but like
elliptical galaxies, they are usually short on
dust, gas, and
pretty spiral arms.
Lenticulars are relatively little
studied, possibly because of their seemingly benign nature.
Famous galaxies historically classified as lenticular include
M84,
M85, and
M86.
Recent pictures and evidence, however, indicate that
lenticulars can be both photogenic and scientifically interesting.
For example, the
above image of
NGC 2787 taken with the
Hubble Space Telescope
shows that the center of
this lenticular galaxy has interesting structure.
The image was taken to help determine how
lenticular galaxies formed,
and what happens in their centers.
The span of
NGC 2787
in the above image is about 4500
light years, while the galaxy lies about 25 million
light years away toward the constellation of
Ursa Major.
APOD: 2002 March 29 - NGC 4631: The Whale Galaxy
Explanation:
NGC
4631 is a big beautiful spiral galaxy seen
edge-on
only 25 million light-years away towards the small northern
constellation
Canes
Venatici.
This galaxy's slightly distorted wedge shape suggests to
some a cosmic herring and to others the popular moniker of
The Whale Galaxy.
Either way, it is similar in size to our own
Milky Way.
In this gorgeous color image,
the Whale's dark interstellar dust clouds, young bright
blue star clusters, and purplish
star forming regions are easy
to spot.
A companion
galaxy, the small elliptical NGC 4627 appears above the
Whale Galaxy.
Out of view off the lower left corner of the
picture lies another distorted galaxy, the hockey stick-shaped
NGC 4656.
The distortions and mingling
trails
of gas and
dust detected at other
wavelengths suggest that all three galaxies have had close encounters
with each other in their past.
The Whale Galaxy is also known to have spouted a halo of hot gas glowing
in
x-rays.
APOD: 2002 March 6 - Simulated Galaxy Cluster View
Explanation:
Stunningly detailed, this picture is a
computer simulated view
of a cluster of galaxies
in the distant cosmos.
A large, elliptical galaxy dominates this hypothetical cluster's
central region surrounded by a swarm of member galaxies.
Other galaxies which lie far behind the cluster are seen as numerous
visible concentric arcs -
lensed by the enormous
gravitational field
dominated by dark matter
within the cluster itself.
Such magnificent images are expected to be achieved by the
Advanced
Camera for Surveys (ACS), one of
the upgrades
being installed on the
Hubble Space Telescope during the
ongoing servicing mission.
Compared to Hubble's workhorse
Wide Field Planetary Camera 2
(WFPC2), whose achievements include the current
deep
field views of the Universe, the new technology ACS
will be twice as sharp an imager with twice the
field of view and five times the sensitivity.
Along with extended views of the distant cosmos,
enthusiastic astronomers also plan to use the ACS to monitor
our own Solar System and to search for
planets orbiting stars beyond the Sun.
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 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: 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 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 June 8 - Three Galaxies in Draco
Explanation:
This intriguing trio of galaxies is sometimes
called the NGC 5985/Draco Group
and so (quite reasonably) is located in the northern
constellation
Draco.
From left to right are
face-on spiral NGC 5985,
elliptical galaxy NGC 5982, and
edge-on spiral NGC 5981 --
all within this single
telescopic
field of view spanning a little more than
half the width of the full moon.
While this grouping is far too small to be a
galaxy cluster
and has not been
cataloged
as a compact group,
these galaxies do lie roughly 100 million light-years from planet Earth.
On close examination with spectrographs, the bright core of the
striking face-on spiral NGC 5985 shows
prominent emission in specific wavelengths of light, prompting astronomers
to classify it as a
Seyfert, a type of active galaxy.
Not as well known as other tight
groupings of galaxies,
the contrast in
visual
appearance makes this triplet an attractive subject for
avid astrophotographers.
APOD: 2001 April 4 - Distant Supernova, Dark Energy
Explanation:
A pinpoint of light from a star that exploded over
10 billion light-years away
is centered in the panel at the lower right,
a cosmic
snapshot of the most
distant supernova.
The ancient stellar detonation
was detected
by digitally subtracting before and
after images of a faint, yellowish, elliptical galaxy
included in the Hubble Space Telescope Deep
Field image illustrated at the top and left.
Remarkable in itself as the
farthest known supernova, its measured
brightness provides astounding
evidence for
a strange universe -
one which eventually
defies gravity and expands at an
accelerating rate.
The unseen force driving
this
expansion is attributed to
"dark energy"
and discovering the fundamental
nature
of dark energy has been called
the challenge of this millennium.
APOD: 2001 March 9 - X-rays From HCG 62
Explanation:
Scanning the skies for galaxies Canadian astronomer
Paul Hickson and colleagues identified some 100 compact
groups
of galaxies, now appropriately called
Hickson Compact Groups (HCGs).
With only a few member galaxies per group, HCGs are much smaller
than the immense
clusters of galaxies
which lurk in the cosmos,
but like the large galaxy clusters, some HCGs seem to be filled with
hot,
x-ray emitting gas.
In fact, groups of galaxies like HCGs may
be the building blocks of the large clusters.
This false-color x-ray image from the orbiting
Chandra
Observatory reveals x-ray emission from the gas in
one
such group, HCG 62, in startling detail.
In the image, black and green colors
represent low intensities while red and purple hues indicate
high x-ray intensities.
Striking features of the
x-ray image are the low brightness blobs
at the upper left and lower right
which symmetrically flank the intense central x-ray region.
HCG 62 lies in Virgo,
and near the group's center resides elliptical
galaxy
NGC 4761.
At optical wavelengths,
some HCGs
make for
rewarding viewing,
even with modest sized telescopes.
APOD: 2001 January 26 - Galaxies Of The Virgo Cluster
Explanation:
Well over a thousand galaxies are known members of
the Virgo Cluster,
the closest large cluster of galaxies to our own
local group.
The galaxy cluster is difficult
to see
all at once because
it covers such a large area
on the sky.
Still,
this excellent
telescopic view records the region of the
Virgo Cluster around its dominant giant elliptical
galaxy M87.
M87 can be seen as a fuzzy patch near the picture's bottom center.
In fact, a close examination of the image will
reveal that many of the "stars" are
actually surrounded by a telltale fuzz, indicating
that they are
Virgo Cluster galaxies.
How many galaxies can you pick out?
Click on the image for an uncropped, labeled version which includes the
NGC catalog
numbers for most of the visible galaxies.
On average,
Virgo Cluster galaxies are measured to be
about 48 million light-years away.
The Virgo
Cluster distance has been used to give an important
determination of the Hubble Constant and
the scale of the Universe.
APOD: 2000 October 23 - Dwarf Elliptical Galaxy NGC 205 in the Local Group
Explanation:
Our Milky Way Galaxy is not alone.
It is part of a gathering of about 25 galaxies known as the
Local Group.
Members include the
Great Andromeda Galaxy (M31),
M32,
M33, the
Large Magellanic Cloud, the
Small Magellanic Cloud,
Dwingeloo 1, several small
irregular galaxies,
and many
dwarf elliptical and
dwarf spheroidal galaxies.
Pictured on the lower left is one of the many
dwarf ellipticals:
NGC 205.
Like
M32,
NGC 205 is a companion to the large M31,
and can sometimes be seen to the south of
M31's center in photographs.
The above image shows
NGC 205 to be unusual for an
elliptical galaxy
in that it contains at least two
dust clouds
(at 1 and 4 o'clock - they are visible but hard to spot)
and signs of recent star formation.
This galaxy is sometimes known as M110,
although it was actually not part of
Messier's original
catalog.
APOD: 2000 August 16 - Unusual Giant Galaxy NGC 1316
Explanation:
Can unusual giant galaxy NGC 1316 help calibrate the universe?
Quite possibly -- if it turns out this atypical galaxy
is composed of typical stars.
NGC 1316,
pictured above, is most obviously strange because
it has a size and shape common for an
elliptical galaxy but
dust lanes and a disk
more commonly found in a
spiral galaxy.
These attributes could be caused by
interactions with
another galaxy over the past billion years.
Most recently,
NGC 1316 has been monitored to find
novae, explosions emanating from
white dwarf stars that should
have a standard brightness.
Again,
NGC 1316 was found atypical in that the nova rate
was unexpectedly high.
If, however, the stars and
white dwarfs that compose
NGC 1316 are typical, then the
novae observed should be just as bright as
novae in other galaxies so that astronomers can
use them to compute an accurate distance.
This distance can then be used to calibrate other
distance indicators and result in a more accurate
scale for distances throughout the
universe.
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 2 - The Secret Spiral Of IC3328
Explanation:
IC3328 is an otherwise unremarkable dwarf
elliptical galaxy
about 50 million light-years away in the
Virgo cluster.
But hidden within IC3328 is a subtle,
beautifully symmetric spiral structure!
A team
of astronomers recently made
this totally surprising discovery using
detailed digital images from the European Southern
Observatory's 8.2 meter Antu telescope.
They numerically modeled the smooth distribution of light for
this galaxy (left) to enable more accurate measurements
of its distance.
When the smooth distribution was
subtracted from the digital image, the startling spiral
structure became apparent (right).
Typical of large, rotating,
disk galaxies with density waves,
spiral structure is unprecedented
in the blob-shaped aggregates of stars normally
classified
as elliptical galaxies.
What created the "secret"
spiral
in IC3328?
Some possibilities under consideration include tidal interactions with
nearby galaxies and amplified internal stellar motions.
APOD: 2000 March 11 - Messier Marathon
Explanation:
Gripped by an
astronomical spring fever,
it's once again time for
many amateur stargazers to embark on
a Messier Marathon!
The Vernal Equinox
occurs March 20, marking the
first day of Spring for the Northern Hemisphere.
It also marks a favorable
celestial situation for
potentially viewing all the objects in 18th century French astronomer
Charles Messier's catalog
in one
glorious dusk to dawn observing run.
This year a bright full moon will interfere with dark skies near
the actual equinox, so good nights near new moon for weekend
marathoners are March 11/12 and April 1/2.
(As an added bonus all the
planets in the solar system can be viewed
on these dates.)
Astronomer Paul Gitto has created this
masterful Messier Marathon grid with 11 rows
and 10 columns of
Messier catalog objects.
In numerical order, the grid begins with
M1, the Crab Nebula,
at upper left
and ends
with M110, a small elliptical galaxy in Andromeda
(lower right).
Gitto's images were made with a digital camera and a 10-inch diameter
reflecting telescope.
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 21, 1999 - Elliptical Galaxy NGC 4881 in Coma
Explanation:
Elliptical galaxies are unlike spiral galaxies
and hence unlike our own Milky Way Galaxy.
The giant elliptical galaxy
named NGC 4881 on the upper left lies at the edge of the giant
Coma Cluster of Galaxies.
Elliptical galaxies are ellipsoidal in shape, contain no
spiral arms,
contain little interstellar gas or
dust,
and are found mostly in rich clusters of galaxies.
Elliptical galaxies appear
typically yellow-red, as opposed to
spirals which have spiral arms that appear quite blue.
Much speculation continues on
how each type of galaxy can form,
on whether ellipticals can evolve from colliding
spirals,
or spirals can be created from colliding ellipticals, or both.
Besides the spiral galaxy on the right, all other images in
this picture are of galaxies that lie well behind the Coma Cluster.
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: November 3, 1999 - M32: Blue Stars in an Elliptical Galaxy
Explanation:
Elliptical galaxies
are known for their old, red stars. But is this old
elliptical up to new tricks?
In recent years, the centers of
elliptical galaxies
have been found to emit unexpectedly high amounts of blue and
ultraviolet light.
Most blue light from
spiral galaxies originates from
massive young hot stars,
in contrast to the red light from the old cool stars
thought to compose ellipticals.
In the
above recently released, false-color photograph by the
Hubble Space Telescope,
the center of nearby dwarf elliptical M32 has actually been
resolved and does indeed show thousands of bright blue stars.
The
answer is probably that
these blue stars are also old and glow blue, reaching relatively
high temperatures by the
advanced process of
fusing helium, rather than
hydrogen, in their cores.
M32 appears in many pictures
as the companion galaxy to the massive
Andromeda Galaxy (M31).
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: September 6, 1999 - HCG 87: A Small Group of Galaxies
Explanation:
Sometimes galaxies form groups. For example, our own
Milky Way Galaxy is part of the
Local Group of Galaxies.
Small, compact groups, like Hickson Compact Group 87 (HCG 87)
shown above, are interesting partly
because they slowly self-destruct.
Indeed, the galaxies of
HCG 87 are gravitationally
stretching each other during their 100-million year
orbits around a common center.
The pulling creates colliding gas that causes bright bursts of
star formation and feeds matter into their
active galaxy centers.
HCG 87 is composed of a large
edge-on
spiral galaxy visible on the lower left, an
elliptical galaxy
visible on the lower right, and a
spiral galaxy visible near the top.
The small spiral near the center might be far in the distance.
Several stars from
our Galaxy are also visible in the foreground.
The above picture was taken in July by the
Hubble Space Telescope's
Wide Field Planetary Camera 2.
Studying groups like HCG 87 allows insight into how all
galaxies form and evolve.
APOD: August 21, 1999 - Galaxies Away
Explanation:
This striking pair of galaxies is far, far away ...
about 350 million light-years
from Earth.
Cataloged
as AM0500-620, the pair is located in the southern
constellation Dorado.
The background elliptical and foreground
spiral galaxy are representative of two of the
three major classes of galaxies which
inhabit our Universe.
Within the disks
of spiral galaxies, like our own
Milky Way,
gas, dust, and young blue star clusters trace out
grand spiral "arms".
The dust lanes
along the arms of this particular
spiral stand out dramatically in this Hubble Space Telescope
image as they obligingly sweep
in front of the background elliptical.
Like the central bulges of spiral galaxies,
elliptical galaxies
tend toward spherical shapes resulting from
more random motions of their stars.
But while spirals produce new stars, star formation in
ellipticals which lack gas and dust seems to have stopped.
How do galaxies evolve with
cosmic time?
Evidence is growing that
graceful galaxy shapes can hide
a violent history.
APOD: March 31, 1999 - PG 1115+080: A Gravitational Cloverleaf
Explanation:
All four blue images in the above photograph are the same object. The gravitational lens effect of the red, foreground,
elliptical galaxy visible near image center creates a
cloverleaf image of the single distant
quasar.
Light from the
quasar is pulled around the massive galaxy in different paths, corresponding to different images.
Light takes many billions of years to reach us from this quasar.
Since light takes a different amount of time to traverse each path,
each image shows the quasar as it appeared at a
slightly different time in the past, creating
time delays on the time scale of days.
Since these time delays are influenced by the
expansion rate of the universe,
analysis of this image helps reveal
Hubble's constant, the parameter that calibrates
universe expansion.
This recent picture by the new
Subaru Telescope
is perhaps the clearest image yet of this
famous optical mirage.
APOD: March 18, 1999 - Messier Marathon
Explanation:
Gripped by an
astronomical spring fever, this week
many amateur stargazers embark on
a Messier Marathon.
The Vernal Equinox
occurs Saturday, March 20, marking the
first day of Spring for the Northern Hemisphere.
It also marks a favorable celestial situation for
potentially viewing all the objects in 18th century French astronomer
Charles Messier's catalog
in one
glorious dusk to dawn observing run.
This year, interference from bright moonlight will be minimal as
the the moon is near its dark or new phase.
Astronomer Paul Gitto has created this
masterful Messier Marathon grid with 11 rows
and 10 columns of
Messier catalog objects.
In numerical order, the grid begins with
M1, the Crab Nebula,
at upper left and ends
with M110, a small elliptical galaxy in Andromeda
(lower right).
Gitto's images were made with a digital camera and a 10-inch diameter
reflecting telescope.
APOD: February 22, 1999 - NGC 1316: After Galaxies Collide
Explanation:
Astronomers turn detectives when trying to
figure out the cause of unusual sites like
NGC 1316.
A preliminary inspection indicates that
NGC 1316 is an enormous
elliptical galaxy
that started devouring a smaller
spiral galaxy
neighbor about 100 million years ago.
Supporting evidence includes the dark
dust lanes
uniquely indicative of a
spiral.
What remains unexplained are the unusually small
globular star clusters,
visible as faint dots on the
above photograph. Most
elliptical galaxies have
more and brighter globular clusters
than evident in
NGC 1316.
Yet the observed
globulars are too old to have been
created by the recent
spiral collision. One
hypothesis therefore holds that these
globulars
survive from an even earlier galaxy
that was subsumed into NGC 1316.
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: 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 2, 1998 - PG 1115: A Ghost of Lensing Past
Explanation:
In this tangle of quasars and galaxies lies a
clue to the expansion rate of the universe.
A diffuse glow evident in the picture
on the left reveals a normal
elliptical galaxy.
Directly behind this galaxy lies a normal
quasar. Because the
quasar is directly
behind the galaxy, however, the gravity of the
galaxy deflects quasar light like a lens, creating
four bright images
of the same distant quasar.
When these images are all digitally subtracted,
a distorted image of the background galaxy that
hosts the quasar appears - here shown on the
right in ghostly white. Each
quasar image traces how the
quasar looked at different times in the past,
with the time between images influenced by the
expansion rate of the universe itself.
Assuming dark matter in the elliptical
lens
galaxy traces the visible matter, this
expansion rate
can be characterized by a Hubble constant of Ho near 65 km/sec/Mpc,
a value close to that determined by
other methods.
Analysis of
this image by itself sheds little
light on whether the
global geometry of the universe
is affected by a
cosmological constant.
APOD: September 29, 1998 - A Peculiar Cluster of Galaxies
Explanation:
Far across the universe, an unusual
cluster of galaxies has been evolving.
A diverse group of galaxies populate this cluster,
including, on the left, an unusual galaxy showing an
equatorial polar ring and a
large spiral.
Above looms a large
elliptical galaxy.
The reason for the small size of galaxies on the
right is not yet known - these galaxies might be smaller
or might just lie even farther in the distance.
Almost every spot in
this picture is a galaxy. Studying
distant clusters
like this may help astronomers better understand
when and how these cosmic giants formed.
APOD: August 27, 1998 - Hercules Galaxies
Explanation:
These are
galaxies of the Hercules Cluster, an archipelago of
"island universes" a mere 650 million light-years distant.
This cluster is loaded with gas and dust rich, star forming,
spiral galaxies
but has relatively few
elliptical galaxies, which lack gas and dust and
the associated newborn stars.
Colors in the composite image show the star forming galaxies
with a blue tint and ellipticals with a slightly yellowish cast.
In this cosmic vista many galaxies seem to be
colliding or
merging
while others seem distorted - clear evidence that
cluster galaxies commonly interact.
Over time, the galaxy interactions are likely to affect the
the content of the cluster itself.
Researchers believe that the
Hercules
Cluster is significantly similar
to young galaxy clusters
in the distant, early Universe
and that exploring galaxy types and their interactions in nearby
Hercules will help unravel the threads
of galaxy and
cluster evolution.
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: April 9, 1998 - Quasar in an Elliptical Galaxy
Explanation:
Where do quasars live?
Quasars
are the brightest objects in the universe,
so bright they can be seen from across the universe.
Observations continue to show that most quasars are
surrounded by a relatively faint nebulous patch.
Astronomers are trying to identify the
nature of these patches. The
above false-color picture shows a central quasar embedded in an unusual
elliptical galaxy.
The galaxy is being
gravitationally distorted
by a neighboring galaxy.
Recent evidence indicates that most
quasars live near the centers of large,
elliptical galaxies - even those
quasars where no host galaxy could be found before.
Quasars themselves are thought to result from matter falling toward
supermassive black-holes.
APOD: March 26, 1998 - Galaxies Away
Explanation:
This striking pair of galaxies is far, far away ...
about 350 million light-years
from Earth.
Cataloged
as AM0500-620, the pair is located in the southern
constellation Dorado.
The background elliptical and foreground
spiral galaxy are representative of two of the
three major classes of galaxies which
inhabit our Universe.
Within the disks
of spiral galaxies, like our own
Milky Way,
gas, dust, and young blue star clusters trace out
grand spiral "arms".
The dust lanes
along the arms of this particular
spiral stand out dramatically in this Hubble Space Telescope
image as they obligingly sweep
in front of the background elliptical.
Like the central bulges of spiral galaxies,
elliptical galaxies
tend toward spherical shapes resulting from
more random motions of their stars.
But while spirals produce new stars, star formation in
ellipticals which lack gas and dust seems to have stopped.
How do galaxies evolve with
cosmic time?
Evidence is growing that
graceful galaxy shapes can hide
a violent history.
APOD: January 16, 1998 - Dusting Spiral Galaxies
Explanation:
How much dust is in spiral galaxies?
Does it block out much of the starlight?
Because astronomers rely on an accurate knowledge
of galaxy properties
to investigate a wide range of problems, like galaxy and
quasar evolution and the
nature of dark matter, answers to simple questions like this
are key.
This striking,
detailed Hubble Space Telescope image of dust in the outer reaches
of a foreground spiral galaxy
(left) back lit by an elliptical galaxy offers
an elegant approach to providing the answers.
As expected, dust lanes in the foreground galaxy seem to be associated
with spiral arms.
But surprisingly, many dust regions are not
completely opaque and the dust is more smoothly distributed
than anticipated.
This "overlapping" pair of galaxies is cataloged as AM1316-241 and is about
400 million light-years away in
the constellation Hydra.
APOD: January 10, 1998 - Disorder in Stephan's Quintet
Explanation:
What are five closely grouped galaxies doing in this image?
The grouping is commonly known as Stephan's
Quintet. Four of the galaxies show essentially the same redshift
suggesting that they are at the same distance from us. The large
bluish spiral below and left of
center actually has a smaller redshift
than the others, indicating it is much closer. It is probably
a foreground object which happens to lie along the line of sight
to the more distant galaxies. Of the four distant galaxies,
three seem to be colliding, showing
serious distortions due to gravitational tidal forces. The fourth
is a normal appearing elliptical galaxy
(at the lower right edge of the field). Recent results suggest
that collisions play an important role in the life cycles of galaxies.
APOD: December 7, 1997 - A Distant Cluster of Galaxies
Explanation:
In
this 1994
Hubble Space Telescope photograph,
every bright
object is a galaxy.
Oddly - most of them are spiral galaxies.
This rich
cluster of galaxies, named
CL 0939+4713, is almost half way
across the visible universe.
Photos like this indicate that
clusters
in the past contained a higher fraction of spirals than do
nearby clusters which are usually dominated by
elliptical galaxies.
APOD: December 5, 1997 - Seeing Through Galaxies
Explanation:
In this dramatic picture,
spiral galaxy
NGC 5091 appears in the foreground.
Tilted nearly
edge-on,
the dust lanes between its spiral arms are clearly visible.
The large
elliptical galaxy NGC 5090 lies just beyond it -
both are about 100 million light years distant in the southern
constellation Centaurus.
Can you see through the spiral galaxy?
The detailed
answer to this question has important implications
for determining
the nature of
dark matter and the measurement of
star formation rates.
Comparing the overlapping and non-overlapping parts of
this and other
pairs of galaxies offers a neat way to find the answer.
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: 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: December 13, 1996 - Disorder in Stephan's Quintet
Explanation: Five closely grouped galaxies are visible
in this image
made using the Kitt Peak National Observatory
2.1 meter telescope. The grouping is commonly known as Stephan's
Quintet. Four of the galaxies show essentially the same redshift
suggesting that they are at the same distance from us. The large
bluish spiral below and left of
center actually has a smaller redshift
than the others, indicating it is much closer. It is probably
a foreground object which happens to lie along the line of sight
to the more distant galaxies. Of the four distant galaxies,
three seem to be colliding, showing
serious distortions due to gravitational tidal forces. The fourth
is a normal appearing elliptical galaxy
(at the lower right edge of the field). Recent results suggest
that collisions play an important role in the life cycles of galaxies.
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 6, 1996 - Elliptical Galaxy NGC 4881 in Coma
Explanation: Elliptical galaxies are unlike spiral galaxies
and hence unlike our own Milky Way Galaxy.
The giant elliptical galaxy
named NGC
4881 on the upper left lies at the edge of the giant Coma Cluster of Galaxies.
Elliptical galaxies are ellipsoidal
in shape, contain no spiral arms,
contain little interstellar gas
or dust, and are found mostly
in rich clusters of galaxies.
Elliptical galaxies appear
typically yellow-red, as opposed to spirals
which have spiral arms that appear quite blue.
Much speculation continues on how each type of galaxy can form,
on whether ellipticals can
evolve from colliding spirals,
or spirals can be created from colliding ellipticals, or both.
Besides the spiral galaxy on the right, all other images in this picture
are of galaxies
that lie well behind the Coma Cluster.
APOD: August 18, 1996 - A Milestone Quasar
Explanation:
Here is a rather typical quasar. But since
quasars
are so unusual it is quite atypical of most familiar objects. Of the two
bright objects in the center of
this photo,
the quasar is on the left. The bright image to
quasar's right is a star, the faint object just above the quasar is an
elliptical galaxy, with an apparently
interacting pair of spiral galaxies near the top.
Quasars appear as unresolved points of light, as do stars, and hence
quasars were thought to be a type of
star until the 1960s.
We now know that the brightest quasars lie far across the
visible
universe from us, and include the most distant objects known. Quasars
may occupy the centers of galaxies
and may even be much brighter than their host galaxies. In fact, the
centers of many nearby galaxies have similarities
to quasars - including the
center of our own Milky Way Galaxy. The exact
mechanism responsible for a
quasar's
extreme brightness is unknown, but thought to involve
supermassive black holes. This picture represents a
milestone for
the six-year-old
Hubble Space Telescope
as it was picture number 100,000, taken on June 22, 1996.
APOD: June 28, 1996 - A Distant Galaxy in the Deep Field
Explanation:
Researchers believe that the faint reddish smudge indicated
by the arrow in the image above is
a candidate for the most distant known galaxy
which may have existed only a few hundred million years after
the Big Bang. The image is part of
the Hubble Deep Field, the Hubble
Space Telescope's deepest yet picture of the Universe. Made in December
1995 by staring for ten consecutive days with the Hubble, astronomers
have been intently
studying the resulting deep field image
filled with remote galaxies for
clues to what galaxies and the Universe looked like in the distant past.
While nearby galaxies are easily detected in the image -
some seen here have visible elliptical and even spiral structures -
the most distant (and therefore oldest) galaxies
must be identified by examining their appearance in
different wavelengths of light.
Based on this technique,
six of the most distant galaxies in the Deep Field appear to be farther
away than even quasars.
APOD: April 19, 1996 - The Virgo Cluster: Hot Plasma and Dark Matter
Explanation:
This ROSAT image of the
Virgo cluster of galaxies reveals a
hot X-ray emitting plasma or gas with a
temperature of 10-100 million degrees pervading
the cluster. False colors have been used to represent
the intensity of X-ray emission.
The large area of X-ray emission, just below and left of center,
is about 1 million light-years across.
The giant elliptical galaxy M87,
the biggest member
of the cluster, is centered in that area while
other cluster members
are scattered around it.
By adding up the amount of
X-ray emitting gas astronomers
have found that its total mass is
up to 5 times the total mass of the cluster galaxies themselves -
yet all this
matter still does not produce nearly enough gravity to keep
the cluster from flying apart! Where is the unseen mass?
Because galaxy clusters are the
largest structures in the Universe, this
mysterious Dark Matter must dominate the cosmos
but its nature is still an
open question.
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: January 14, 1996 - A Distant Cluster of Galaxies
Explanation:
Every bright object in
this 1994
photograph by the
Hubble Space Telescope is a
galaxy. Oddly - most of the objects are
spiral galaxies. This rich
cluster of galaxies, named
CL
0939+4713, is almost half way
across the visible universe. Photos like this indicate that
clusters in the past contained a higher fraction of
spirals than do
nearby clusters, which are usually dominated by
elliptical galaxies.
APOD: January 8, 1996 - Local Group Galaxy NGC 205
Explanation:
The
Milky Way Galaxy is not alone. It is part of a
gathering of about 25 galaxies known as the
Local Group.
Members include the
Great Andromeda Galaxy (M31),
M32,
M33, the
Large Magellanic Clouds, the
Small Magellanic Clouds,
Dwingeloo 1, several small
irregular galaxies, and many
dwarf elliptical
galaxies. Pictured is one of the many dwarf ellipticals: NGC 205. Like
M32, NGC 205 is a companion to the large M31, and can sometimes be seen to
the south of
M31's center in
photographs. The
above image
shows this galaxy to be unusual for an
elliptical galaxy
in that it contains at least two
dust clouds (at 7 and 11
o'clock - they are visible but hard to spot) and signs of recent star
formation. This galaxy is sometimes known as M110, although it was
actually not part of
Messier's original catalog.
APOD: January 6, 1996 - Dwarf Elliptical Galaxy M32
Explanation:
Being the largest galaxy around can sometimes make you popular. Pictured
is M31's companion galaxy
M32.
M31, the Andromeda galaxy, is the largest
galaxy in our
Local Group of galaxies
- even our tremendous
Milky Way Galaxy is smaller.
Little M32 is visible in most pictures of M31 - it is
the small circular spot north of M31's center. M32 is a
dwarf elliptical
galaxy.
Elliptical galaxies have little or no measurable
gas or
dust -
they are composed completely of
stars and typically appear more red than
spiral galaxies.
Elliptical galaxies do not have disks -
they generally have oblong shapes and therefore show elliptical profiles on
the sky.
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 15, 1995 - A Quintet of Galaxies
Explanation:
Five closely grouped galaxies are visible in this image
made using the
Kitt Peak National Observatory 2.1 meter telescope.
The grouping is commonly known as
Stephan's Quintet.
Four of the galaxies show essentially the same
redshift suggesting
that they are at the same distance from us.
The large bluish spiral below and left of center actually has
a smaller redshift than the others, indicating it is much closer.
It is probably a foreground object which happens to lie
along the line of sight to the more
distant galaxies.
Of the four distant galaxies, three seem to be colliding, showing serious
distortions due to gravitational tidal forces.
The fourth is a normal appearing elliptical galaxy (at the lower
right edge of the field).
Recent results suggest that collisions play an important
role in the life cycles of galaxies.
APOD: November 13, 1995 - Virgo Cluster Galaxies
Explanation:
Pictured are several galaxies of the Virgo Cluster, the closest
cluster of galaxies to the
Milky Way. The Virgo Cluster spans more than
5 degrees on the sky - about 10 times the angle made by a full
Moon. It contains over 100 galaxies of many
types - including
spirals,
ellipticals, and
irregular galaxies. The Virgo
Cluster is so massive that it is noticeably pulling our Galaxy toward it.
The above picture includes two galaxies that are also
Messier objects: M84
and M86. M84 is the bright elliptical galaxy just above the center of the
photograph, and M86 is the bright elliptical galaxy to its right.
APOD: September 13, 1995 - Elliptical Galaxy M87
Explanation:
Elliptical galaxy M87 is a type of
galaxy that looks much different
than our own
Milky Way Galaxy. But even for an
elliptical galaxy M87 is peculiar.
M87 is much bigger than an average
galaxy, appears at the center of a whole cluster of galaxies known as the
Virgo Cluster, and shows a very high number of
globular clusters.
These globular clusters are visible as faint
spots surrounding the bright center of M87. In general, elliptical galaxies
contain similar numbers of stars as
spiral galaxies, but are ellipsoidal in
shape (spirals are mostly flat), have no spiral structure, and little
gas and
dust.
This picture is number sixty on a publicly posted list of
images from the
Anglo-Australian
Telescope (AAT).
APOD: July 28, 1995 - M82: An Irregular Galaxy
Explanation:
Not all galaxies have spiral structure like our Milky Way. Many have
smooth elliptical shapes, but also many have irregular shapes such
as the bright sky object M82, the 82nd object on
Messier's
list. The strange structure of this galaxy is thought to be caused by
young stars ejecting gas in energetic bubbles, and by lanes of absorbing
dust.