Astronomy Picture of the Day |
APOD: 2008 December 23 - Collinder 399: The Coat Hanger
Explanation:
Is this coat hanger a star cluster or an asterism?
This
cosmic hang-up has been debated over much of last century,
as astronomers wondered whether this binocular-visible object is
really a physically associated
open cluster or a chance projection.
Chance star projections are known as
asterisms, an example of which is the popular
Big Dipper.
Recent precise measurements from different vantage points in the Earth's orbit around the Sun
have uncovered discrepant angular shifts indicating that the
Coat Hanger
is better described as an
asterism.
Known more formally as
Collinder 399,
this bright stellar grouping is wider than the
full moon and lies in the
constellation of the
Fox
(Vulpecula).
On the far right of the image is the
open cluster of stars
NGC 6802.
APOD: 2008 December 18 - La Superba
Explanation:
Y Canum Venaticorum (Y CVn) is a very rare star
in planet Earth's night sky.
It's also very red, exhibiting such a remarkable spectrum
of light, 19th century astronomer
Angelo
Secchi dubbed it
La Superba.
Located 710 light-years away in northern constellation
Canes Venatici
(the Hunting Dogs), the star varies in brightness over
a period of about half a year.
Near maximum, it becomes just bright enough to see with the
unaided eye, but the star's beautiful red hue is
easy
to see in binoculars or a small telescope.
In fact, La Superba is among the brightest of the
carbon stars -
cool, highly evolved
red
giant stars with exceptional abundances of carbon.
The carbon is created
by helium fusion near the stellar core
and dredged up into the stars' outer layers.
The resulting overabundance of simple carbon molecules
(like CO, CN, C2) in the atmospheres of carbon stars strongly absorbs
bands of bluer light and gives these stars a deep red color.
La Superba is losing its carbon-rich atmosphere
into the surrounding
interstellar material through a strong stellar wind,
and could be near the beginning of a transition
to a planetary nebula phase.
APOD: 2007 November 15 - M13: The Great Globular Cluster in Hercules
Explanation:
M13 is
modestly recognized as
the Great Globular Cluster
in Hercules.
A system of stars numbering in the hundreds of thousands,
it is one of the brightest
globular
star clusters in the northern sky.
At a distance of 25,000 light-years, the cluster stars
crowd
into a region 150 light-years in diameter, but
approaching the cluster core
over 100 stars would be contained
in a cube just 3 light-years on a side.
For comparison, the
closest star to the Sun is over
4 light-years away.
This stunning view of the cluster combines recent telescopic
images of the cluster's dense core with
digitized photographic plates recorded between 1987 and 1991
using the Samuel Oschin Telescope, a wide-field
survey instrument at Palomar Observatory.
The resulting composite highlights both inner and outer reaches
of the giant star cluster.
Among the distant background galaxies also visible,
NGC
6207
is above and to the left of the Great Globular Cluster
M13.
APOD: 2007 August 9 - Star Cluster Messier 67
Explanation:
Gathered
at the center of this sharp skyview are the stars of
Messier 67,
one of the oldest known
open star clusters.
In fact, though open star clusters are usually much younger, the
stars of M67
are likely around 4 billion years old, about
the same age and with about the same elemental abundances as
the Sun.
Open clusters are almost always
younger because they are dispersed
over time as they encounter other stars,
interstellar clouds, and experience gravitational tides while
orbiting
the center of our galaxy.
Still, M67 contains over 500 stars or so and lies some
2,800 light-years away in the
constellation
Cancer.
At that estimated distance, M67 would be about 12 light-years across.
APOD: 2007 May 11 - LDN 1622: Dark Nebula in Orion
Explanation:
The silhouette of an intriguing
dark nebula
inhabits this cosmic scene, based on images from the
Palomar
Observatory Sky Survey.
Lynds' Dark Nebula
(LDN) 1622 appears against a faint background
of glowing hydrogen gas only easily seen in long telescopic
exposures of the region.
LDN 1622 lies near the plane of our Milky Way Galaxy,
close on the sky to
Barnard's
Loop - a large cloud surrounding the rich
complex of emission nebulae found
in the Belt and Sword
of Orion.
But the obscuring dust of LDN 1622 is thought to be much closer
than Orion's more famous nebulae, perhaps only 500 light-years away.
At that distance, this 1 degree wide field of view
would span less than 10 light-years.
APOD: 2007 January 4 - Central Cygnus
Explanation:
Supergiant star Gamma Cygni lies at the center
of the Northern Cross, famous
asterism
in the constellation
Cygnus
the Swan.
Known by the proper
name Sadr,
the bright star also lies at the
center of this gorgeous skyscape, featuring
a complex of
stars, dust clouds, and glowing nebulae along the
plane of our
Milky Way galaxy.
The field of view spans over 3 degrees
(six Full Moons) on
the sky and
includes emission nebula IC 1318 and open star cluster NGC 6910.
Left of Gamma Cyg and shaped
like two glowing cosmic wings divided by a long dark
dust lane, IC 1318's popular name is understandably the
Butterfly Nebula.
Above and left of Gamma Cyg, are the young, still tightly
grouped
stars of NGC 6910.
Some distance estimates for Gamma Cyg place it at around 750 light-years
while estimates for IC 1318 and
NGC 6910
range from 2,000 to 5,000 light-years.
APOD: 2006 December 11 - IC 2118: The Witch Head Nebula
Explanation:
Double, double toil and trouble;
Fire burn, and cauldron bubble -- maybe
Macbeth should have consulted the Witch Head Nebula.
This suggestively shaped
reflection nebula
is associated with the
bright star Rigel in the
constellation Orion.
More formally known as
IC 2118,
the Witch Head Nebula glows primarily by light reflected from bright
star Rigel, located just
off the upper right edge of the
full image.
Fine dust
in the nebula reflects the light.
The blue color is caused not only by
Rigel's blue color but because the
dust grains reflect blue light more efficiently than red.
The same
physical process causes
Earth's daytime sky to appear blue, although the scatterers in
Earth's atmosphere are molecules of
nitrogen and
oxygen.
The nebula lies about 1000 light-years away.
APOD: 2006 September 7 - Colorful Moon Mosaic
Explanation:
No single exposure can easily capture faint stars along with the
subtle
colors of the Moon.
But this dramatic composite view highlights both.
The mosaic digitally stitches together fifteen
carefully
exposed high resolution images of a bright,
gibbous
Moon and a representative background star field.
The fascinating color differences along
the lunar surface are real, though highly exaggerated,
corresponding to regions with different
chemical
compositions.
And while these color differences are not visible to the eye
even with
a telescope, moon watchers can still see a dramatic
lunar presentation
tonight.
A partial eclipse of the Moon will be visible
from Europe, Africa, Asia, and Australia.
APOD: 2006 March 17 - The Big Dipper Cluster
Explanation:
A well-known
asterism
in northern skies, The Big Dipper
is easy to recognize even
when
viewed upside down.
Part of the larger constellation of
Ursa Major,
the bright
dipper stars above
are named
(left to right along the dipper)
Dubhe, Merak, Phecda, Megrez, Alioth, Mizar/Alcor, and Alkaid.
Of course,
stars in any given
constellation
are unlikely to be physically
related.
But surprisingly, most of the
big dipper stars do seem to
be headed in the same direction as they
plough through
space, a property they share with other stars spread out
over an even larger area across the sky.
Their
measured common motion suggests that they all belong
to a loose, nearby
star cluster, thought to
be on average only
about 75 light-years away and up to 30 light-years across.
The cluster is more properly known as the
Ursa Major
Moving Group.
APOD: 2005 November 18 - The 37 Cluster
Explanation:
For the mostly harmless denizens of planet Earth, the
brighter stars of open cluster
NGC 2169
seem to form a cosmic 37.
(Did you
expect 42?.)
Of course, the improbable numerical
asterism
appears solely by chance and lies at an estimated distance of 3,600
light-years toward the
constellation Orion.
As far as galactic or open star clusters go,
NGC 2169
is a small one, spanning about 7 light-years.
Formed at the same time from the same cloud of dust and gas,
the stars of
NGC 2169 are only about 8 million years old.
Such clusters
are expected to disperse over time as they
encounter other stars, interstellar clouds, and
experience gravitational tides while traveling through
the galaxy.
Over four billion years ago, our own Sun was likely formed
in a similar open
cluster of stars.