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Andromeda Galaxy Andromeda Galaxy  Home | X-Objects | Stars | Habitability | Life |
© Robert
Gendler,
Astroimaging
Gallery
(Used with permission)
Larger mosaic
(more).
Andromeda is the
largest member of
the Local Group of
galaxies, which
includes the Milky
Way and its satellite
galaxies. Andromeda's
own satellites include
M32, at center left, and
M110, at lower right
(more from
APOD).
A Large Spiral Galaxy
Wider and possibly brighter than our own Milky Way, the
Andromeda
Galaxy was once thought to be the dominant member of the
Local
Group of galaxies. Although it is Milky Way's nearest
large galactic neighbor, this large spiral galaxy
(type
Sb with two arms) lies around 2.52 ± 0.14 million
light-years (ly) from the Solar System
(Ribas et
al, 2005).
It can be found in (0:40:27+40:40:12, J2000; and
0:42:44.3+41:16:9.4, ICRS 2000) Constellation
Andromeda,
the Chained Maiden. It is located northwest of Mu and
Beta Andromedae
(Mirach);
west of Nu Andromedae; northeast of Theta and Sigma
Andromedae; north of Pi, Delta, and Epsilon Andromedae;
and south of Theta and Omega Cassiopeiae. Andromeda can
be seen by Human eyes from Earth without a telescope as
a "little cloud" (see
Akira
Fujii's photo to better relate the galaxy's location
to the brightest stars of Constellation Andromeda).
Andromeda has a bright disk that is now believed to span
as much as 228,000 ly in width
(Chapman
et al, 2005). In 2005, astronomers announced that
Andromeda's disk actally extends far further out, so that
the disk spans at least 260,000 light-years -- almost
twice the size of the bright disk seen in photographs
(Ibata
et al, 2005). The outer disk emits nearly 10 percent of
the galaxy's total light and may be comprised of
metal-poor stars stripped from smaller galaxies that
strayed too close. On January 7, 2007, a team of
astronomers announced the discovery of low-metallicity,
red giant stars up to some 500,000 light-years from
Andromeda's core which suggests that the galaxy is much larger
than originally thought, so that Andromeda's luminous halo
may actually overlap with that of the Milky Way
(BBC
News -- more below).
© Jason Ware,
www.galaxyphoto.com
(Used with permission)
Larger image
(more).
Andromeda has a bright
yellowish nucleus, dark
winding dustlanes, and
bluish spiral arms and
star clusters (more
from
APOD).
In the venerable
Star
Names: Their Lore and Meaning, Richard Hinckley
Allen noted that: "... the Great Nebula, the Queen of the
Nebulae, ..., is said to have been known as far back as A.D.
905; was described by
[Abd-al-Rahman]
Al Sufi as the Little Cloud before 986; and appeared on
a Dutch star-map of 1500." According to
Robert
Burnham, Jr. (1931-93): "The first hint of the true
nature of the Andromeda Galaxy came late in 1923 when several
[C]epheid variable stars were identified in the system [by
Edwin
Powell Hubble (1889-1953) who thus] ... definitively
established the great spiral as an extra-galactic object
...."The galaxy is frequently referred to as M31 because
it was the 31st object in the
Messier Catalogue
of diffuse objects that
Charles
Messier (1730-1817) found not to be comets. Subsequently,
the "nebula" was also designated as NGC 224 by
John
Louis Emil Dreyer (1852-1926) in his
New
General Catalogue (NGC) of Nebulae and Clusters of
Stars, which was first published in 1887 and later
supplemented with Index Catalogue (IC) I in 1895 and
IC II in 1907.
Bill Schoening,
Vanessa
Harvey,
REU program,
NOAO/AURA/NSF
Larger red, green, and blue composite image.
Recent observations indicate that,
although the spiral disk of Andromeda
may be much larger than that of the
Milky Way, the galaxy appears to be
much less dense, with a smaller mass
counting its dark matter halo (box
view at page
bottom -- more at
NOAO).
Careful estimates of Andromeda's angular diameter obtained
using 2-inch binoculars by Robert Jonckhere from 1952 to 1953
indicated that Andromeda's disk had a diameter of over 200,000
ly (assuming a distance of 2.9 million ly). Hence, Andromeda's
spiral disk may as much as twice as large as the Milky Way's.
Although Andromeda was long thought to be the most massive galaxy
in the Local Group, recent data suggest that Andromeda's visible
mass may total around 300 to 400 billion Solar-masses. This is
considerably less than more recent estimates of the Milky Way's
visible mass of as much as 600 billion or more Solar-masses,
which suggests that the Milky Way may be much denser than
Andromeda. These results apparently have been confirmed by
recent estimates of the total halo masses of the two spirals
that account for the gravitational effects of their invisible
dark matter, which suggest that Andromeda has a total of around
700 billion to 1.2 trillion Solar-masses compared to 0.93 to 1.9 trillion
or more for the Milky Way (more discussion from
(Xue
et al, 2008;
Evans
et al, 2000; and
Evans
and Wilkinson, 2000).
IRAS,
IPAC,
NASA
Larger infrared image.
Young stars are probably being born
in many dusty regions of Andromeda
that are bright in infrared wavelengths,
with many short-lived but massive, blue
stars in the more intense white and
yellow areas (more from
IPAC).
The brightest star cloud in Andromeda has its own NGC number,
NGC
206. One of the largest star-forming regions known in the
Local Group of galaxies,
Sir William
Friedrich Wilhelm Herschel (1738-1822,
portrait)
noted it in his catalog as H V.36 on his discovery of the
diffuse object on October 17, 1786.
Located
next to a dark nebula towards the southwestern, outer edge of
Andromeda's spiral disk (another
photo),
the cloud's bright blue stars give an indication of the massive
star cluster's youth (more from
Astronomy
Picture of the Day).
B.J. Mochejska
(Warsaw University),
The DIRECT Project,
FLWO,
MDM
Larger blue, visual, and infrared,
composite image.
One of the largest clusters of young blue stars
in the Local Group of galaxies, NGC 206 is
located in one of Andromeda's dusty spiral arms
(more from
APOD
and
CfA).
So far, only one supernova has been recorded in the Andromeda
Galaxy, but it was the first to be detected outside the Milky
Way. Known as Supernova 1885 for the year of its appearance,
it has also been designated as
S
Andromedae.
Ernst
Albrecht Hartwig (1851-1923) observed it on August 20, 1885 at
Dorpat Observatory in Estonia. While found independently by
several other observers, only Hartwig realized its significance.
The supernova reached a magnitude of six between August 17th and
20th but then faded to magnitude 16 by February 1890.
Philip Choi,
Puragra
Guhathakurta,
UCSC,
KPNO --
larger blue and infrared image.
Andromeda has an "extreme" warp in its outer spiral disk,
possibly from interactions
with satellite galaxies (more from
UCSC
and
UCOLICK),
as well as debris trails
from past mergers with other galaxies
(more).
Astronomers have been finding evidence of a warp in Andromeda's spiral
disk for some time. The faint outer parts of a spiral galaxy are more
susceptible to warping because they are less strongly bound by the
gravitational and other forces that keep disk stars in a plane and are
also more susceptible to the influence of neighboring galaxies. As a
result, the outer regions of a rotating body of stars and gas can
deviate from the plane of the disk, like an old record album exposed to
too much heat. Such a warp tends to occur at the outer edges, while the
inner part of the spiral disk continues to look reasonably flat.
Andromeda's warp is especially pronounced on the northeast (left) side
of its major axis. Such galactic warps are very difficult to demonstrate
conclusively because the outer portions of a spiral disk are extremely
faint compared to their bright central regions. However, the warp in
Andromeda may be the most extreme case of a warped spiral galaxy found
thus far. Possible causes of the warp include interactions between
Andromeda and its smaller satellite galaxies
(more discussion).
GALEX
Team,
CalTech,
NASA
Larger ultraviolet image.
This ultraviolet image highlights
a 150,000-ly-wide ring of young
and hot, blue stars that surrounds
Andromeda's central bulge (more
from
APOD
and
GALEX).
On October 18, 2006, astronomers using NASA's
Spitzer Space Telescope
announced the discovery of two dust rings (or "holes") in Andromeda's
dust disk using infrared light that provide evidence of an ancient
head-on collision with neighboring dwarf galaxy along its polar axis
Messier 32 (M32)
some 210 million years ago. Computer simulations support the hypothesis
that the passage of the much smaller galaxy created violent waves of
gravitational interactions that left rings of gas and dust propagating
outward from the site of the impact. Since Andromeda is much more
massive than M32, the larger galaxy was not substantially disrupted, but
M32 lost more than half its initial mass in the course of the collision
(more).
Pauline Barmby,
CfA,
JPL,
NASA --
larger infrared image
Holes in Andromeda's disk may be from an ancient collision with
satellite galaxy M32
(more).
Active Galactic Nucleus
STScI,
NASA
Larger image.
The central 30 light-years of Andromeda
contains two galactic nuclei, which suggests
that the great spiral consumed a major
galactic companion whose substance has
been mostly merged except for its central
core (more from
APOD
and
STScI).
In the 1990s, astronomers using the
Hubble Space
Telescope found that Andromeda has a nucleus with a double structure.
The "nuclear hot-spots" are located close together, considering that the
galaxy's spiral disk has been estimated to be anywhere from 150,000 to
more than 200,000 ly across while the observed central area measures only
around 30 ly wide. Subsequent ground-based observations led some
astronomers to speculate that two galactic nuclei do indeed exist, are
moving with respect to each other, and that one nucleus is slowly
disrupting the other through tidal forces. As a result, some astronomers
believed that that one nucleus may be the remains a smaller satellite
galaxy that was "eaten" by Andromeda
(Corbin
et al, 2001;
Gerssen
et al, 1995; and
Lauer
et al, 1993). In 2005, astronomers using the Hubble Space Telecope
announced that the two two bright blobs are actually composed of an
elliptical ring of older red stars and a smaller, brighter, and denser
disk of young blue stars of around 200 million years old around the
galaxy's central black hole (NASA
press
release).
Michael Garcia,
Stephen Murray,
Palomar
Sky Survey
Larger image.
The black square in the center
of Andromeda's spiral disk has
been observed with x-rays to
reveal a supermassive black
hole, as as well as smaller
ones (more from
CXC).
Andromeda's core has a supermassive central black hole of
around 140 million Solar-masses (latest NASA
press
release). Recent observations with
the Chandra X-Ray Observatory also reveal numerous other
bright X-ray sources, most of which are probably due to
binary systems where a star is feeding gas into a neutron
star and black hole. A very cool X-ray source has been
identified about 10 light years south of the galactic center.
A second, hotter X-ray source was found to be at a position
consistent with the position of the super-massive black hole.
Garcia
et al, 2001;
T. Brown et al, 2001;
CXC,
SAO,
NASA
Larger x-ray image.
The blue dot is an unusually "cool"
million degree X-ray source that
lies just below an black hole
(yellow) that may be X-ray bright
from matter swirling toward a
supermassive black hole of 30
million Solar-masses (more
from
CXC).
Andromeda's satellite (or "companion") galaxies include
M32
and
M110,
two bright dwarf elliptical galaxies that are the
brightest of a swarm of smaller companions. By late
1999, however, at least 10 satellite galaxies of Andromeda
were known, including NGC 185 (which was discovered by
William Herschel), and NGC 147 (discovered by
Heinrich
Ludwig d'Arrest, 1822-1875) as well as the very faint
dwarf systems And I, And II, And III, possibly And IV (which
may be a cluster or a remote background galaxy), And V, And
VI (also called the Pegasus dwarf), and And VII (also the
Cassiopeia dwarf).
Satellite galaxy M32 may be interacting to distort the disk
structure of Andromeda itself, whose spiral arms of neutral
hydrogen are displaced from those consisted of stars by
around 4,000 light-years and so cannot be continuously
followed in the area closest to its smaller neighbor.
Computer simulations have shown that such disturbances can
be modelled by assuming a recent close encounter with a
small companion of the mass of M32, which also suggest M32
has lost many stars from such an encounter to be spread out
in Andromeda's halo.
Very Large Galactic Halo
Using the Hubble Space Telescope, astronomers had previously
announced in 2003 that they had obtained the deepest
visible-light image ever taken of the sky to resolve
approximately 300,000 stars in Andromeda's luminous
halo.
By capturing both faint dwarf stars and bright giant stars,
astronomers were able to estimate the age of many members
of Andromeda's halo population by analyzing color and brightness
distributions. Describing an initial hypothesis subsequently
contested in January 2007, the astronomers indicated that they had
found many stars spanning a wide range of ages from six to 13
billion years old, which is much wider than that of the stellar
population of the Milky Way's halo where 11- to 13-billion-year-old,
metal-poor stars reside. (More discussion and close-up images
from
Alan
M. MacRobert at
Sky and Telescope).
STScI,
NASA
Larger image.
Although Andromeda's luminous halo
was thought to include many younger
stars around six to 13 billion years
old in 2003, new observations of
old red giant, halo stars up to
500,000 light-years away from
Andromeda's core were announced
in 2007 (2003;
and
2007
findings).
On January 7, 2007, astronomers announced finding old
low-metallicity, red giant stars up to some 500,000
light-years from Andromeda's center which suggests that the
galaxy is up to five times larger than originally thought,
so that its luminous halo may actually overlap with that of
the Milky Way. The new finding also suggests that previous
observers mis-identified relatively metal-rich red giants
in Andromeda's galactic bulge as halo stars. Based on
observations of the Milky Way and other galaxies, the
metallicity of stars farther from the galactic center should
fall with distance from the core
(more).
Ann Feild,
STScI,
NASA
Larger image.
Although a 2003 study of 300,000 stars
in Andromeda's halo indicated that their
age range was wider than those found in
the Milky Way's (more from
STScI
and
APOD),
this observation has been revised
by subsequent observations that suggest
that Andromeda's luminosity may be five
times larger than originally thought
(more).
In addition, a giant stream of metal-rich stars was recently
detected in Andromeda's halo
(Ibata
et al, 2001). The presence of younger
stars in Andromeda's halo may the result of a more violent
phase of the galaxy past involving mergers with smaller
satellite galaxies. Furthermore, numerical simulations of the
movements of Andromeda and the Milky Way suggest that the two
big spiral galaxies themselves may eventually collide and merge
within five to 10 billion years.
Old but Bright Globular Cluster
A small and compact satellite of Andromeda,
G1
is the brightest
globular star
cluster in the
Local Group.
Also known as Mayall II, G1 contains at least 300,000 old stars.
Despite its globular appearance, however, G1 may actualy be the
stripped down core of a dwarf spheroidal galaxy (like
SagDEG,
the Milky Way's satellite) that has been "shredded" by its larger
host. G1 may have at least 10 to 18 million Solar-masses, at
least twice the mass of
Omega
Centauri, the Milky Way's largest globular
(Meylan
et al, 1998). It is located around 130,000 to 170,000 ly
from Andromeda's nucleus.
Michael
Rich,
Kenneth Mighell,
James D. Neill,
Wendy Freedman,
Columbia University,
Carnegie
Observatories, STScI,
NASA
Larger image.
One of Andromeda's more compact satellites
is G1,
the brightest globular star cluster in
the Local Group (more from
STScI and
APOD).
G1 appears to be nearly as old as the oldest of the roughly 250
known globulars in the Milky Way Galaxy and so probably was formed
shortly after the birth of the first stars
at the beginning of the universe. Unlike many other globulars, it
has a "rather high mean metallicity of [Fe/H] = --0.95, somewhat
similar to
47 Tucanae"
which may be the result of self enrichment during an early phase of
cluster evolution
(Meylan
et al, 2001). Recently, some astronomers detected a 20,000
Solar-mass black hole in G1's core
(more from
STScI
and
Gebhardt
et al, 2002).
Satellite Galaxies
On January 11, 2006, astronomers announced their discovery that
many of Andromeda's faint companion galaxies lie within a thin
sheet running perpendicular through the galaxy's Andromeda disk.
Nine out of 14 low-mass satellites lying with 1.3 million
light-years from Andromeda are found within this sheet, whose
typical width is only 52,000 light years (about two percent of
the distance between Milky Way and Andromeda). The sheet runs
through Andromeda's core and is almost exactly aligned with its
polar axis.
Eva Grebel,
Andreas Koch,
University of Basel,
NOAO/AURA/NSF,
Keck Observatory
Larger illustration.
Many of Andromeda's
satellite galaxies
are located within
a plane pendicular
to its disk
(more).
Similar polar planes containing contain many of the Milky Way's
companion galaxies were found around three decades ago by William
Kunkel and Donald Lynden-Bell. One hypothesis is that such satellite
galaxies are tiny left-overs from the break-up of a more massive
galaxy which has since been swallowed by their host but still move
within the orbital plane of their predecessor, as galactic
mergers are believed to be a main mechanism of galactic growth.
A second possibility is that the observed alignment with the
poles of spiral galaxies' disks traces the otherwise invisible
distribution of non-luminous, so-called dark matter around
these massive galaxies. Finally, it is also possible that the
observed orientation along a plane is a consequence of the
infall of satellites along dark matter filaments, as
cosmological models predict density fluctuations or matter
concentrations which would attract neighboring clumps and
continued growth that lead to streams of dark and luminous
matter along filamentary features of the so-called cosmic web.
Small galaxies forming in dark matter streams could end up in
preferred sheets determined by their infall direction toward
massive galaxies. Indeed, the Andromeda satellite plane
points to the nearby spiral galaxy M33 as well as to the M81
group of galaxies
(more).
Other Information
More information and images of Andromeda may be available
at NASA and IPAC's
Extragalactic Database.
Up-to-date technical summaries on Andromeda may be
available at: NASA's
ADS
Abstract Service for the
Astrophysics Data
System; the
SIMBAD
Astronomical Database mirrored from CDS, which may require
an account to access; and the NSF-funded,
arXiv.org Physics e-Print archive's
search
interface.
In Greek mythology, Andromeda was rescued from Cetus, the Whale, by
Perseus who also married her. This constellation is most easily seen
in Autumn for observers in the Northern Hemisphere, but may be visible
from June through February.
For more information on stars and other objects in Constellation
Andromeda and an illustration, go to Christine Kronberg's
Andromeda.
For another illustration, see David Haworth's
Andromeda.
© 2003-2008 Sol Company. All Rights Reserved.
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