© 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).
Breaking News
Stefan Immler,
Erin Grand, Swift,
GSFC, NASA
Larger UV image.
This new high-resolution
UV image highlights the
hottest and youngest stars
and energetic stellar
remnants in Andromeda's
spiral arms, densest
clusters, and violent
core around its central
black hole (more).
On September 16, 2009, NASA's Swift Satellite Mission released the highest resolution ultra-violet (UV) image of Andromeda available. Covering an area some 200,000 light-years (ly) wide and 100,000 ly high, the image was compiled from 330 UV images made at wavelengths of 192.8, 224.6, and 260 nanometers. The result covers some 20,000 UV sources among the hottest and youngest stars and stellar remnants in Andromeda's spiral arms, densest clusters, and violent core around its central, supermassive black hole (more from NASA and Astronomy Picture of the Day).
McConnachie et al, 2009;
PAndAS
Larger composite image and
composite illustration
.
Andromeda has been growing
by pulling stars (as well as
gas) from smaller satellite
galaxies, such as bright
neighbor Triangulum (more).
On September 2, 2009, a team of astronomers (using the Canada-France Hawaii Telescope for the Pan-Andromeda Archaeological Survey or PAndAS) announced the discovery of stars and coherent structures that are almost certainly remnants of dwarf galaxies destroyed by the tidal field of Andromeda (M31). They also found that the halo of the giant spiral galaxy has non-native stars and "coherent structures" (such as stellar streams or "tidal tails" and a gaseous as well as stellar warp in its spiral disk). M31's nearest bright neighbor, satellite galaxy Triangulum (M33), moreover, has a stellar structure (including a warped disk) suggestive of a recent encounter with M31 around 2.5 billion years ago, and model simulations indicate that the two galaxies will have an even more violent encounter in about 2 billion years). Their observations supports the hierarchical galaxy formation model through the apparent interactions of M31 and M33 in the continuing growth of galaxies in the modern era (more from PAndAS; UBC news release; Lisa Grossman, New Scientist, September 2, 2009; McConnachie et al, 2009; and Martin et al, 2009).
Courtesy of PAndAS
Larger illustration (more).
The two galaxies will have an
even more violent encounter
in about 2 billion years (more).
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).
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