Messier Index/M31

Andromeda Galaxy 320px A visible light image of the Andromeda Galaxy. J2000 SA(s)b 00 42 44.3 +41 16 9 2.54 ± 0.06 Mly(778 ± 17 kpc)Jensen, Joseph B.; Tonry, John L.; Barris, Brian J.; Thompson, Rodger I.; Liu, Michael C.; Rieke, Marcia J.; Ajhar, Edward A.; Blakeslee, John P. Measuring Distances and Probing the Unresolved Stellar Populations of Galaxies Using Infrared Surface Brightness Fluctuations Astrophysical Journal February 2003 583 2 712–726 http://adsabs.harvard.edu/abs/2003ApJ...583..712J 10.1086/345430A a −301 ± 1 km/s 4.4 190′ × 60′ Andromeda −20.0B b M31, NGC 224, UGC 454, PGC 2557, 2C 56 (Core), LEDA 2557 The Andromeda Galaxy (also known as Messier 31, M31, or NGC 224; often referred to as the Great Andromeda Nebula in older texts) is a spiral galaxy approximately 2.5 million light-years awayI. Ribas, C. Jordi, F. Vilardell, E.L. Fitzpatrick, R.W. Hilditch, F. Edward First Determination of the Distance and Fundamental Properties of an Eclipsing Binary in the Andromeda Galaxy Astrophysical Journal 2005 635 L37–L40 http://adsabs.harvard.edu/abs/2005ApJ...635L..37R 10.1086/499161 in the constellation Andromeda. It is the nearest spiral galaxy to our own, the Milky Way. As it is visible as a faint smudge on a moonless night, it is one of the farthest objects visible to the naked eye, and can be seen even from urban areas with binoculars. It is named after the princess Andromeda (Greek: Ανδρομέδη - Andromédē) in Greek mythology. Andromeda is the largest galaxy of the w:Local Group, which consists of the Andromeda Galaxy, the Milky Way Galaxy, the Triangulum Galaxy, and about 30 other smaller galaxies. Although the largest, it may not be the most massive, as recent findings suggest that the Milky Way contains more dark matter and may be the most massive in the grouping.Dark matter comes out of the cold BBC News February 5, 2006 http://news.bbc.co.uk/2/hi/science/nature/4679220.stm 2006-05-24 The 2006 observations by the Spitzer Space Telescope revealed that M31 contains one trillion (1012) stars, greatly exceeding the number of stars in our own galaxy. Young Kelly 2006-06-06 http://www.newscientist.com/article/dn9282-andromeda-galaxy-hosts-a-trillion-stars.html Andromeda galaxy hosts a trillion stars NewScientist 2006-06-08 While the 2006 estimates put the mass of the Milky Way to be ~80% of the mass of Andromeda, which is estimated to be 7.111 [solar masses,Karachentsev, I. D.; Kashibadze, O. G. Masses of the local group and of the M81 group estimated from distortions in the local velocity field Astrophysics 2006 49 1 3–18 http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=2006Ap.....49....3K 10.1007/s10511-006-0002-6 a 2009 study concluded that Andromeda and the Milky Way are about equal in mass., CfA Press Release No.: 2009-03 For Release: Monday, January 05, 2009 01:00:00 PM EST. At an apparent magnitude of 4.4, the Andromeda Galaxy is notable for being one of the brightest Messier objects,Frommert, H.; Kronberg, C. August 22, 2007 http://seds.lpl.arizona.edu/messier/dataMag.html Messier Object Data, sorted by Apparent Visual Magnitude SEDS 2007-08-27 http://web.archive.org/web/20070712184703/http://seds.lpl.arizona.edu/messier/dataMag.html 2007-07-12 making it easily visible to the naked eye even when viewed from areas with moderate light pollution. Although it appears more than six times as wide as the full moon when photographed through a larger telescope, only the brighter central region is visible with the naked eye. Observation history The earliest recorded observation of the Andromeda Galaxy was in 964 CE by the Persian astronomer, Abd al-Rahman al-Sufi (Azophi), Kepple George Robert Glen W. Sanner The Night Sky Observer's Guide, Volume 1 Willmann-Bell, Inc. 1998 0-943396-58-1 18 who described it as a "small cloud" in his Book of Fixed Stars. Other star charts of that period have it labeled as the Little Cloud. The first description of the object based on telescopic observation was given by Simon Marius in 1612. Charles Messier catalogued it as object M31 in 1764 and incorrectly credited Marius as the discoverer, unaware of Al Sufi's earlier work. In 1785, the astronomer William Herschel noted a faint reddish hue in the core region of the M31. He believed it to be the nearest of all the "great nebulae" and, based on the color and magnitude of the nebula, he incorrectly estimated that it was no more than 2,000 times the distance of w:Sirius.W. Herschel William Herschel On the Construction of the Heavens Philosophical Transactions of the Royal Society of London 75 1785 213–266 10.1098/rstl.1785.0012 w:William Huggins in 1864 observed the w:spectrum of M31 and noted that it differed from a gaseous nebula.William Huggins William Huggins On the Spectra of Some of the Nebulae Philosophical Transactions of the Royal Society of London 154 1864 437–444 10.1098/rstl.1864.0013 The spectra of M31 displayed a w:continuum of frequencies, superimposed with dark absorption lines that help identify the chemical composition of an object. The Andromeda nebula was very similar to the spectra of individual stars, and from this it was deduced that M31 had a stellar nature. In 1885, a supernova (known as "S Andromedae") was seen in M31, the first and so far only one observed in that galaxy. At the time M31 was considered to be a nearby object, so the cause was thought to be a much less luminous and unrelated event called a nova, and was named accordingly "Nova 1885".Backhouse T. W. 1888 nebula in Andromeda and Nova, 1885 Monthly Notices of the Royal Astronomical Society 48 108 1888MNRAS..48..108B http://adsabs.harvard.edu//abs/1888MNRAS..48..108B 2009-07-27 right|thumb|200px|Great Andromeda Nebula by w:Isaac Roberts. The first photographs of M31 were taken in 1887 by w:Isaac Roberts from his private observatory in w:Sussex, England. The long-duration exposure allowed the spiral structure of the galaxy to be seen for the first time.Isaac Roberts A Selection of Photographs of Stars, Star-clusters and Nebulae, Vol. II The Universal Press London 1899 However, at the time this object was commonly believed to be a nebula within our galaxy, and Roberts mistakenly believed that M31 and similar spiral nebulae were actually solar systems being formed, with the satellites nascent planets. The w:radial velocity of this object with respect to our w:solar system was measured in 1912 by w:Vesto Slipher at the w:Lowell Observatory, using w:spectroscopy. The result was the largest velocity recorded at that time, at 300 kilometres per second (186 miles/s.), moving in the direction of the Sun.V.M. Slipher Vesto Melvin Slipher The Radial Velocity of the Andromeda Nebula Lowell Observatory Bulletin 1913 1 2.56–2.57 http://adsabs.harvard.edu/abs/1913LowOB...1b..56S left|thumb|M31 in proximity to Pegasus constellation Island universe In 1917, w:Heber Curtis observed a nova within M31. Searching the photographic record, 11 more novae were discovered. Curtis noticed that these novae were, on average, 10 magnitudes fainter than those that occurred within our Galaxy. As a result he was able to come up with a distance estimate of 500,000 light-years. He became a proponent of the so-called "island universes" hypothesis, which held that spiral nebulae were actually independent galaxies.Heber D. Curtis Heber Doust Curtis Novae in Spiral Nebulae and the Island Universe Theory Publications of the Astronomical Society of the Pacific January 1988 100 6 10.1086/132128 http://adsabs.harvard.edu/abs/1988PASP..100....6C In 1920 the w:Great Debate between w:Harlow Shapley and w:Heber Curtis took place, concerning the nature of the w:Milky Way, spiral nebulae, and the dimensions of the w:universe. To support his claim that Great Andromeda Nebula (M31) was an external galaxy, Curtis also noted the appearance of dark lanes resembling the dust clouds in our own Galaxy, as well as the significant w:Doppler shift. In 1922 w:Ernst Öpik presented a very elegant and simple astrophysical method to estimate the distance of M31, his result (450 kpc) put Andromeda Nebula far outside our Galaxy.Öpik Ernst Ernst Öpik An estimate of the distance of the Andromeda Nebula Astrophysical Journal 1922 55 406–410 http://adsabs.harvard.edu/abs/1922ApJ....55..406O 10.1086/142680 w:Edwin Hubble settled the debate in 1925 when he identified extragalactic w:Cepheid variable stars for the first time on astronomical photos of M31. These were made using the 2.5 metre (100 in) w:Hooker telescope, and they enabled the distance of Great Andromeda Nebula to be determined. His measurement demonstrated conclusively that this feature was not a cluster of stars and gas within our Galaxy, but an entirely separate galaxy located a significant distance from our own.E. P. Hubble Edwin Hubble A spiral nebula as a stellar system, Messier 31 Astrophysical JournalEngl 1929 69 103–158 http://adsabs.harvard.edu/cgi-bin/bib_query?1929ApJ....69..103H 10.1086/143167 This galaxy plays an important role in galactic studies, since it is the nearest giant spiral (although not the nearest galaxy). In 1943, w:Walter Baade was the first person to resolve stars in the central region of the Andromeda Galaxy. Based on his observations of this galaxy, he was able to discern two distinct populations of stars based on their w:metallicity, naming the young, high velocity stars in the disk Type I and the older, red stars in the bulge Type II. This nomenclature was subsequently adopted for stars within the Milky Way, and elsewhere. (The existence of two distinct populations had been noted earlier by w:Jan Oort.)W. Baade Walter Baade The Resolution of Messier 32, NGC 205, and the Central Region of the Andromeda Nebula Astrophysical Journal 1944 100 137 http://adsabs.harvard.edu/abs/1944ApJ...100..137B 10.1086/144650 Dr. Baade also discovered that there were two types of Cepheid variables, which resulted in a doubling of the distance estimate to M31, as well as the remainder of the Universe.John R. Gribbin 2001 The Birth of Time: How Astronomers Measure the Age of the Universe 151 Yale University Press 0300089147 Radio emission from the Andromeda Galaxy was first detected by w:Grote Reber in 1940. The first radio maps of the galaxy were made in the 1950s by John Baldwin and collaborators at the Cambridge Radio Astronomy Group.van der Kruit, P. C.; Allen, R. J. The Radio Continuum Morphology of Spiral Galaxies Annual Review of Astronomy and Astrophysics 14 417-445 1976 10.1146/annurev.aa.14.090176.002221 The core of the Andromeda Galaxy is called 2C 56 in the 2C radio astronomy catalogue. In 2009, the first planet may have been discovered in the Andromeda Galaxy. This candidate was detected using a technique called w:microlensing, which is caused by the deflection of light by a massive object.Ingrosso, G.; Calchi Novati, S.; De Paolis, F.; Jetzer, Ph.; Nucita, A. A.; Zakharov, A. F. arXiv Pixel-lensing as a way to detect extrasolar planets in M31 http://arxiv.org/abs/0906.1050 2009-07-10 General The Andromeda Galaxy is approaching the w:Sun at about 300 kilometers per second (186 miles/s.), so it is one of the few w:blue shifted galaxies. The Andromeda Galaxy and the Milky Way are approaching one another at a speed of 100 to 140 kilometers per second (62–87 miles/s.; 223,200–313,200mph). Malik Tariq 2002-05-07 http://www.space.com/scienceastronomy/astronomy/galaxy_collides_020507-1.html Crash Course: Simulating the Fate of Our Milky Way SPACE.com 2006-09-18 http://web.archive.org/web/20020606060654/http://www.space.com/scienceastronomy/astronomy/galaxy_collides_020507-1.html 2002-06-06 The collision is predicted to occur in about 2.5 billion years. In that case the two galaxies will likely merge to form a giant w:elliptical galaxy. Cox, T.J., Loeb, A. The collision between the Milky Way and Andromeda Monthly Notices of the Royal Astronomical Society 386 1 461–474 2008 http://adsabs.harvard.edu/abs/2008MNRAS.tmp..333C 10.1111/j.1365-2966.2008.13048.x However, Andromeda's tangential velocity with respect to the Milky Way is only known to within about a factor of two, which creates uncertainty about the details of the collision. The Grand Collision The Sky At Night November 5, 2007 Such events are frequent among the galaxies in w:galaxy groups. The fate of the w:Earth and the w:Solar System in the event of a collision are presently unknown, but there is a small chance that the Solar System could be ejected from the Milky Way or join Andromeda.When Our Galaxy Smashes Into Andromeda, What Happens to the Sun? Cain, Fraser Universe Today http://www.universetoday.com/2007/05/10/when-our-galaxy-smashes-into-andromeda-what-happens-to-the-sun/ 2007 2007-05-16 The measured distance to the Andromeda Galaxy was doubled in 1953 when it was discovered that there is another, dimmer type of w:Cepheid. In the 1990s, measurements of both standard w:red giants as well as w:red clump stars from the w:Hipparcos satellite measurements were used to calibrate the Cepheid distances. Holland Stephen The Distance to the M31 Globular Cluster System The Astronomical Journal 115 5 1916–1920 1998 http://adsabs.harvard.edu/abs/1998astro.ph..2088H 10.1086/300348 Stanek, K.Z., Garnavich, P.M. Distance to M31 With the HST and Hipparcos Red Clump Stars Astrophysical Journal Letters 503 131–141 1998 http://arxiv.org/abs/astro-ph/?9802121 thumb|left|The Andromeda Galaxy pictured in w:ultraviolet light by w:GALEX Recent distance estimates At least four distinct techniques have been used to measure distances to the Andromeda Galaxy. In 2003, using the infrared w:surface brightness fluctuations (I-SBF) and adjusting for the new period-luminosity value of Freedman et al. 2001 and using a metallicity correction of -0.2 mag dex−1 in (O/H), an estimate of 2.57 ± 0.06 Mly (787 ± 18 kpc) was derived. Using the Cepheid variable method, an estimate of 2.51 ± 0.13 Mly (770 ± 40 kpc) was achieved in 2004.I. D. Karachentsev, V. E. Karachentseva, W. K. Hutchmeier, D. I. Makarov A Catalog of Neighboring Galaxies Astronomical Journal 2004 127 2031–2068 http://adsabs.harvard.edu/abs/2004AJ....127.2031K 10.1086/382905 In 2005, a group of astronomers consisting of w:Ignasi Ribas (CSIC, IEEC) and his colleagues announced the discovery of an eclipsing binary star in the Andromeda Galaxy. The binary star, designated M31VJ00443799+4129236,C c has two luminous and hot blue stars of types O and B. By studying the eclipses of the stars, which occur every 3.54969 days, the astronomers were able to measure their sizes. Knowing the sizes and temperatures of the stars they were able to measure the w:absolute magnitude of the stars. When the visual and absolute magnitudes are known, the distance to the star can be measured. The stars lie at the distance of 2.52 ± 0.14 Mly (770 ± 40 kpc) and the whole Andromeda Galaxy at about 2.5 Mly. This new value is in excellent agreement with the previous, independent Cepheid-based distance value. Andromeda is close enough that the w:Tip of the Red Giant Branch (TRGB) method may also be used to estimate its distance. The estimated distance to M31 using this technique in 2005 yielded 2.56 ± 0.08 Mly (785 ± 25 kpc).McConnachie, A. W.; Irwin, M. J.; Ferguson, A. M. N.; Ibata, R. A.; Lewis, G. F.; Tanvir, N. Distances and metallicities for 17 Local Group galaxies Monthly Notices of the Royal Astronomical Society 2005 356 4 979–997 http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=2005MNRAS.356..979M 10.1111/j.1365-2966.2004.08514.x Averaged together, all these distance measurements give a combined distance estimate of 2.54 ± 0.06 Mly (778 ± 17 kpc).A a Based upon the above distance, the diameter of M31 at the widest point is estimated to be 141 ± 3 kly.D d Mass and luminosity estimates Mass estimates for the Andromeda halo (including w:dark matter) give a value of approximately 1.2312 M☉N. W. Evans & M. I. Wilkinson The mass of the Andromeda galaxy Monthly Notices of the Royal Astronomical Society 2000 316 4 929–942 http://adsabs.harvard.edu/cgi-bin/bib_query?2000MNRAS.316..929E 10.1046/j.1365-8711.2000.03645.x (or 1.2 million million w:solar masses) compared to 1.912 M☉ for the Milky Way. Thus M31 may be less massive than our own galaxy, although the error range is still too large to say for certain. Even so, the masses of the Milky Way and M31 are comparable, and M31's spheroid actually has a higher stellar density than that of the Milky Way. Kalirai, J.S. et al. The Metal-Poor Halo of the Andromeda Spiral Galaxy (M31) Astrophysical Journal 648 389–404 2006 10.1086/505697 In particular, M31 appears to have significantly more common stars than the Milky Way, and the estimated w:luminosity of M31, ~2.610 L☉, is about 25% higher than that of our own galaxy. Sidney van den Bergh The local group of galaxies The Astronomy and Astrophysics Review 9 3–4 273–318 1999 10.1007/s001590050019 However the rate of star formation in the Milky Way is much higher, with M31 only producing about one solar mass per year compared to 3–5 solar masses for the Milky Way. The rate of w:supernovae in the Milky Way is also double that of M31.W. Liller, B. Mayer The Rate of Nova Production in the Galaxy Publications Astronomical Society of the Pacific July 1987 99 606–609 http://adsabs.harvard.edu/abs/1987PASP...99..606L 10.1086/132021 This suggests that M31 has experienced a great star formation phase in its past, but is now relatively w:quiescent, whereas the Milky Way is experiencing more active star formation. Should this continue, the luminosity in the Milky Way may overtake that of M31 in the future. Structure Based on its appearance in visible light, the Andromeda galaxy is classified as an SA(s)b galaxy in the de Vaucouleurs-Sandage extended classification system of spiral galaxies.NASA/IPAC Extragalactic Database Results for Messier 31 http://nedwww.ipac.caltech.edu/ 2006-11-01 However, data from the w:2MASS survey showed that the bulge of M31 has a box-like appearance, which implies that the galaxy is actually a barred galaxy with the bar viewed almost directly along its long axis.R.L. Beaton, E. Athanassoula, S.R. Majewski, P. Guhathakurta, M.F. Skrutskie, R.J. Patterson, M. Bureau Unveiling the Boxy Bulge and Bar of the Andromeda Spiral Galaxy Astrophysical Journal Letters 2006 http://adsabs.harvard.edu/abs/2006astro.ph..5239B 10.1086/514333 658 L91 thumb|right|300px|The Andromeda Galaxy seen in w:infrared by the w:Spitzer Space Telescope, one of w:NASA's four Great Space Observatories In 2005, astronomers used the w:Keck telescopes to show that the tenuous sprinkle of stars extending outward from the galaxy is actually part of the main disk itself.S. C. Chapman, R. Ibata, G. F. Lewis, A. M. N. Ferguson, M. Irwin, A. McConnachie, N. Tanvir A kinematically selected, metal-poor spheroid in the outskirts of M31 Astrophysical Journal 2006 http://adsabs.harvard.edu/cgi-bin/bib_query?astro-ph/0602604 653 255 10.1086/508599 Also see the press release, CalTech Media Relations February 27, 2006 Andromeda's Stellar Halo Shows Galaxy's Origin to Be Similar to That of Milky Way http://pr.caltech.edu/media/Press_Releases/PR12801.html 2006-05-24 This means that the spiral disk of stars in Andromeda is three times larger in diameter than previously estimated. This constitutes evidence that there is a vast, extended stellar disk that makes the galaxy more than 220,000 light-years in diameter. Previously, estimates of Andromeda's size ranged from 70,000 to 120,000 light-years across. The galaxy is inclined an estimated 77° relative to the Earth (where an angle of 90° would be viewed directly from the side). Analysis of the cross-sectional shape of the galaxy appears to demonstrate a pronounced, S-shaped warp, rather than just a flat disk.UC Santa Cruz January 9, 2001 Astronomers Find Evidence of an Extreme Warp in the Stellar Disk of the Andromeda Galaxy http://www.ucsc.edu/news_events/press_releases/archive/00-01/01-01/andromeda.html 2006-05-24 A possible cause of such a warp could be gravitational interaction with the satellite galaxies near M31. The galaxy M33 could be responsible for some warp in M31's arms, though more precise distances and radial velocities are required. Spectroscopic studies have provided detailed measurements of the rotational velocity of M31 at various radii from the core. In the vicinity of the core, the rotational velocity climbs to a peak of 225 kilometres per second (140 miles/s.) at a radius of 1,300 w:light-years, then descends to a minimum at 7,000 light-years where the rotation velocity may be as low as 50 kilometres per second (31 miles/s.). Thereafter the velocity steadily climbs again out to a radius of 33,000 light-years, where it reaches a peak of 250 kilometres per second (155 miles/s.). The velocities slowly decline beyond that distance, dropping to around 200 kilometres per second (124 miles/s.) at 80,000 light-years. These velocity measurements imply a concentrated mass of about 69 M☉ in the nucleus. The total mass of the galaxy increases w:linearly out to 45,000 light-years, then more slowly beyond that radius.V. C. Rubin, W. K. J. Ford Rotation of the Andromeda Nebula from a Spectroscopic Survey of Emission Astrophysical Journal 1970 159 379 http://adsabs.harvard.edu/abs/1970ApJ...159..379R 10.1086/150317 The w:spiral arms of Andromeda are outlined by a series of w:H II regions that Baade described as resembling "beads on a string". They appear to be tightly wound, although they are more widely spaced than in our galaxy.H. Arp Halton Arp Andromeda Nebula from a Spectroscopic Survey of Emission Astrophysical Journal 1964 139 1045 http://adsabs.harvard.edu/abs/1964ApJ...139.1045A 10.1086/147844 Rectified images of the galaxy show a fairly normal spiral galaxy with the arms wound up in a clockwise direction. There are two continuous trailing arms that are separated from each other by a minimum of about 13,000 light-years. These can be followed outward from a distance of roughly 1,600 light-years from the core. The most likely cause of the spiral pattern is thought to be interaction with M32. This can be seen by the displacement of the neutral hydrogen clouds from the stars.R. Braun The distribution and kinematics of neutral gas, w:HI region in M31 Astrophysical Journal 1991 372, part 1 54–66 http://adsabs.harvard.edu/abs/1991ApJ...372...54B 10.1086/169954 thumb|350px|left|Image of Andromeda Galaxy (M31) taken by Spitzer in infrared, 24 w:micrometres (Credit:w:NASA/w:JPL-w:Caltech/K. Gordon (w:University of Arizona) In 1998, images from the w:European Space Agency's w:Infrared Space Observatory demonstrated that the overall form of the Andromeda galaxy may be transitioning into a w:ring galaxy. The gas and dust within Andromeda is generally formed into several overlapping rings, with a particularly prominent ring formed at a radius of 32,000 light-years from the core.Esa Science News October 14, 1998 ISO unveils the hidden rings of Andromeda http://www.iso.vilspa.esa.es/outreach/esa_pr/andromed.htm 2006-05-24 This ring is hidden from visible light images of the galaxy because it is composed primarily of cold dust. Close examination of the inner region of Andromeda showed a smaller dust ring that is believed to have been caused by the interaction with M32 more than 200 million years ago. Simulations show that the smaller galaxy passed through the disk of Andromeda along the latter's polar axis. This collision stripped more than half the mass from the smaller M32 and created the ring structures in Andromeda.Busted! Astronomers Nab Culprit in Galactic Hit-and-Run Harvard-Smithsonian Center for Astrophysics October 18, 2006 http://www.cfa.harvard.edu/press/pr0628.html 2006-10-18 Studies of the extended halo of M31 show that it is roughly comparable to that of the Milky Way, with stars in the halo being generally "metal-poor", and increasingly so with greater distance.J. S. Kalirai, K. M. Gilbert, P. Guhathakurta, S. R. Majewski, J. C. Ostheimer, R. M. Rich, M. C. Cooper, D. B. Reitzel, R. J. Patterson The Metal-Poor Halo of the Andromeda Spiral Galaxy (M31) Astrophysical Journal 2006 http://adsabs.harvard.edu/abs/2006astro.ph..5170K 648 389 10.1086/505697 This evidence indicates that the two galaxies have followed similar evolutionary paths. They are likely to have accreted and assimilated about 1–200 low-mass galaxies during the past 12 billion years.J.S. Bullock and K.V. Johnston Tracing Galaxy Formation with Stellar Halos I: Methods Astrophysical Journal 2005 635 2 931–949 http://adsabs.harvard.edu/abs/2005ApJ...635..931B 10.1086/497422 The stars in the extended halos of M31 and the Milky Way may extend nearly one-third the distance separating the two galaxies. Nucleus right|thumb|200px|HST image of Andromeda galaxy core showing possible double structure. w:NASA/w:ESA photo. right|thumb|200px|Artist's concept of Andromeda galaxy core showing a view across a mysterious disk of young, blue stars encircling a supermassive black hole. w:NASA/w:ESA photo. M31 is known to harbor a dense and compact star cluster at its very center. In a large telescope it creates a visual impression of a star embedded in the more diffuse surrounding bulge. The luminosity of the nucleus is in excess of the most luminous globular clusters.July 2009 In 1991 w:Tod R. Lauer used WFPC, then on board the w:Hubble Space Telescope, to image Andromeda's inner nucleus. The nucleus consists of two concentrations separated by 1.5 w:parsecs. The brighter concentration, designated as P1, is offset from the center of the galaxy. The dimmer concentration, P2, falls at the true center of the galaxy and contains a 108 M☉ w:black hole.Lauer, T. R. et al. Planetary camera observations of the double nucleus of M31 Astronomical Journal 106 4 1436–1447, 1710–1712 1993 10.1086/116737 w:Scott Tremaine has proposed that the observed double nucleus could be explained if P1 is the projection of a disk of stars in an eccentric orbit around the central black hole. Tremaine Scott Scott Tremaine An Eccentric-Disk Model for the Nucleus of M31 Astronomical Journal 110 628–633 1995 http://adsabs.harvard.edu/abs/1995AJ....110..628T 10.1086/117548 The eccentricity is such that stars linger at the orbital apocenter, creating a concentration of stars. P2 also contains a compact disk of hot, spectral class A stars. The A stars are not evident in redder filters, but in blue and ultraviolet light they dominate the nucleus, causing P2 to appear more prominent than P1.Hubble news desk STScI-1993-18 July 20, 1993 Hubble Space Telescope Finds a Double Nucleus in the Andromeda Galaxy http://hubblesite.org/newscenter/newsdesk/archive/releases/1993/18/text/ 2006-05-26 While at the initial time of its discovery it was hypothesized that the brighter portion of the double nucleus was the remnant of a small galaxy "cannibalized" by Andromeda,Schewe, Phillip F.; Stein, Ben The Andromeda Galaxy has a Double Nucleus Physics News Update American Institute of Physics http://www.aip.org/pnu/1993/split/pnu138-2.htm 2009-07-10 this is no longer considered to be a viable explanation. The primary reason is that such a nucleus would have an exceedingly short lifetime due to tidal disruption by the central black hole. While this could be partially resolved if P1 had its own black hole to stabilize it, the distribution of stars in P1 does not suggest that there is a black hole at its center. Discrete sources Multiple X-ray sources have been detected in the Andromeda Galaxy, using observations from the ESA's w:XMM-Newton orbiting observatory. w:Robin Barnard et al. hypothesized that these are candidate black holes or w:neutron stars, which are heating incoming gas to millions of kelvins and emitting X-rays. The spectrum of the neutron stars is the same as the hypothesized black holes, but can be distinguished by their masses.R., Barnard; U. Kolb; J.P. Osborne Timing the bright X-ray population of the core of M31 with XMM-Newton A&A August 2005 http://adsabs.harvard.edu/abs/2005astro.ph..8284B There are approximately 460 w:globular clusters associated with the Andromeda galaxy.P. Barmby, J.P. Huchra M31 Globular Clusters in the Hubble Space Telescope Archive. I. Cluster Detection and Completeness Astronomical Journal 2001 122 2458–2468 http://www.iop.org/EJ/article/1538-3881/122/5/2458/201285.html 10.1086/323457 The most massive of these clusters, identified as w:Mayall II, nicknamed Globular One, has a greater luminosity than any other known globular cluster in the w:local group of galaxies.Hubble news desk STSci-1996-11 April 24, 1996 Hubble Spies Globular Cluster in Neighboring Galaxy http://hubblesite.org/newscenter/newsdesk/archive/releases/1996/11/ 2006-05-26 It contains several million stars, and is about twice as luminous as w:Omega Centauri, the brightest known globular cluster in the w:Milky Way. Globular One (or G1) has several stellar populations and a structure too massive for an ordinary globular. As a result, some consider G1 to be the remnant core of a w:dwarf galaxy that was consumed by M31 in the distant past.G. Meylan, A. Sarajedini, P. Jablonka, S.G. Djorgovski, T. Bridges, R.M. Rich G1 in M31 - Giant Globular Cluster or Core of a Dwarf Elliptical Galaxy? Astronomical Journal 2001 122 830–841 http://www.iop.org/EJ/article/1538-3881/122/2/830/201075.html 10.1086/321166 The globular with the greatest apparent brightness is w:G76 which is located in the south-west arm's eastern half. In 2005, astronomers discovered a completely new type of star cluster in M31. The new-found clusters contain hundreds of thousands of stars, a similar number of stars that can be found in globular clusters. What distinguishes them from the globular clusters is that they are much larger – several hundred light-years across – and hundreds of times less dense. The distances between the stars are, therefore, much greater within the newly discovered extended clusters.A.P. Huxor, N.R. Tanvir, M.J. Irwin, R. Ibata A new population of extended, luminous, star clusters in the halo of M31 Monthly Notices of the Royal Astronomical Society 2005 360 993–1006 http://arxiv.org/abs/astro-ph/0412223 10.1111/j.1365-2966.2005.09086.x Satellites Andromeda's satellite galaxies Like the Milky Way, Andromeda Galaxy has satellite galaxies, consisting of 14 known dwarf galaxies. The best known and most readily observed satellite galaxies are M32 and M110. Based on current evidence, it appears that M32 underwent a close encounter with M31 (Andromeda) in the past. M32 may once have been a larger galaxy that had its stellar disk removed by M31, and underwent a sharp increase of w:star formation in the core region, which lasted until the relatively recent past.K. Bekki, W.J. Couch, M.J. Drinkwater, M.D. Gregg A New Formation Model for M32: A Threshed Early-type Spiral? Astrophysical Journal 2001 557 1 L39–L42 http://adsabs.harvard.edu/abs/2001ApJ...557L..39B 10.1086/323075 M110 also appears to be interacting with M31, and astronomers have found a stream of metal-rich stars in the halo of M31 that appears to have been stripped from these satellite galaxies.R. Ibata, M. Irwin, G. Lewis, A.M. Ferguson, N. Tanvir A giant stream of metal-rich stars in the halo of the galaxy M31 Nature July 5, 2001 412 6842 49–52 10.1038/35083506 M110 does contain a dusty lane, which may indicate recent or ongoing star formation.Young L. M. November 2000 Properties of the Molecular Clouds in NGC 205 The Astronomical Journal volume 120 5 2460–2470 10.1086/316806 In 2006 it was discovered that nine of these galaxies lay along a plane that intersects the core of the Andromeda Galaxy, rather than being randomly arranged as would be expected from independent interactions. This may indicate a common tidal origin for the satellites.A. Koch and E.K. Grebel The Anisotropic Distribution of M 31 Satellite Galaxies: A Polar Great Plane of Early-Type Companions Astronomical Journal 2006 131 3 1405–1415 http://adsabs.harvard.edu/abs/2005astro.ph..9258K 10.1086/499534 Notes A aaverage(787 ± 18, 770 ± 40, 772 ± 44, 783 ± 25) = ((787 + 770 + 772 + 783) / 4) ± ((182 + 402 + 442 + 252)0.5 / 4) = 778 ± 17 B bApparent Magnitude of 4.36 - w:distance modulus of 24.4 = −20.0 C cJ00443799+4129236 is at celestial coordinates R.A. 00 44 37.99, Dec. +41 29 23.6. D ddistance × tan( diameter_angle = 190′ ) = 141 ± 3 kly diameter References 2 External links Andromeda Galaxy Simbad data on M31 Messier 31, SEDS Messier pages Astronomy Picture of the Day: July 18, 2004 • October 17, 1998 • December 22, 2005 Globular Clusters in M31 at The Curdridge Observatory First direct distance to Andromeda − Astronomy magazine article Andromeda galaxy at SolStation.com Andromeda Galaxy at The Encyclopedia of Astrobiology, Astronomy, & Spaceflight M31, the Andromeda Galaxy at NightSkyInfo.com Ker Than http://space.com/scienceastronomy/060123_andromeda_plane.html Strange Setup: Andromeda's Satellite Galaxies All Lined Up SPACE.com January 23, 2006