The “neighboring” galaxy

1. Andromeda has a bright disk that is now believed to span as much as 228,000 ly in width. In 2005, astronomers announced that Andromeda's disk actually extends far further out, so that the disk spans at least 260,000 ly. The Andromeda Galaxy also known as Messier 31, M31, or NGC 224 is a spiral galaxy approximately 2.5 million ly away in the constellation Andromeda. It is the nearest spiral galaxy to our own, the Milky Way Galaxy. The “neighboring” galaxy

2. Data • Distance estimate of 2.53 ± 0.07 Mly (775 ± 22 kpc) • The diameter of M31 at the widest point is estimated to be 140 ± 4 kly. • The estimated luminosity of M31 is double that of the Milky Way. • There are approximately 460 globular clustersassociated with the Andromeda galaxy. • Recent observations by the Spitzer Space Telescope revealed that M31 contains one trillion (1012) stars, greatly exceeding the number of stars in our own galaxy. • Current mass estimates for the Andromeda halo (including dark matter) give a value of approximately 1.23 × 1012M (or 1.2 million million solar masses) compared to 1.9 × 1012M for the Milky Way.

3. 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.

4. Observational history in years • 905 A.D. or earlier - known by Persian Astronomers • 964 A.D. - described by the Persian astronomer Abd-al-Rahman Al-Sufi in his Book of Fixed Stars. • 1500 - appears on a Dutch star map. • 1612- the first description of the object based on telescopic observation was given by Simon Marius. • 1764 - Charles Messier catalogued it as object M31 • 1885 - a supernova (known as "SAndromedae") was seen in the Andromeda Galaxy, the first and so far only one observed in that galaxy. At the time, it was thought to be a much less luminous and unrelated event called a nova, and was named accordingly Nova 1885. • 1887 - The first photographs of this galaxy were takenby Isaac Roberts from his private observatory in Sussex. • 1912- The radial velocity of this object with respect to our solar system was measuredby Vesto Slipher at the Lowell Observatory , using spectroscopy. • 1923 - Edwin Hubble found the first Cepheid variable in the Andromeda galaxy and thus established the intergalactic distance and the true nature of M31 as a galaxy • 1929- Hubble published his epochal study of the Andromeda "nebula" as an extragalactic stellar system (galaxy).

6. Double Nucleus • The Hubble Space Telescope has revealed that the Andromeda galaxy M31 has a double nucleus. This suggests that either it has actually two bright nuclei, probably because it has "eaten" a smaller galaxy which once intruded its core, or parts of its only one core are obscured by dark material, probably dust. In the first case, this second nucleus may be a remainder of a possibly violent dynamical encountering event in the earlier history of the Local Group. In the second case, the duplicity of Andromeda's nucleus would be an illusion causes by a dark dust cloud obstructing parts of a single nucleus in the center of M31.

7. Supermassive Black Hole • Andromeda's core has a supermassive central black hole of around 140 million Solar-masses . 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.

8. Andromeda’s Large Galactic Halo • Although Andromeda's luminous halo was thought to include many younger stars around 6 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 • 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.

9. The Bright Globular Cluster • A small and compact satellite of Andromeda, G1is the brightest globular star cluster in the Local Group. Also known as Mayall II, G1 contains at least 300,000 old stars. It is located around 130,000 to 170,000 ly from Andromeda's nucleus. It outshines even the brightest globular in our Milky Way, Omega Centauri, and can be glimpsed even by better equipped amateurs under very favorable conditions, with telescopes starting at 10-inch aperture. Recently, some astronomers detected a 20,000 Solar-mass black hole in G1's core.

10. Satellite Galaxies • 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, NGC 147, as well as the very faint dwarf systems And I, And II, And III, possibly And IV, And V, And VI, and And VII

11. 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.

12. On October 18, 2006, astronomers using NASA's Spitzer Space Telescopeannounced 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 (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.

13. The Great Collision • The Andromeda Galaxy is approaching the Sun at about 300 kilometres per second, so it is one of the few blue shifted galaxies. Given the motion of the Solar System inside the Milky Way, one finds that the Andromeda Galaxy and the Milky Way are approaching one another at a speed of 100 to 140 kilometres per second. However, this does not mean it will definitely collide with the Milky Way, since the galaxy's tangential velocity is unknown. If it is on a collision course, the impact is predicted to occur in about 3 billion years. In that case the two galaxies will likely merge to form a giant elliptical galaxy.Such events are frequent among the galaxies in galaxy groups.