Great Andromeda Spiral Galaxy - astronomy.

Great Andromeda Spiral Galaxy - astronomy. I INTRODUCTION Great Andromeda Spiral Galaxy, also known as M31 and as the Andromeda Galaxy, large spiral galaxy in the constellation Andromeda, about 2.5 million light-years from Earth. The Great Andromeda Spiral Galaxy is the largest nearby galactic neighbor to the Milky Way Galaxy, Earth's home galaxy, and is part of the Local Group. Because it is so near, it appears very bright, with a total magnitude (a measure of its brightness as seen from Earth) of 3.4, and is easily visible to the naked eye on a clear, dark night. Its immense diameter of about 200,000 light-years makes it appear five times larger than the full Moon in our sky. II STUDYING THE ANDROMEDA GALAXY The Great Andromeda Spiral Galaxy is the most studied of external galaxies because astronomers can view features in it that they believe are also present in the Milky Way, but are made invisible by the Milky Way's thick intervening clouds of dust. Astronomers study the Andromeda Galaxy's spiral arms; the birth of stars, dense spherical groups of stars called globular clusters, as well as looser star groupings called open clusters; interstellar matter; and supernova remnants. The mass of the Andromeda Galaxy is believed to be equivalent to between 300 billion and 400 billion Suns. From the 1970s to the 1990s, observations made with ground-based telescopes and with the Hubble Space Telescope showed that the galaxy appears to have a double nucleus--two bright areas near its center, instead of the usual single bright spot. In 1999 astronomers mapping the motion of stars near the galaxy's center discovered that the stars orbit the center in an elliptical path instead of in the more usual circular path found in most other galaxies. The stars move much more slowly at one end of the ellipse than at the other. Astronomers believe that stars bunch up at the slow point in the orbit, like cars in a traffic jam, creating one of the bright areas. The other bright area, at the other end of the ellipse, occurs because stars pass close to the black hole (a region of space so dense that not even light can escape its gravitational pull) at the center of the galaxy. Astronomers believe that all galaxies have a similar, giant black hole at their center. Matter from the stars is more likely to fall into the black hole at this close point in the stars' orbit, and matter accelerating toward the black hole glows to produce the second bright area. The Andromeda Galaxy and our Milky Way Galaxy are moving toward each other at about 120 km (75 mi) per second. Calculations indicate that the two spiral galaxies will eventually collide to form a single, giant elliptical galaxy. A series of close passes that may begin in about 2 billion years will first pull their spiral arms into long streams of stars, with complete merger occurring by about 5 billion years from now. The black holes at the heart of each galaxy will combine as a single supermassive black hole, possibly resulting in a powerful quasar. Recent computer models suggest our Sun and its solar system will survive but likely will be thrown into the distant outer halo of the new combined galaxy. III HISTORY OF OBSERVATION The first recorded observation of this galaxy was in AD 905 by Persian astronomer al-Sufi, who described it as the "little cloud" in his Book of Fixed Stars (964). The telescopic discovery of this object is often attributed to German astronomer Simon Marius who described the soft glow of the object in 1611 or 1612. French astronomer Charles Messier entered the object as the 31st item in his Catalogue of Nebulae and Star Clusters (1771-1784) on August 3, 1764. The name M31 is referred to as the object's Messier number, in which the M is followed by the number in Messier's catalog. Messier studied the galaxy with several instruments, including an excellent telescope, but could see no individual stars within, so he theorized that the object was a nebula, or cloud of gas and dust. It was not until 1923 that the American astronomer Edwin Hubble measured the distance to M31 and showed that it was not a nearby nebula but actually a distant galaxy like the Milky Way. Microsoft ® Encarta ® 2009. © 1993-2008 Microsoft Corporation. All rights reserved.