The Andromeda Galaxy (also known as Messier 31 or M31) is a large spiral galaxy located about 2.5 million light-years from the Milky Way. It contains about one trillion stars, and is one of the most distant objects which can readily be seen with the naked eye from the surface of Earth. Astronomers currently believe that the Andromeda and Milky Way galaxies will collide in several billion years, with unknown consequences for Sun-like stars in both galaxies.
According to NASA, the Andromeda galaxy is one of the largest galaxies in the Local Group, which also includes the Milky Way (in which Earth resides). Also like the Milky Way, Andromeda is a spiral galaxy – meaning that most of the stars and dust can be found either in a small, dense “core” region or within a number of long tendrils or “arms” which appear to twist away from the core in spiral shapes. According to the Smithsonian Museum of Natural History, astronomers estimate that Andromeda may contain more than a trillion stars.
At that size, luminosity and distance, Andromeda is unusual in that it can be readily seen with the naked eye, provided that the night sky is clear. (For observers in the northern hemisphere, California astronomer Arthur Huffman says that Andromeda can be found by locating the Great Square of Pegasus constellation.) The earliest known observation of Andromeda was made by a medieval Persian, Abd al-Rahman al-Sufi, although it would not be until the 20th century that other astronomers realized the strange cloud was actually a distant galaxy filled with stars of its own. Today, scientists studying Andromeda make use of a wide range of ground-based and orbital telescopes.
As with most, if not all, similar galaxies, Andromeda owes its shape to a supermassive black hole somewhere within its core. Recent research, however, indicates that Andromeda’s core arrangement is unusual. Andromeda appears to have a cluster of numerous black holes – one recent survey counted 26 of them. The spiral arms stretch out from the core to a diameter of between 100,000 and 220,000 light-years.
Studies of the so-called redshift have confirmed that most galaxies observable from Earth are travelling away from each other at high speed. The Andromeda galaxy is an exception. Pulled together by their immense gravitational forces, Andromeda and the Milky Way are currently speeding together at a combined velocity of about 250,000 miles per hour. At that pace, they will strike one another in several billion years’ time. What happens next is still unknown. The most likely possibility, according to Tim Stephens of the University of California, Santa Cruz, is that a new, merged galaxy will form. Instead of the graceful spiral arms of Andromeda or Milky Way today, it will form into an ovoid shape known as an “elliptical galaxy.”
NASA’s Jet Propulsion Laboratory has published a large number of images of the Andromeda Galaxy, viewed via visual light as well as telescopes in the infrared and X-ray spectrum.