Andromeda Galaxy, cataloged as M31 and NGC 224, the closest large galaxy to the Milky Way and the only one visible to the naked eye in the Northern Hemisphere. It is also known as the Great Nebula in Andromeda. It is 2.2 million light-years away and is part of the Local Group of several galaxies that includes the Milky Way, which it largely resembles in shape and composition, although the Milky Way is a barred spiral galaxy and Andromeda is a spiral galaxy. It has a diameter of about 165,000 light-years and contains at least 200 billion stars. Its two brightest companion galaxies are M32 and M110. The light arriving at earth from the Andromeda Galaxy is shifted toward the blue end of the spectrum, whereas the light from all other cosmic sources exhibits red shift.
The Columbia Electronic Encyclopedia, 6th ed. Copyright © 2012, Columbia University Press. All rights reserved.