M31 - Andromeda Galaxy

The Andromeda Galaxy, M31, is the nearest major galaxy to our Milky Way at 2.5 million light years distant.  At 220,000 light years in diameter, it is significantly larger than the Milky Way and recent data indicates that it contains greater than twice the number of stars at nearly 1 trillion making M31 the most massive of the Local Group galaxies.  The bright central bulge of the galaxy has an apparent magnitude of 3.4 making this one of the most distant objects visible to the naked eye.  Blue light emanates from young stars which is contrasted by the dark dust lanes that make the spiral shape of the galaxy more evident.  The diffuse bright ellipses above and below the plane of the galaxy are two of M31's satellite dwarf galaxies known as M32 (smaller above) and M110 (larger below).