Wednesday, December 10, 2008

Andromeda Galaxy

Andromeda is the nearest major galaxy to the Milky Way. These two galaxies dominate the Local Group of galaxies. Also known as M31, Andromeda is a spiral galaxy which lies about 2.5 million light years away in the constellation Andromeda. It is visible with the naked eye as a faint smudge on a moonless night. Although Andromeda is the largest galaxy, it may not be the most massive, as recent astronomical observations suggest that the Milky Way contains more dark matter and, in consequence, may be the most massive in the group.

The first description of the Andromeda Galaxy based on telescopic observation was given by Simon Marius in 1612, and Charles Messier catalogued it as object M31 in 1764. In 1785, the astronomer William Herschel noted a faint reddish hue in the core region of the M31. He believed it to be the nearest of all the great nebulae and, based on the color and magnitude of the nebula, he wrongly estimated that it was no more than 2,000 times the distance of Sirius. In 1864, William Huggins observed that the spectrum of M31 differed from a gaseous nebula. The spectra of M31 displayed a continuum of frequencies, superimposed with dark lines. This was very similar to the spectra of individual stars. From this it was deduced that M31 had a stellar nature and, therefore, that it was a galaxy.