Andromeda galaxy is also known as M31, referring to its position as the thirty-first non-star object to be catalogued by French astronomer Charles Messier in 1774. Messier credited astronomer Simon Marius with the 1612 discovery of Andromeda. Marius was the first to describe the galaxy as seen through a telescope. Records show, however, that ancient Persian astronomer Al-Sufi observed the galaxy as early as A.D. 905 and called it the "little cloud." Not until the present century was Andromeda shown to be a galaxy. Previously, astronomers had considered it a nebula within the Milky Way. In 1912, American astronomer Vesto Melvin Slipher analyzed Andromeda with a spectrometer, an instrument that breaks light down into its component wavelengths. He discovered that Andromeda's spectrum did not match that of any known gas, but was more like the pattern made by starlight.
In 1924, American astronomer Edwin Powell Hubble first proved that Andromeda is indeed a separate galaxy. Hubble identified twelve cepheid variable stars in Andromeda and determined that they were at least eight hundred thousand light-years 69 away. This distance is much greater than the farthest reaches of the Milky Way. Thus, Hubble concluded that Andromeda was a separate galaxy. Twenty-five years later German-born American astronomer Walter Baade looked through a telescope twice as powerful as the one used by Hubble and came up with a more accurate measurement of its distance of two million light-years away.
