About the Subject

At a distance of roughly two and a half million light years, the Andromeda Galaxy (M31) is the closest spiral galaxy to our own Milky Way. It is also the largest member of our own Local Group of galaxies. It is approximately 150,000 light years in diameter and has a mass of approximately one trillion Suns. With an apparent magnitude of 3.4, for most observers it is the most distant object visible to the naked eye.

Because M31 is so close to the Milky Way, it was the first galaxy other than the Milky Way in which Cepheid Variable stars were observed. The cool thing about Cepheids is that their absolute luminosity varies with rate of change of their apparent luminosities. In other words, if you know how quickly it is flickering, you know how bright it is. Careful measurements of the brightnesses of Cepheids in Andromeda in 1925 allowed Edwin Hubble to finally determine the approximate distance to Andromeda, conclusively setting the debate as to whether so-called “spiral nebulae” were really separate galaxies.