NASA's Hubble traces hidden history of Andromeda galaxy

In the years following the launch of NASA's Hubble Space Telescope, astronomers have tallied over 1 trillion galaxies in the universe. But only one galaxy stands out as the most important nearby stellar island to our Milky Way -- the magnificent Andromeda galaxy (Messier 31). It can be seen with the naked eye on a very clear autumn night as a faint cigar-shaped object roughly the apparent angular diameter of our Moon. A century ago, Edwin Hubble first established that this so-called 'spiral nebula' was actually very far outside our own Milky Way galaxy -- at a distance of approximately 2.5 million light-years or roughly 25 Milky Way diameters.