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).

Messier 31 is located 2.5 million light-years away from Earth and was a much bigger target in the sky than the galaxies Hubble routinely observes.

Andromeda is the closest large galaxy to the Milky Way. And it is getting closer, as the two will merge in several billion years. It has been an object of study for centuries, but we have never seen it like this.