The Andromeda galaxy, also known as M31, is a spiral galaxy about 2.5 million light years away from the Milky Way. It gets its name because it's found in Andromeda, a northern hemisphere constellation. A naked-eye object, the Andromeda galaxy is a long, hazy patch in the sky that looks like a smudge of light. Using binoculars helps viewers to clearly see its shape.
The Andromeda galaxy is about 260,000 light years long, making it almost three times longer than the Milky Way. Scientists believe that it holds a trillion stars, which is many more than the Milky Way holds. Scientists also believe that the Milky Way is a more massive galaxy due to the sheer amount of dark matter it contains.
Andromeda is also the home of the first Cepheid variables to be discovered. These are brilliant stars, and their brightness changes over time in a predictable way. They allowed astronomer Edwin Hubble to determine that Andromeda was another galaxy that sits very far away from the Milky Way.
Another interesting fact about the Andromeda galaxy is that it is supposed to collide with the Milky Way in about 5 billion years. After a period of time, the two galaxies are expected to merge into an elliptical galaxy.