The Andromeda Galaxy (also known as Messier 31 or M31) is the nearest large galaxy to the Milky Way. The spiral galaxies are around 2.5 million light years apart and on a collision course with each other. During the collison, the two supermassive black holes at the centers of both galaxies will then spiral toward each other. When they finally merge after billions of years, they'll unleash a titanic burst of energy in the form of gravitational waves, rippling through spacetime itself.
Image: NASA/ESA/CSA & kk
