Clusters of stars set the interstellar medium ablaze in the Andromeda Galaxy about 2.5 million light-years away. Also known as M31, Andromeda is the Milky Way’s closest major galaxy. It measures approximately 152,000 light-years across and, with almost the same mass as our home galaxy, is headed for a collision with the Milky Way in 2-4 billion years. In the meantime, Andromeda remains an object of study for many astronomers.
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