Andromeda Galaxy

Andromeda lies about 2.5 million light-years away and can be seen with the naked eye on a dark night. It is the biggest large galaxy close to our own.

Distance estimates have been refined over the years and may shift a little more.

Context

Andromeda, also called M31, is visible as a faint smudge from dark sites and has been recorded by observers for roughly a thousand years. For centuries it was thought to be a nebula inside our own galaxy. In the 1920s Edwin Hubble used Cepheid variable stars, whose pulsation rate reveals their true brightness, to show that it lay far outside the Milky Way.

That measurement opened up the idea that the universe is full of other galaxies. Modern estimates, using Cepheids and other distance markers, put Andromeda at about 2.5 million light-years. Values quoted in the literature differ slightly, which is why the figure is given as approximate.

It is a large spiral galaxy, wider than the Milky Way, and is thought to hold on the order of a trillion stars. It is moving toward us at roughly 110 kilometres per second. Astronomers expect the two galaxies to merge in several billion years, although the exact timing and whether a collision is certain are still being studied.

Because it is so near, Andromeda is a key laboratory for studying how spiral galaxies form, what their stars are like and how dark matter is spread out.