Andromeda galaxy

At a distance of about 2.5 million light years, the Andromeda galaxy (also known as NGC 224 and M31) is the nearest galaxy to the Earth apart from smaller companion galaxies such as the Magellanic Clouds.
Like the Milky Way, Andromeda is a spiral galaxy. It can be spotted with the naked eye and so has been known to humans for a very long time.
In the 1920s astronomer Edwin Hubble confirmed that Andromeda is another galaxy and not just a gas cloud in the Milky Way as was previously thought.
Image: A mosaic view of the Andromeda galaxy created from 10 Galaxy Evolution Explorer images (credit: NASA/JPL-Caltech)