Andromeda galaxy

Galaxy 2.2 million light years away from Earth in the constellation Andromeda, and the most distant object visible to the naked eye. It is the largest member of the Local Group of galaxies.

Like the Milky Way, it is a spiral orbited by several companion galaxies but contains about twice as many stars as the Milky Way. It is about 200,000 light years across.

In 1993 US astronomers detected two components at the centre of Andromeda, indicating that it may have a double nucleus. The fainter of the two components lies at the exact centre of Andromeda while the rest of the galaxy orbits it. A black hole with a mass tens of millions times that of the Sun has been detected here. The other, brighter component of the ‘double nucleus’ may be a remnant of a galaxy that collided with Andromeda, although it was not fully understood by the end of the twentieth century.