Number 31 in Messier's famous catalogue of "Things not to be confused with Comets"! Originally described in the year 964 and thought to be a nebula but despite persistent theorising by many astronomers and philosophers over the centuries it was not until 1925 that Edwin Hubble proved conclusively that the "Andromeda Nebula" is a whole separate galaxy like our own. At a distance of 2.5 million light years from the Milky Way, M31 has a diameter of approximately 220,000 light years and contains upwards of 1 trillion stars. It is the most distant object in the night sky that is visible with the naked eye. All the other structures visible to the unaided eye are actually within our own galaxy.

The Andromeda Galaxy is approaching the Milky Way at about 110 kilometres per second (68 miles per second) and the two will collide and combine in 4 to 5 billion years.

More details to be found of course on Wikipedia at https://en.wikipedia.org/wiki/Andromeda_Galaxy.