The nearest big spiral galaxy to the Milky Way is the Andromeda galaxy. Appearing as a smudge of light to the naked eye in the constellation Andromeda, this galaxy is about twice as big as the Milky Way but very similar in many ways. At the moment, it is about 2.2 million light years away from us but the gap is closing at 500,000 km/hour. While most galaxies are rushing away as the universe expands, Andromeda is the only big spiral galaxy moving towards the Milky Way. The best explanation is that the two galaxies are in fact a bound pair in orbit around one another. Both galaxies formed close to each other shortly after the Big Bang initially moving apart with the overall expansion of the universe. But since they are bound to one another, they are now falling back together and one very plausible scenario puts them on a collision course in 3 billion years.
Galaxies collide and interact occasionally and there are several well-known examples in the vicinity of the Milky Way. We see interacting pairs as snapshots in time and the results are often very dramatic. Long streams of stars thrown off in beautiful open spiral patterns are characteristic of these collisions and are known as tidal tails and bridges because of their origin in the strong mutual gravitational tides of the two interacting galaxies. Colliding galaxies also tend to merge with one another and the final outcome after some violent convulsions lasting a few hundred million years is another kind of galaxy called an elliptical. During this period, the gas in these galaxies can be ignited violently in a starburst creating stars at rates hundreds of times greater than normal. Galaxy interactions are not that common an event in the local neighbourhood (maybe one in a hundred galaxies) but the rates of merging and interaction is much larger at early times in the universe. Galaxy merging is fundamental to building up structure in the universe and explains many of the peculiar features of young galaxies seen by the Hubble Space Telescope.
The visible fuzzy patch of stars stretches about as long as the width of the full moon, and half as wide; only with significant magnification can you tell it stretches six times that length in fullness.
A spiral galaxy like the Milky Way, Andromeda contains a concentrated bulge of matter in the middle, surrounded by a disk of gas, dust, and stars 260000 light-years long, more than 2.5 times as long as the Milky Way. Though Andromeda contains approximately a trillion stars to the quarter to half a billion in the Milky Way, our galaxy is actually more massive, because it is thought to contain more dark matter.
Amazingly, this stretch of stars, which in our sky appears about as long as the full moon and half as wide, lies 2.5 million light-years away, further than any star you can see with your eyes. Also known as M31, it is the closest galaxy to the Milky Way – and it’s moving closer every day.
Of all members of the Local Group M31 is considered to have the closest external resemblance to the Milky Way, thus it is often referred to as a ‘sibling galaxy’. M31 is an ‘island universe’ – a gigantic collection of billions of stars estimated to be 2.54 million light years distant. It has been observed since ancient times and was first catalogued as long ago as 905 AD. The common name of M31 derives from Charles Messier’s entry # 31 in his famous Messier catalogue in August 1764.