The Andromeda galaxy, our Milky Way's closest neighbor, is the most distant object in the sky that you can see with your unaided eye — but only on a clear night from a location with a very dark sky. The galaxy is a beautiful sprial, but one fact you may not be aware of: We’re safe for a few billion years, but Andromeda is headed our way and on a collision course with the Milky Way. Here we explain that cosmic train wreck, plus explain Andromeda's location, shape and other facts.

The Andromeda Galaxy's Coat of Many Colors
The Andromeda Galaxy's Coat of Many Colors
Credit: ESA

Location, location, location

This mosaic of M31 merges 330 individual images taken by the Ultraviolet/Optical Telescope aboard NASA's Swift spacecraft. It is the highest-resolution image of the galaxy ever recorded in the ultraviolet. The image shows a region 200,000 light-years wide and 100,000 light-years high (100 arcminutes by 50 arcminutes).
This mosaic of M31 merges 330 individual images taken by the Ultraviolet/Optical Telescope aboard NASA's Swift spacecraft. It is the highest-resolution image of the galaxy ever recorded in the ultraviolet. The image shows a region 200,000 light-years wide and 100,000 light-years high (100 arcminutes by 50 arcminutes).
Credit: NASA/Swift/Stefan Immler (GSFC) and Erin Grand (UMCP)

Located just to the north of the constellation bearing its name, the Andromeda galaxy appears as a long, hazy patch in the sky. It should appear as a smudge in the sky, even with moderate light pollution. If you live in a more populated place, you may have more trouble. Binoculars will clearly reveal its shape. [How to Find Andromeda]

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 260,000 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. [Andromeda Galaxy Photos: Amazing Pictures of M31]

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.

Collision course

Artist's conception of the Milkomeda galaxy a trillion years from now.
Artist's conception of the Milkomeda galaxy a trillion years from now.
Credit: David A. Aguilar (CfA)

Andromeda's proximity will be deadly to our galaxy. The two galaxies are rushing closer to one another at about 70 miles per second (112 kilometers per second). Astronomers estimate that it will collide with the Milky Way in about 5 billion years. By that time, the sun will have swollen into a red giant and swallowed up the terrestrial planets, so Earth will have other things to worry about. [Milky Way Galaxy's Head-On Crash with Andromeda in Pictures (Gallery)]

Still, the fresh influx of dust should boost star formation in the new Milkomeda galaxy, and the Earthless sun may well leave the Milky Way for good. After a messy phase, where arms project crazily from the combined pair, the two should settle into a smooth elliptical galaxy.

Galaxy collisions are a normal part of the universe's evolution. In fact, both Andromeda and the Milky Way bear signs of having already crashed into other galaxies.  Andromeda boasts a large ring of dust in its center, giving it an interesting shape. Astronomers believe this dust may have formed when it swallowed an existing galaxy.

More Andromeda facts: The galaxy that changed the universe

In 964, the Persian astronomer Abd al-Rahman al-Sufi described the galaxy as a "small cloud" in his Book of Fixed Stars, the first known report of our nearest neighbor. When Charles Messier labeled it M31 in 1764, he incorrectly credited the discovery of what was then called a nebula to the German astronomer, Simon Marius, who provided the first telescopic observation of the object.

This image of the Andromeda Galaxy is a composite of an infrared photo from ESA's Herschel space telescope and the XMM-Newton’s X-ray telescope. The infrared frame shows rings of dust that trace gaseous reservoirs where new stars are forming and the X-ray image shows stars approaching the ends of their lives.
This image of the Andromeda Galaxy is a composite of an infrared photo from ESA's Herschel space telescope and the XMM-Newton’s X-ray telescope. The infrared frame shows rings of dust that trace gaseous reservoirs where new stars are forming and the X-ray image shows stars approaching the ends of their lives.
Credit: ESA/Herschel/PACS/SPIRE/J.Fritz, U.Gent/XMM-Newton/EPIC/W. Pietsch, MPE

In the 1920s, the distant galaxy became part of the Great Debate between American astronomers Harlow Shapley and Heber Curtis. At the time, astronomers believed the Milky Way completely composed the whole of universe, and the strange patches known as nebulae lay inside of them. Curtis had spotted various nova in Andromeda, and argued instead that it was a separate galaxy.

The discussion wasn't concluded until 1925, when Edwin Hubble identified a special kind of star known as a Cepheid variable - a star whose characteristics allow for precise measurements of distance - within Andromeda. Because Shapley had previously determined that the Milky Way was only 100,000 light-years across, Hubble's calculations revealed that the fuzzy patch was too far away to lay within the Milky Way.

Hubble went on to use his measurements of the Doppler shifts of the galaxies to determined that the universe was expanding.

— Nola Taylor Redd