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 The Andromeda Galaxy

Although several dozen minor galaxies lie closer to our Milky Way, the Andromeda galaxy is the closest spiral galaxy to ours. Excluding the large and small Magellanic Clouds, visible from Earth's southern hemisphere, the Andromeda galaxy is the brightest external galaxy you can see. At 2.5 million light-years, it's the most distant thing most of us humans can see with the unaided eye. Astronomers sometimes call the Andromeda galaxy by the name Messier 31, or M31. Its diameter is approximately 200,000 light-years, making it the largest member of the local group, and it shares various characteristics with the Milky Way system. Andromeda galaxy was the 31st on a famous list of fuzzy objects compiled by the French astronomer Charles Messier (1730-1817). Most Messier objects are star clusters or gas clouds in our Milky Way galaxy. But the Andromeda galaxy is a whole separate galaxy, even bigger than our Milky Way. In a dark sky, you can see that it's like a smudge of distant light larger than a full moon. The galaxy's name stems from the area of Earth's sky in which it appears: the constellation of Andromeda. The mass of both, Andromeda and Milky Way, is difficult to estimate with accuracy, but it was long thought that the Andromeda galaxy is more massive than the Milky Way by a margin of some 25% to 50%. The number of stars contained in the Andromeda galaxy is estimated at one trillion, or roughly twice the number of stars in the Milky Way.

At one time, the Andromeda galaxy was called the Great Andromeda Nebula. Astronomers thought this patch of light was composed of glowing gases or was perhaps a solar system in the process of formation. It wasn't until the 20th century that astronomers were able to resolve the Andromeda spiral nebula into individual stars. This discovery lead to a controversy about whether the Andromeda spiral nebula and other spiral nebulae lie within or outside the Milky Way. In the 1920s, Edwin Hubble finally put the matter to rest, when he proved that the Andromeda galaxy indeed resides beyond the bounds of our Milky Way galaxy. The Andromeda galaxy has formed roughly 10 billion years ago from the collision and subsequent merger of small protogalaxies. During this time, its rate of star formation would have been very high, to the point of becoming a luminous infrared galaxy for roughly 100 million years. Andromeda and the Triangulum galaxy has a very close passage 2-4 billion years ago. This event produced high rates of star formation across the Andromeda galaxy's disk. Over the past 2 billion years, however, the star formation throughout Andromeda's disk is thought to have decreased to the point of near inactivity. Like the Milky Way, Andromeda galaxy has satellite galaxies, consisting of over 20 known dwarf galaxies. The best known and most readily observed satellite galaxies are M32 and M110. There have been interactions with satellite galaxies like M32, M110, or others that have already been absorbed by the Andromeda galaxy. The Triangulum galaxy is a non-dwarf galaxy that lies 750,000 light-years from Andromeda. It is currently unknown whether it is a satellite of Andromeda or not.

Our understanding of the size of the Andromeda galaxy has grown bigger in recent years. In 2015, observations from the Hubble Space Telescope found that a halo of material surrounding Andromeda is six times larger and 1,000 times more massive than what was previously measured. Scientists released the most detailed photo of Andromeda ever using a mosaic of images from the Hubble Space Telescope. This image revealed more than 100 million stars within the galaxy, as well as dust structures and other features. Blackhole activities within Andromeda also came under scrutiny. A search using NASA's Chandra X-ray telescope yielded 26 black hole candidates in Andromeda in 2013, making this the biggest catch of sudden candidates ever found in another galaxy besides our own Milky Way. Another 40 black holes were tracked down in 2016 using NuSTAR. Scientists tracked down a probable pulsar (a dead star that spins rapidly) in Andromeda in 2017. The newer observations found that this object's spectrum of light appears similar to pulsars in the Milky Way.

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. Andromeda galaxy is approaching The Milky Way at about 110 kilometers per second; and they are expected to collide in around 4-5 billion years, merging to form a giant elliptical galaxy or a large lenticular galaxy. Andromeda galaxy's tangential or sideways velocity with respect to the Milky Way is relatively much smaller than the approaching velocity and therefore it is expected to collide directly with the Milky Way in about 4 billion years. The fate of Earth and the Solar system in the event of collision is currently unknown. Before the galaxies merge, there is a small chance that the Solar system could be ejected from the Milky Way or join the Andromeda Galaxy.