The Andromeda galaxy: our cosmic neighbour

The Andromeda galaxy, also known as M31, is a spiral galaxy located about 2.5 million light-years away from the Milky Way, making it the closest galaxy to us. It is also the farthest object that can be seen with the naked eye on a dark night. It is for this reason that we have references to observations of this galaxy since many centuries ago, as we will explain later.

Characteristics of Andromeda

Andromeda is the largest galaxy in the Local Group. The latter is a group formed by more than 30 galaxies, including the Milky Way. With a diameter of about 150,000 light-years, Andromeda is 50% larger than our galaxy, and contains more than a trillion stars, while the Milky Way is formed by about 400 million stars.

Despite this clear difference in size, our galaxy is more massive. The reason for this lies in dark matter. This type of matter is invisible (it doesn’t interact with light). The only way to detect it nowadays is through the gravitational effects it has on galaxies or other celestial bodies. This type of matter is the most common in the universe, representing 85% of the existing matter. The Milky Way contains much more dark matter than Andromeda, making it more massive.

Like most galaxies in the universe, Andromeda has a supermassive black hole at its centre, with a mass millions of times greater than that of the Sun. In addition, this galaxy possesses what we call satellite galaxies. In the same way that the Moon orbits the Earth, there are at least 13 small galaxies that orbit Andromeda. The largest of them is M110, which can be seen with a basic telescope.

Discovery and observations

As we explained earlier, the Andromeda galaxy can be easily seen on a dark night without needing a telescope. The oldest record in which Andromeda is mentioned dates back to 965 AD, attributed to the Persian astronomer Abd al-Rahman al-Sufi. In 1612, shortly after the invention of the telescope, the German astronomer Simon Marius gave the first description of the galaxy, based on observations with a telescope.

For centuries, astronomers considered Andromeda a component of the Milky Way, as it was believed to be a nebula. Hence its first name: the Andromeda Nebula. It was not until the 1920s that the American astronomer Edwin Hubble conclusively determined that Andromeda was actually a galaxy beyond the Milky Way.

Formation of the galaxy

The formation of the Andromeda galaxy began about 10 billion years ago, with the collision and subsequent merger of small protogalaxies. These were galaxies in the process of formation.

These collisions formed most of the galaxy’s halo and its disk. During this time, star formation would have been very frequent. In addition, there is strong evidence that one of Andromeda’s satellite galaxies (M32) was a normal-sized, massive galaxy, but has lost much of its stars due to the action that Andromeda has on it.

In the next 4 billion years, Andromeda and the Milky Way will merge, giving rise to a larger galaxy. Although this process may seem very violent, the space between stars is so great that the probability of two stars colliding is very low.

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