Chapter 1: Introduction to Andromeda

In our ongoing series aimed at novice astronomers, we delve into key facts about various celestial objects. Today, we focus on the Andromeda galaxy, notable for being the closest galaxy to our Milky Way (excluding dwarf satellite galaxies) and for its impending collision with our galaxy.

Andromeda stands out as one of the few galaxies that can be seen with the naked eye.

Its visibility made Andromeda the first galaxy identified by astronomers, with records tracing back to the 10th century. Initially mistaken for a nebula, it earned the name "Andromeda Nebula." British astronomer William Huggins was the first to realize in 1864 that Andromeda was composed of numerous stars, although it was still thought to be part of the Milky Way at that time.

In the early 20th century, American astronomer Heber Curtis observed a supernova in Andromeda. His analysis of earlier photographs revealed around ten additional supernova events. These explosions were, on average, ten times less powerful than those seen in other galaxies, prompting Curtis to propose that Andromeda might be much more distant than previously thought. He estimated its distance to be 500,000 light-years, though his findings were met with skepticism.

The debate over Andromeda's location was ultimately resolved by Edwin Hubble in 1925. By discovering Cepheids, a type of variable star, within Andromeda, he was able to accurately measure its distance, arriving at a figure close to the current estimate of approximately 2.5 million light-years. Notably, Andromeda is significantly larger than the Milky Way, with a radius twice that of our galaxy and at least double the number of stars.

The supermassive black hole at the center of Andromeda has a mass about 25 times greater than the one in our galaxy, estimated at around 100 million solar masses. Furthermore, Andromeda possesses a more substantial halo of dark matter than the Milky Way, although the formation of new stars occurs at a rate four times slower than in our galaxy.

As Andromeda approaches us at roughly 300 kilometers per second, a merger between the two galaxies is predicted to occur in about 4.5 billion years.

The collision, while seemingly dramatic, will unfold without significant effects on most stars and planets due to the vast distances involved. The likelihood of direct collisions between stars or planets during this merger is nearly zero, and the entire process will take hundreds of millions of years.

For a more detailed exploration of the impending collision between our galaxy and Andromeda, check out our previous articles.

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