Updated: Oct 24, 2019
The Andromeda Galaxy or Messier 31 (M31) is approximately 2.5 million light years away from Earth and its the nearest major galaxy to the Milky Way. Before It was estimated to contain approximately 1 trillion stars, more then the double of stars in the Milky Way, around 300 billion stars. And it is also the largest galaxy in our Local Group. But according to new studies by Monthly Notices of the Royal Astronomical Society, the new method of measurement changes completely the understanding of our Local group. And now Andromeda Galaxy is believed to have a similar size to the Milky Way.
The Galaxy was named after Princess Andromeda, wife of Perseus in the Greek Mythology. Andromeda is also a constellation, the 19 largest in the sky and it is so bright that it can be seen by naked eyes on moonless nights. By 964 the persian astronomer had described the galaxy in his “book of fixed stars”. By 1887 is was thought to be a nebula in our galaxy. By in 1917, Heber Curtis became a proponent of the so called “island universes” after studying novas in Andromeda, stating that those spiral nebulae might actually be independent galaxies. And finally in 1925, Edwin Hubble demonstrated that Andromeda was actually a galaxy.
Though Andromeda known for being a huge galaxy, our Galaxy is more massive. The explanation given before was that the Milky Way contains more dark matter. But now it may be the fact that they have a very similar size, but the Milky way has more dark matter.
According to a photograph taken by the telescope Hubble in 1991, Andromeda has a double Nucleus, there at two black holes in its center. There are 2 events that might explain this fact. One being that The Andromeda Galaxy was formed, approximately between 6000 and 10000 million years ago, by the collision of 2 smaller galaxies and because of that there is a double nucleus. Another one is that the dust around its nucleus is so dense that cover the vision and therefore it seems to be a double nucleus.
Although the collision of the 2 precursor galaxies of Andromeda happened between 6 to 10 billion years ago, it is estimated that they were only fully forged into Andromeda around 1.5 to 3 billion years ago.
Andromeda is approaching the Milky Way by around 110 Kms⁻¹ and the collision will probably happen in about 4 billion years. The galaxy product of the collision has been nicknamed Milkomeda or Milkdromeda. Although this huge collision will happen, it will hardly affect our solar system. But by that time Earth is estimated to be already uninhabitable due to increase in sun’s size and luminosity.