Galaxies: Everything you need to know

You are reading this blog sitting on a Pale Blue Dot, which is part of a Solar System, a part of an even larger structure called the Milky Way Galaxy or “Aakash Ganga” in Indian Astronomy.

The Milky Way was seen since around 400 BC but the earliest identification of the Andromeda galaxy was during the 10th Century as a “small cloud”. Such visible galaxies were first considered nebulas. In 1920 a debate took place between Harlow Shapley and Heber Curtis, are these nebulas or galaxies outside our own Milky Way Galaxy?

Edwin Hubble then resolved the debate by using a new 100-inch Mt. Wilson telescope, identifying Cepheid variables (stars with changing brightness) thus determining the distance to the “Andromeda Nebula”. He concluded that it was too distant to be situated in our galaxy, thus it became the first galaxy to be known other than Milky Way and our Universe just got bigger way bigger!

What exactly is a galaxy?

A galaxy is a massive collection of dust, gas, and stars held together by gravity. A typical galaxy consists of tens to hundreds of billions of stars out of which some may have their own solar system. It is also home to Black holes, Supernovas, Pulsars, Asteroids, and Comets.

https://stargazingmumbai.in/galaxies/

There are other types of galaxies due to different parameters of classification, they are Starburst Galaxy, Interacting, Active Galaxy (Blazars, LINERS, Seyfert, Quasar), and Luminous Infra-Red Galaxies.

How are they Formed?

Galaxies are estimated to be formed in the “Cosmic Dark Ages” after BIG BANG (which was 13.8 billion years ago). There are two models.

Top-Down Approach: – Galaxies were formed from huge gas clouds larger than the resulting Galaxies. If the gas cloud was slowly rotating, then the stars would form first then the disc resulting in an elliptical galaxy and if it was rotating faster then the disc would form first giving us a spiral galaxy.

Bottom-Up approach: – Large gas cloud was fragmented into smaller lumps and then these lumps combined and formed galaxies.

Irregular galaxies are formed due to gravitational interaction and the merging of other galaxies. Through interactions, galaxies get distorted and lose their shapes thus having irregular shapes.

Almost all galaxies have a Super Massive Black Hole (SMBH) at their center. Our own Milky Way has an SMBH in its center which is roughly 4 million times more massive than our sun. These SMBHs play a significant role in the evolution of galaxies. Another route of evolution is mergers; they are thought to be the reason for the formation of large elliptical galaxies.

Our Milky Way is going to collide with Andromeda Galaxy 2.5 Billion years from now, still, there is very less probability that the stars will collide with each other. The SMBHs of both galaxies will interact with each other and this Cosmic dance of two black holes will create Gravitational Waves from the merger. The resulting structure will be an elliptical galaxy and would be called “Milkomeda”!!