Galaxies

A galaxy is a collection of stars, gas, dust and dark matter bound together by gravity. The word galaxy is derived from a Greek word "galaxias". Astronomers believe that there are about 170 billion galaxies in the observable universe. 

Galaxies differ enormously, yet they can be categorized according to their shape. There are spiral, elliptical, barred spiral or irregular shaped galaxies in the universe. Spiral galaxies like the Pinwheel Galaxy and the NGC 4414 galaxy consists of a flat rotating disc and a central bulge. The disc contains stars, gas and dust while the term bulge is used for a central concentration of stars in the galaxy. The spiral galaxies are so called due to the presence of spiral arms that extend from the centre to the disk. New stars are formed in the spiral arms hence they are brighter than the surrounding disk. According to Wikipedia, spiral galaxies have the following characteristics. 

• A flat rotating disc of stars
• A central bulge of older stars
• A near- spherical halo of stars
• A supermassive black hole at the centre .  (Sagittarius A* in the Milky Way)
• A near - spherical dark matter halo

 

Pinwheel Galaxy

Our own galaxy, Milky Way has proven to be a barred spiral galaxy recently. In barred spiral galaxies, spiral arms wind out from a central bar. The galaxy NGC 1300 is also another example for this type of galaxy.

Milky Way galaxy

Elliptical galaxies are shaped like a ball or an egg and they're mostly made up of old stars. They don't have much star formation activity within them. Some examples for this type of galaxies are M87, M32 and IC 1101, which is one of the largest galaxies in the observable universe.

IC 1101 galaxy : an elliptical galaxy

Irregular galaxies have no specific regular shape and they contain lots of gas and dust so most of them are helpful for the formation of new stars.

Our milky way is a part of a galaxy group known as the Local Group. A group of galaxies is the smallest collection of galaxies. Normally, galaxy groups don't contain more than 50 galaxies and they are the most common structures of galaxies in the universe. I will state more about group of galaxies later in my other posts.

The Laniakea Supercluster or the Local Supercluster is where the Local Group belongs to. Superclusters are very large structures which contain large groups of galaxy clusters or groups. 

The Local Supercluster