Galaxy

Galacy is coming from Greek word “galaxi” and is a vast collection of billions of stars, dust and hydrogen gas, isolated in space from similar systems. Galaxies are the building blocks of the universe. There are about 100 billion galaxies in the universe, and each galaxy has on an average 100 billion stars. The two important galaxies in the universe are:

• Milky way galaxy
• Andromeda galaxy

Milky Way galaxy comprises of billions of stars, hydrogen gas and dust. It is 100,000 light years in diameter. The solar system is a part of the Milky Way galaxy. The galaxies were formed by the shrinking of vast over dense clouds of hydrogen gas and dust.

Classification of Galaxies

Galaxies have several different shapes and sizes. The galaxies are commonly classified on the basis of their shapes. There are different types of galaxies in the universe are spiral galaxies, elliptical galaxies, and irregular galaxies. Sone of the brightness galaxies are elliptical galaxies but spiral galaxies are commonly much bigger than others. The example of spiral galaxy is Milky Way, which is about 100,000 light years in diameter and is slowly rotating. Three types of galaxies are given below

1. Elliptical galaxies – These galaxies are almost egg-shaped, which is present in galaxy clusters and smaller compact groups. Elliptical galaxies may have hundred million to hundred trillions stars, and they can change in size from a few thousand light years across to more than a few hundred thousand. The scientists think that elliptical galaxy haa a central super massive black hole, which is related with the mass of the galaxy. Messier 87 is an example of an elliptical galaxy. There are a number of subgroups of ellipticals including ‘dwarf ellipticals”. The tightly knit groups of stars are called as globular clusters.
2. Spiral galaxies – Spiral galaxies are the most familiar type in the universe. Our Milky Way is an example of spiral galaxy. Spiral gakaxies are large rotating disks of stars and nebulae, which are bounded by a shell of dark matter. The central bright region at the core of a galaxy is called as the ‘galactic bulge’. Several spiral galaxies have a halo of stars and star clusters arrayed above and below the disk.
3. Irregular galaxies – Irregular galaxies are irregular in shape. Small Magellanic Cloud is an example of irregular galaxy. Generally Irregular galaxies do not have sufficient structure to describe them as spirals or ellipticals. They may have active area of star arrangement and a number of smaller ones are listed as “dwarf itregulars’. It is very similar to the very earliest galaxies, which created of about 13.5 billion years ago. Irregulars are describe by their structures