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In this article we are discussing the types and classification of galaxies.

Galaxy: It is a large congregation of stars bound together by gravitational force. The stars in the respective galaxy keep rotating around its own center of mass. The diameters of individual galaxies range from 80,000-150,000 light years.

Formation of stars:

a) A galaxy begins with the accumulation of a large hydrogen cloud in the form of Nebula. 

b) This growing Nebula gets developed into localised clumps of gas. 

c) These clumps keep growing into denser gaseous bodies, which eventually begin the process of formation of stars. 

Based on their shape, the galaxies can be classified in three categories that are as follows:

Spiral Galaxy : 

a) About 70% of galaxies discovered are in spiral shape, including ours i.e. the Milky Way.

b) These are flat and disk-shaped with a bulge in the center. This bulge is produced by tightly packed group of stars in the center of the galaxies. The outer form, a bar-like structure is formed due to the congregation of new stars.

c) Our Sun is a new star. Our solar system is located in the outer periphery of our galaxy the Milky Way. Solar system took around 250 million years to revolve around the center of the Milky Way. So our cosmic year is of 250 million years.

d) The closest galaxy to ours is Andromeda.

Elliptical Galaxy :

a) Elliptical galaxies are more three-dimensional in structure.

b) The stars in these galaxies are located in random orbits around the center. They comprise more older and low-mass stars.

c) These galaxies contain various masses. The ecliptic dwarf type are one of the smallest galaxies with approximately 1 million solar masses, whereas the ecliptic giant type of galaxies are worth ten thousand billion solar galaxies.

d) Around 14% galaxies are elliptical galaxies.

Irregular Galaxy: 

a) Such galaxies do not have any distinct or regular shape. Some of these were once spiral or elliptical but were deformed by disorders in the gravitational pull.

b) They contain abundant amount of gas and dust. The amount of gas present in them is around 10-20%.

c) Only 3% of galaxies are irregular.

d) They are divided into 2 major Hubble types. 

e) They are Irr I (with some structure) and Irr II (without any structure).

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