The collection of billions of the star at one place is called a galaxy. Milky Way is the best example of a galaxy. There is a strong gravitational force among the celestial bodies and this is the force which holds together the dust stars and gases. The cluster of these three forms a galaxy. Galaxies are different in size. The largest galaxy has diameter around 192000 light years while the smallest has the diameter 6600 light year. Some galaxies are in scattered form in the space while other are found in a group. Galaxies travel at the speed of light. They take just 2 seconds to move from earth to moon. Galaxies can be called as a system which turns gases into stars and stars back into gases Collision usually take place between two galaxies and the reason for this is that there is very limited space between two galaxies.

There is different classification of galaxies and this classification is on the base of their evolution history. There are three basic classes of galaxies i.e.

1) Elliptical
2) Spiral
3) Irregular
4) Quasars

Elliptical
These galaxies are named so because of their shapes. They look like football. They have smooth varying brightness. All Elliptical galaxies are almost of same colour.They are redder than the sun. The largest galaxies of the universe is the are giant elliptical galaxies.

Spirals
Spiral galaxies are found in the shape of flat disc of stars. They have a bright bulging ball in the mid. This bulging part is called nuclei. There is a spiral arms which wrap around this bulging parts. That’s why they are called Spiral galaxies. These spiral shape arms around the disc are formed because of the movement of waves over the flat disc.

Irregular
The galaxies which are neither Elliptical nor spiral are called Irregular galaxies. In irregular galaxies stars gas and dust are spread over randomly. Irregular galaxies do not have any identified form. Irregular galaxies are known as the small galaxies. They contain around one million stars.

Quasars
Quasars galaxies are discovered in the early 1960’s.Quasars have the energetic nuclei. And the radiation of these nuclei is so strong that it overwhelms the light of other galaxies. This nuclei is small in size but it emits radiation 100 times of the radiation of the entire galaxies.