A galaxy is a gravitationally bound system or collection of stars, gas rotating around super massive black holes, dust, and dark matter.
There are 20 crore or more galaxies in the Universe. The Galaxy's can be range in the number of stars and its size.
Some are small galaxies, such as dwarf galaxies, can contain tens of millions of stars. Some are big galaxies, such as massive galaxies can contains hundreds of trillions of stars. The diameter of an average galaxy can reach 326,000 light years.
According to Hubble Classification System, galaxies categorized into three main categories:
spiral, irregular, and Elliptical galaxy
Spiral galaxies are broken into two main types, normal and barred. The Milky Way, is thought to be a barred spiral galaxy with a black hole at its center. Spiral galaxies we comprise up to half of all the galaxies in the universe.
Irregular galaxies are classified into three main categories; Irr-I, Irr-II, and dIrr. Some irregular galaxies are deformed by the gravity of other galaxies.
Elliptical galaxies make up only 10% - 15% of all galaxies in the universe. That's formation is not fully understood, it is believed that elliptical galaxies are sometimes formed from the merging of two or more galaxies.
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