Galaxy 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 many different shapes and size. The galaxies are usually classified on the basis of their shape. Three types of galaxies are discussed below:
- Spiral galaxies: They are the most familiar type in the universe. Our Milky Way is an example of a spiral galaxy. Spirals are large rotating disks of stars and nebulae which are bounded by a shell of dark matter.
- Elliptical galaxies: Elliptical galaxies are almost egg-shaped which is present in galaxy clusters and smaller compact groups. Elliptical may have hundred million to hundred trillion stars, and they can range in size from a few thousand light-years across to more than a few hundred thousand.
- Irregular galaxies: Irregular galaxies are irregular in shape. Small Magellanic Cloud is an example of the irregular galaxy. Irregulars generally do not have sufficient structure to describe them as spirals or ellipticals. They may have the active area of star arrangement, and a number of smaller ones are listed as “dwarf irregulars”.