A large group of stars is called galaxy. There are large number of galaxies having different sizes – regular and irregular shape. Galaxies can be divided into two categories
(i) Normal galaxies
(ii) Radio galaxies
(i) Normal galaxies. These galaxies emit comparatively small amount of radio radiations as compared to the total radiations emitted. These galaxies are bright from the centre and gradually dim towards the edges. Each normal galaxy contains billions of stars in the form of a band, travelling together in the universe. Depending upon their shapes, the normal galaxies may be divided into three groups; namely :
(a) Eliptical galaxies
(b) Spiral galaxies
(c) Irregular galaxies
About 18% of the galaxies are elliptical, 80% of the galaxies are spiral and only 2% of the galaxies are irregular galaxies. The irregular galaxies are youngest, spiral galaxies are middle aged and elliptical galaxies are quite old.
Example of normal galaxies is Milky Way and Andromeda galaxy.
(ii) Radio galaxies. These galaxies emit million times more radio radiations than normal galaxies. The radio radiations do not come from the galaxy itself but are believed to be coming from two large radio sources.
Milky way is the name of the galaxy to which our earth belongs. The Milky Way is the glowing belt of the sky formed by the combined light of a very large number of stars. If one looks at the sky on a clear night, a hazy band of white light stretching across a great circle is seen, which is called the Milky Way or Akash Ganga. It is so called because the light from the various stars together gives the impression of stream of milk flowing of a stream of milk flowing across the sky.