A star is a huge, more or less spherical mass of glowing gas emitting large amount of radiant energy. Billions of stars form a galaxy. There are three types of stars. They are (i) double and multiple stars (ii) intrinsically variable stars and (iii) Novae and super novae.
In a galaxy, there are only a few single stars like the Sun. Majority of the stars are either double stars (binaries) or multiple stars. The binary stars are pairs of stars moving round their common centre of gravity in stable equilibrium. An intrinsically variable star shows variation in its apparent brightness. Some stars suddenly attain extremely large brightness, that they may be seen even during daytime and then they slowly fade away. Such stars are called novae. Supernovae is a large novae.
The night stars in the sky have been given names such as Sirius (Vyadha), Canopas (Agasti), Spica (Chitra), Arcturus (Swathi), Polaris (Dhruva) ... etc. After the Sun, the star Alpha Centauri is nearest to Earth.
The Sun is extremely hot and self−luminous body. It is made of hydrogeneous matter. It is the star nearest to the Earth. Its mass is about 1.989 × 1030 kg. Its radius is about 6.95 × 108 m. Its distance from the Earth is 1.496 × 1011 m. This is known as astronomical unit (AU). Light of the sun takes 8 minutes 20 seconds to reach the Earth. The gravitational force of attraction on the surface of the Sun is about 28 times that on the surface of the Earth.
Sun rotates about its axis from East to West. The period of revolution is 34 days at the pole and 25 days at the equator. The density of material is one fourth that of the Earth. The inner part of the Sun
is a bright disc of temperature 14 × 106 K known as photosphere. The outer most layer of the Sun of temperature 6000 K is called chromosphere.
Most of the stars appear to be grouped together forming interesting patterns in the sky. The configurations or groups of star formed in the patterns of animals and human beings are called constellations. There are 88 constellations into which the whole sky has been divided.
If we look towards the northern sky on a clear moonless night during the months of July and August, a group of seven bright stars resembling a bear, the four stars forming a quadrangle form the body, the remaining three stars make the tail and some other faint stars form the paws and head of the bear. This constellation is called Ursa Major or Saptarishi or Great Bear. The constellation Orion resembles the figure of a hunter and Taurus (Vrishabha) resembles the shape of a bull.
A large band of stars, gas and dust particles held together by gravitational forces is called a galaxy. Galaxies are really complex in nature consisting of billions of stars. Some galaxies emit a comparatively small amount of radio radiations compared to the total radiations emitted. They are called normal galaxies. Our galaxy Milky Way is a normal galaxy spiral in shape.
The nearest galaxy to us known as Andromeda galaxy, is also a normal galaxy. It is at a distance of 2 × 106 light years. (The distance travelled by the light in one year [9.467 × 1012 km] is called light year). Some galaxies are found to emit millions of times more radio waves compared to normal galaxies. They are called radio galaxies.