Stars are the heavenly bodies like the sun that are extremely hot and have light of their own. Every star is a huge mass of hot gases and big flames are coming out of it. Stars are made up of vast clouds of hydrogen gas, some helium and dust. In all the stars (including the sun), hydrogen atoms are continuously being converted into helium atoms and a large amount of nuclear energy in the form of heat and light is released during this process. It is this light which makes a star shine. Thus, a star is a hydrogen nuclear energy furnace, so big that it holds together by itself.
There are billions and billions of stars in the sky but only about 2000 stars can be seen with the naked eye on a clear moonless night. All the stars are very big in size. They appear small because they are very far-off from us. The stars twinkle at night, that is, the intensity of their light appears to increase and decrease continuously. The stars are classified according to their physical characteristics like size, color, brightness and temperature. The star which lies immediately above the earth’s north pole and around which all other stars seem to revolve is called pole star. The Indian name of pole star is a Dhruva Tara. The pole star appears to be stationary in the sky and does not change its position at all. It has been observed that all the stars, except the pole star, appear to move from east to west.
This can be explained as follows. It has been found that the earth itself rotates on its axis from west to east direction due to which the stars appear to move in the opposite direction from east to west. Thus, the apparent motion of stars in the sky is due to the rotation of the earth on its axis. Since we are ourselves on the earth, the earth appears to be stationary to us and the stars appear to be moving in the sky. The pole star appears to be stationary and does not change its position with time because it lies on the axis of rotation of earth which is fixed and does not change with time.
The sun is also a star. It is the star around which the earth and other planets revolve. As compared with the millions of other known stars, the sun is small and of average brightness. The sun appears to be larger and brighter because it is much more nearer to the earth than any other star. Thus, the star nearest to the earth is the sun. Being nearer to us, the sun does not twinkle like other stars. The sun is extremely hot. The temperature at the surface of the sun is about 6000°C, while the temperature at its centre is about 20 million degrees celsius.