4. Stars

Stars are the heavenly bodies like the sun that are extremely hot and have light of their own. 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 heat and light which makes a star shine. Thus, a star is a hydrogen nuclear energy furnace, so big that it holds together by itself. The stars are classified according to their physical characteristics like size, colour, brightness and temperature.

Stars are of three colours: red, white and blue. The colour of a star is determined by its surface temperature. The stars which have comparatively low surface temperature are red, the star having high surface temperature are white whereas those stars which have very high surface temperature are blue on colour. Some of the important example of the stars are: Pole (or Polaris), Sirius, Vega, Capella, Alpha centauri, Beta centauri, Proxima centauri, Spica, Regulus, Pleiades, Aldebaram, Arcturus, Betelgeuse, and of course, the Sun.

All the stars (except the pole star) appear to move from east to west in the night sky. This can be explained as follows: the earth itself rotates on its axis from west to east. So, when the earth rotates on its axis from west to east, the stars appear to move in the opposite direction, from east to west. Thus, the apparent motion of the 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 but the stars appear to be moving in the sky. Thus, it is due to the rotation of earth on its axis that we see the stars changing their positions in the sky as the night progresses.