Stars are the heavenly bodies that are extremely hot and incandescent (have light of their own). Stars are in reality huge glowing balls of hot gases. Stars are mainly made up of hydrogen. They also contain some helium and dust. In stars, hydrogen is continuously converted into helium, and this process is accompanied by the Liberation of tremendous amount of energy. The stars radiate this energy in the form of heat and light.
There are billions and billions of stars in the Universe, however only about 2000 stars can be seen with naked eye. At first sight, all the stars appear white; however, closer examination reveals that they have different colours ranging from red to blue. The colour of the star gives us information about its temperature. The reddest stars are the coolest with the surface temperature as low as 30000 C. The blue stars are the hottest with surface temperatures as high as 250000 C or more. The sun is a yellow- white star. Its surface temperature is about 60000 C.
The stars have wide range of sizes. The largest stars (such as Betegleuse in the constellation of Orion) have diameters about 3000 times that of our sun. The smallest stars (white dwarfs) have diameters about the one hundredth the diameter of the sun. Our sun is a medium-sized having medium, temperatures and medium brightness. Since the stars have different physical characteristics such as size, colour and brightness, they are often classified on the basis of these characteristics.
If we observe the stars in the night sky, we notice that the stars appear to be moving from east to west with time. This due to the rotation of the earth around its axis. The earth rotates around its axis from west to east and as a result, the stars appear to be moving from east to west. Thus, the apparent motion of the stars in the sky is due to the rotation of the earth around its axis. The pole star is located almost vertically above the north pole (of the earth’s axis of rotation) and remains almost stationary in the sky. All other stars appear to be revolving around this star.
The Stars are not permanent
We observe that stars remain unchanged year after year. This may lead us to the conclusion that the stars we see are unchanging and are permanent.
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However, this is not true. Stars, like people, are born, mature, grow old and finally come to an end. During this ageing the physical characteristics, such as colour, brightness and size of the star also change. The changes in the stars take place very slowly.
The stars appear to be permanent because a star may not undergo any observable change during the short period of our life span. It is not possible to study the changes taking place in stars by observing a particular star because a star may not undergo any appreciable change during the short period of observations. Scientists have overcome this difficulty by studying many stars which are at different stages of development.
The universe contains millions and millions of stars which are of different ages; some are middle aged whereas still others are in the final stages of their life. Our sun is a middle aged star. From the study of the various stars which are in different stages of life we can find out how the stars evolve. Now let us study the various stages in the life of a star.