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| Stars |
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| A star is a dense cloud of hydrogen and helium gas contracting under its own gravitational pull. In all the stars hydrogen atoms are continuously converted into helium atoms and this process is accompanied by a large amount of energy. This energy is radiated in the form of heat and light. |
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| The stars appear to be permanent to us because the changes taking place in them are very small to be noticed by us during our life span. However, the scientists have overcome this difficulty by studying millions of stars, which are at different stages of development. Stars like us are born, they mature, grow old and finally die. Our Sun is a middle-aged star. |
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| The stars are classified according to their physical characteristics like: |
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 Brightness |
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 Colour |
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 Temperature |
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 Size |
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| Star is a huge ball of glowing gas in the sky. The sun is a star. It is the only star close enough to the earth to look like a ball. The other billions of stars are so far away that they appear to be no more than pinpoints of light - even through powerful telescopes. Scientists estimate that there are about 10 billion trillion stars in the universe. Stars are enormous objects. Even large stars look like tiny dots because they are so far away. The nearest star - other than the sun - is more than 40 million kilometers away. |
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| Stars differ greatly in colour and brightness, because they differ in temperature and size. Some stars look yellow like the sun. Others have a blue or red glow. |
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| Stars twinkle because starlight comes to us through moving layers of air that surround the earth. The stars shine by day and night, but we can see them only when the sky is dark and clear. |
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| A star is made up mainly of two gases: hydrogen and helium. The great weight of a star makes the temperature at its center high enough for a nuclear reaction between hydrogen atoms to take place. The energy released by the reaction keeps stars shining until much of the hydrogen in the star's center is used up. Stars produce nuclear energy by changing hydrogen into helium through a series of nuclear reactions. As a result of the reactions, a helium nucleus is created out of four hydrogen nuclei. When this nuclear fusion takes place, energy is released. |
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| 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 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. |
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| This is because; the earth 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. The pole star appears to be stationary and does not change its position with time because it lies on the axis of rotation of the earth, which is fixed and does not change with time. |
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