There are two reasons why all the stars in the night sky do not shine equally brightly. Firstly, the stars are at different distances from the Earth. Secondly, their luminosity depends on their age. Young stars are bright, hot and shine bluish-white, and older stars which have already given out a lot of their energy are red in colour and are cooler. Our sun is middle aged (about 4.8 billion years old), with a temperature of around 5500'C on its surface, and a yellow shine. Stars are like a window to the past. If a star is 10 light years away, its light takes 10 years to reach us. Consequently, we see the star not as it is 'now', but as it was 10 years ago.
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