Astronomers bave been able to classify stars according to colour. They have discovered that stars with different colours have different sizes,and that their surface tempera tures are also different. The larger a star is, the higher surface temperature it has.

From the above table it can be seen that stars are generally divided into five kinds: blue, white, yellow, orange and red. Blue stars are the largest ones, with a diameter of 16,000,000 kilometres. Their surface tetnperature is also the highest, as high as 25,000℃,Then there are yellow stars, 1,600,000 kilometres in diameter and 6,000℃ in surface temperature. Orange stars are smaller,They have a diameter of 1,000,000 kilometres and a surface temperature of 5,000℃.

The smallest kind of stars appear red in colOUr. Their diameter is only one-twentieth of that of blue stars, that is, 800,000 kilometres, and their surface temperature is only, 3,000℃.

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