聚变), which happens exactly where it's the hottest. The sun is about 4.5 billion years old and it will still last six billion years or so. When it runs out of its fuel (燃料), it would develop into a red star when increasing in size.

　　Since stars are billions of light years away, it may be hard to recognize that stars are in continuous battles with themselves. All the stars have their own gravitational pull (引力) that pulls them. The surprising part is that the nuclear fusion occurring inside the star truly produces an outward (向外的) push to resist (抵抗) the gravitational force, which keeps its present shape.

　　Stars have different colors. The hottest stars are surprisingly the smallest ones and are blue in color. Their temperatures go to about 12,000 Kelvin. Although the yellow­colored stars are the middle­sized ones just like the sun, their temperatures fall to about 6,000 Kelvin. And lastly, the coolest and greatest ones are colored red and the temperatures of them are about 3,000 Kelvin.

　　When looking at the night sky, we may think that stars are up there alone. But don't be surprised that these heavenly bodies in fact come in pairs. And stars can even come in groups of 3 or even 4!

　　语篇解读：本文是说明文。作者在文中告诉我们一些有关星星的有趣事实。

　　4.What will happen when the sun runs out of its fuel?

　　A．It will disappear slowly.

　　B．It will last six billion years.

　　C．It will become a white star.

　　D．It will become larger in size.

　　解析：选D　细节理解题。根据第二段的"When it runs out of its fuel (燃料), it would develop into a red star when increasing in size."可知，当太阳用完了它的能量，它就变成了红色的星球，体积增大了。

　　5．What can we learn from Paragraph 3?

　　A．All stars are in a state of balance.

B．It's hard for the stars to keep their shapes.