[9급 국가직 영어 2014년 15번]

The earth is a planet full of life. One of the reasons for this is that our sun is the kind of star that can support life on a planet. All the time the sun continues to send out a steady supply of heat and light. For our sun is a stable star. ① This means that it stays the same size. And its output of energy (heat and light) does not change much. ② Some stars are not stable. They grow bigger and hotter and then smaller and cooler. ③ The heat and light they send out vary greatly. If our sun behaved like that, the earth would boil and freeze repeatedly. ④ Life could exist under these great changes. We are here because a steady amount of energy pours forth from our sun.