14、The sun is the closest star to earth. It takes 8 minutes for the light from the sun to reach our earth. The distance from the sun to earth is one astronomical unit（天文单位）. It is about 150 million kilometers.
Energy from the sun heats the earth's surface. The earth would be too cold for life without our star's heat energy. Other stars are too far away to give the earth enough heat energy for life. Scientists wonder if there is life near other stars.
Our sun is an average（平均的） size star. It appears like the largest one, because it is the closest star. Some stars are smaller than our sun. Very small stars are called dwarf（短小的）stars. Some of them are no larger than our small planets. Some stars are much larger than our sun. The largest stars are called giant（巨大的）stars.
Not all stars are the same distance from the earth. The different distances make some giant stars appear very small. Astronomers （天文学家）use light-year to talk about distance in space. A light-year is the distance the light travels in one year. The star, Alpha Centauri is the closest star besides our sun. It is over 4 light-year away from earth. It takes light from this star over four years to reach the earth.