Sharon Carter, age 12, of Don Mills, Ont., Canada, for her question:
WILL THE STARS EVER BURN OUT?
The stars in our sky inhabit just a small corner in the mighty galaxy. They were there in more or less the same patterns thousands of years ago, all the while burning themselves out. However, even when our old favorites all die, there still will be stars in the sky. For new stars are being born to replace them.
The average star burns up its basic fuel at a fantastic rate. It is, of course, a stupendous nuclear furnace, and its basic fuel is hydrogen gas. Our sun, for example, uses about 564 million tons of hydrogen fuel every second. It has been blazing away at this extravagant rate for about 5 billion years. So far it has consumed no more than half of its original fuel.
An average star starts out with enough hydrogen fuel to last perhaps 12 billion years. But this depends on how fast it burns. A huge star tends to be hot and bright, and it burns up its fuel at a great rate. If it is about 10 times more massive than our sun, it may burn itself out in a few million years.
A star that lives a long time is usually a small one, perhaps about one tenth as massive as our sun. It burns more slowly, and its fiery furnace may have a life expectancy of 500 billion years. However, at long last every star comes to the end of its fuel and the blazing furnace finally becomes a dark ball of cold, dead ashes.
Astronomers tell us that the final phases of a star's life may be very dramatic. As the fuel runs low, the core of the star shrinks and becomes even hotter. The outer shell of gases becomes cooler and spreads out to form a giant red star. This phase lasts until the whole star collapses, growing hotter and heavier.
Perhaps the collapsed star will explode. For a short while it becomes a supernova, bright enough to outshine all the other stars in the galaxy. Its shells of outer gases continue to expand through outer space. The core becomes a white dwarf star, so dense that a thimble full of its material weighs 100 tons. The white dwarf, no bigger than a planet, continues to burn up the last of its fuel. Then at last its fiery furnace goes out and the star dies.
Meantime, dramatic events are happening in other parts of the universe. Here and there a vast cloud of hydrogen condenses into a huge ball, perhaps surrounded by several smaller balls of material. When things are just right, the big ball ignites a nuclear furnace and becomes a brand new blazing star. Perhaps it sheds its first beams on a family of planets and a whole new solar system comes into being.