Kaye Frances Mebane, age 13, of Burlington, N.C., for her question:
How do we know the life histories of the stars?
As our minds stretch out into the universe, our Earth bound ideas of time and space must be left behind. Everything is on a staggering scale of grandeur. Stars live and die, always on the MOVE, but none of these cosmic changes can be observed in the short span of a human lifetime:
The life of a star may last millions or billions of years. WE cannot watch them develop from youngsters to old timers. Nevertheless, scientists have figured out their histories and the changing phases of their lives. They can estimate the age of most stars and when they can be expected to wink out their lights. This star dating was learned in the past 15 years.
Stars come in different types. And the basic work of dating them was done by charting the whereabouts of two common types. Expert knowledge of chemistry, physics and other sciences was used to Explain these charts. One chart tabulated the halo type stars that tend to populate the dust free edges Of our galaxy. Another tabulated the disk type stars that tend to populate the cloudy, spiraling arms of the galaxy.
Since stars are formed from the dust and gas of cosmic clouds, the two charts held a lot of information. In the dust free regions, the cosmic clouds were swept up long ago and formed into stars. Hence the halo stars must be old timers. The bright disk stars must be youngsters in regions where infant stars are still being formed from cosmic clouds.
All stars must obey certain laws of the universe which make them change from phase to phase to phase. Their changing phases are governed by a tug of war between the universal laws of gravitation and the laws of nuclear reaction.
As the original cosmic cloud contracts, its whirling dust and gases become dense and heavy. Its own gravitation then pulls it together and a star is born. Its dense core gets hot Enough to FUSE hydrogen fuel into helium, and the young star starts its own nuclear furnace. Its later phases are controlled by its mass and its energy. The gravitation of its mass pulls inward as its nuclear energy pushes outward. But as the fuel is used up its energy decreases. The final phase is won by gravitation, and the worn out star collapses.
The mass of a star is the amount of material packed into its size. A small star may be more massive than a big star made of lighter materials. The star's quota of gravitation depends upon its mass. And stars burn up their fuel at different rates. A hot, massive star may burn up fast and collapse in a million years. A cool star may burn slowly for billions of years.