Bright Stars Die Young

Stars are bright because of a combination of two reasons. First, they may be really dim stars, but they are very close. Our Sun is one such star. Or they really may be intrinsically very bright, and this factor outweighs their great distance: Spica in Virgo (arc to Arcturus; speed on to Spica) is this type of star. Stars like this don't last very long.

Why? Science is more than a collection of facts: there are usually reasons for these facts.

Stars burn because the hydrogen gas of which they're made is fused into helium. For this fusion to take place, the hydrogen atoms have to be pressed tightly together, overcoming their natural tendency to fly apart. This pressing requires great force.

The force pressing them together is gravity. To see how this works, let's first look at our own planet. Here on Earth, about ten miles of atmosphere towers above us. Over each square inch, there is a tower of air weighing fifteen pounds. Each of us is holding up about fifteen thousand pounds of air! This force is counteracted by the pressure of the fluids in our bodies, so we are not crushed.

On Jupiter, though, the massive weight of an atmosphere forty thousand miles high makes the pressure on the surface millions of times greater.

Finally, on the Sun, a half million miles of atmosphere presses the atoms of hydrogen at the center so close together that they fuse into helium. The Sun blazes into light.

On a star bigger than the Sun, an even larger atmosphere bears down on the center. The fusion reaction, depleting the hydrogen, goes much faster.

Even though the star is larger, the fusion, using up the hydrogen fuel, proceeds so rapidly that the star burns out much faster than the Sun.

For comparison, a small star, like the Sun, has a lifetime lasting billions of years. A really large and bright star has a lifetime of only several million years.

Profligacy has its price, in stars as well as in people.

(05/16/07)