Preliminaries

Abstract

If you want an important insight into what makes stars work, go out and look at them for a few nights. You will find that they appear to do nothing much at all except shine steadily. This is certainly true from a historical perspective: taking the sun as an example, from fossil evidence, we can extend this period of “inactivity” to roughly three billion years. The reason for this relative tranquility is that stars are, on the whole, very stable objects in which self-gravitational forces are delicately balanced by steep internal pressure gradients. The latter require high temperatures. In the deep interior of a star these temperatures are measured in (at least) millions of degrees Kelvin and, in most instances, are sufficiently high to initiate the thermonuclear fusion of light nuclei. The power so produced then laboriously works its way out through the remaining bulk of the star and finally gives rise to the radiation we see streaming off the surface. The vast majority of stars spend most of their active lives in such an equilibrium state, converting hydrogen into helium, and it is only this gradual transmutation of elements by the fusion process that eventually causes their structure to change in some marked way.

“Cosmologists have, nonetheless, made real progress in recent years. This is because what makes things baffling is their degree of complexity, not their sheer size —and a star is simpler than an insect.”

—Martin Rees in Scientific American (Dec. 1999)