Stars: A Very Short Introduction by Andrew King

Author:Andrew King [King, Andrew]
Language: eng
Format: epub
ISBN: 9780199602926
Publisher: Oxford University Press
Published: 2012-01-15T07:00:00+00:00

8. Evolutionary tracks on the Hertzsprung–Russell diagram. Main-sequence stars move to the right and become red giants after ending core hydrogen burning. The speed of this motion depends on the star’s mass: slow for low-mass stars, and high for massive stars. Lower-mass stars move up the red giant branch as their degenerate helium cores grow, and eventually evolve from red giants into white dwarfs (lower left-hand corner, track not shown). Massive stars move back and forth across the diagram before ending their lives as neutron stars or black holes

As we have by now come to expect, igniting these reactions causes structural changes in the stars involved, and makes them alternately expand and contract, swinging back and forth across the HR diagram. If the star’s mass is large, all the helium is eventually exhausted in the core. Then this contracts still further, forcing an even larger expansion of the envelope. These huge stars are called supergiants. Their cores are layered like a giant onion: each layer has a different chemical composition, with lighter elements lying above heavier ones. Outside this core is a relatively homogenous envelope, still mainly hydrogen, but now enriched (or polluted) with many of the heavier elements produced within. This has happened because convection – the boiling motions that can transport energy outwards in a star – has stirred the various onion layers, and particularly the outer envelope, whenever this gets very large and cool. We will see in the next chapter how these elements are spread through space, producing all the phenomena familiar to us, including our own bodies.

The vast sweep of this chain of discovery is worth some thought. Starting from a simple ball of hydrogen and helium, the laws of physics inexorably drive the creation of all the other chemical elements. At each stage, the iron dictates of the virial theorem – just gravity and thermodynamics – force the star to look for new energy sources. Each one makes new elements. So the stars shine, and make the stuff that makes us.

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