The timescale of stellar evolution is very long. The Sun, for example took a few million years just to contract from an interstellar gas cloud to become a main-sequence star. Its lifetime as a main-sequence star is about ten thousand million years, of which it has used about four and a half thousand million years. In about five and a half thousand million years it will exhaust its central hydrogen and become a red giant. In agreement with popular expectation its further expansion will probably be sufficient to bring the Earth into its outer layers, and the Sun’s large luminosity then will destroy organic life as we know it. The Sun will probably be a red giant or supergiant for a few million years.

There are many uncertainties in stellar evolution theory not least of which are the end points of stellar evolution or stellar death. Some stars, the more massive stars, probably die violently in a supernova explosion. After many stages of nuclear burning the core becomes unstable and implodes, while the outer layers of the star explode. There is a massive output of energy: a supernova often outshines the entire light output of its parent galaxy for a week or more. Neutrons created from the decomposition of the core move through the outer layers and nucleosynthesis of heavy elements may occur. The outer layers of gas are thrown off and become a supernova remnant which may shine and emit radio waves for 104yr or more . The core implodes and may leave a remnant, such as a NEUTRON STAR, an object made largely of neutrons, of typical radius 10km, at an average density of about 1016kgm-3. A cubic centimetre of such a star weighs over ten million tonnes! The neutron star rotates rapidly and may be observable as a radio pulsar. It is also possible that the remnant could be a black hole, but nobody knows for certain.

Since supernovae are rare events we must conclude that most stars die peacefully. Within our Galaxy they probably occur once every 50 years on average. Many stars probably end up as WHITE DWARFS. When nuclear burning has ceased they simply contract and radiate away their thermal energy. A typical white dwarf has a radius, similar to the Earth, of about 5000km, an average density of about 109kgm~3, and a surface temperature of 104K. At such high densities the stellar material is degenerate.

There is a maximum mass which a white dwarf is permitted to have. If it were to have a larger mass than this upper limit, it would collapse under its own weight, presumably to a neutron star or black hole. The critical mass is about 1.4 solar masses. However, observationally we know the important additional fact that stars can lose mass during their lifetime. Red giants lose mass in stellar winds. Radiation pressure blows away some of the outer layers at a rate as high as 10~6 to 10~7 solar masses a year. Mira, or long-period variables (see chapter 4), probably expel mass after each pulsation. There is also an instability in which a shell containing a large amount of mass is thrown off to expose the deeper regions of the star, producing a planetary nebula. A planetary nebula consists of a very hot (105K) central star and a cool expanding envelope. Eventually the envelope fades away and the central star cools down to become a white dwarf. In this way stars more massive than the critical mass for a white dwarf m ay die peacefully.

In summary of the chapter we may say that many of the theoretical aspects of stellar evolution have found observational confirmation. Of special importance is the evolution to the red-giant stage: stars that were fully mixed chemically would not get to that stage, for example, and there is good agreement of theory and observations for clusters. We specially point out that the old theory of stellar evolution, that stars at the top of the main sequence evolved by losing mass and contracting down the main sequence can be ruled out. The mass-loss rates observed for main-sequence stars are too small to have any noticeable effect during a star’s lifetime and the old theory fails to explain the number and properties of red giants. Nevertheless there are uncertainties in the present theory and the outlines given here are for the simple case of non-rotating single stars. Rotation, magnetic fields, mixing and mass loss must all eventually be considered in detail. The fundamental problems of what the mass of a Cepheid is, how convection really works and whether the solar neutrino experiment agrees with theory must be resolved by further painstaking research.

It is fitting to end where we started, by remarking, with Eddington, that ‘It is reasonable to hope that in a not too distant future we shall be competent to understand so simple a thing as a star’.

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