Beyond Maturity

Stellar Evolution Beyond the Main Sequence

Simon Jeffery : Armagh Observatory

Adapted from an article published in Astronomy Now Magazine, June 1998

Contents

The Sun has shone virtually unchanged for over four billion years, but it won't shine for ever. In about six billion years time, the hydrogen in its core will have been completely converted into helium, and there will be no nuclear fuel left to make it shine. No longer supported by nuclear energy, the Sun's core will start to collapse.

A remarkable series of changes will then take place. First, the Sun will expand a hundred-fold to become a "red giant". Later it will collapse to become a "white dwarf", a hundred times smaller than it is now. All of this within one tenth of the time the Sun has been shining so far.

What will happen inside the Sun and why? Does it happen to all stars? What other changes take place? These are questions about "stellar evolution", the history of stars as they are transformed by nuclear processes deep inside.

Many things affect the way a star will evolve, but the most important one is its mass. Simply, this is because the heavier a star is the brighter it shines and the more rapidly it consumes its fuel supplies. Other important factors are how much of a star's initial mass was made up of 'metals' - or elements heavier than helium, how fast it was spinning when it was formed and whether it has a strong magnetic field. How do we know what happens inside stars?

Modelling stellar evolution

In most cases, it is impossible to see inside a star, and it is impossible to create a real star in the laboratory to find out how it behaves. Most of what we know about stellar evolution comes from computer models of stars. Just a few properties can be compared with observations of real stars. The models use numbers to describe the properties of the gas inside a star, its temperature, pressure, and so on, all the way from the surface of the star, which we can see, down into the centre. These models obey physical laws and use data that have been tested in the laboratory. By simulating changes in chemical composition caused by nuclear reactions, the models can evolve in time. If the physics and data are right, then the model stars should evolve in exactly the same way as real stars. 

The first successful models were built during the 1950's and 1960's, when the first electronic computers became available to astronomers such as Martin Schwarzschild, Fred Hoyle, C. Hayashi, and Icko Iben Jr. These calculations established most of what we know today about how stars evolved.

The HR diagram

Astronomers talk about three main types of Hertzsprung-Russell diagram. The original HR introduced by E. Hertzsprung and H.N. Russell in 1913 showed the brightness and spectral types of stars with known distances. It represented a huge step forward for understanding stellar evolution.

 Today, most HR diagrams compare properties of stars which can be observed directly, such as the apparent colour and relative brightness of stars in a cluster. Cluster colour-magnitude diagrams are extremely useful for comparing stars that were probably formed almost at the same time, but differ in mass. We learn about the evolution of high-mass stars from HR diagrams of young galactic open clusters, and the evolution of low-mass metal-poor stars from old globular clusters.

However, it is easiest for us to think about and make computer models of stars in terms of their luminosity and radius relative to the Sun. Since the colour-magnitude diagram is really a measure of luminosity and surface temperature, it is convenient to do a quick conversion using a relation true for all stars, L = R2T4, where L, R and T are luminosity, radius and temperature measured relative to the Sun. Therefore, when theoreticians make models of evolving stars, they usually start off with a diagram that shows luminosity and temperature, a luminosity-temperature diagram. Stellar luminosities and temperatures differ by such large amounts that it is necessary to use special scales in the diagram, called logarithms. These diagrams can be converted into a colour-magnitude diagram later for comparing with the observations.

 The term HR diagram is widely used to refer to all three different types of diagram.

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Simon Jeffery (csj@star.arm.ac.uk)
Last modified: 09/06/00

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