1.3 Tracks on the HR diagram
After a long period of stability on the main sequence, these later stages of a star’s evolution are dramatically less stable, with rapid changes in a star’s diameter, temperature and luminosity as different phases of hydrogen and helium fusion are triggered in different regions in and around the star’s collapsing core.
As you saw at the end of last week, the HR diagram provides a very useful way of visualising the progress of these changes as tracks on the diagram.
Figure 4 shows the evolution of a star of the same mass as our own Sun after leaving the main sequence, first expanding to form a red giant, powered by helium fusion and hydrogen shell fusion. Eventually though, the supply of helium will also be exhausted.
Activity 2 Lifetime of the red giant
Since helium fusion is less efficient than hydrogen fusion, the lifetime of the red giant phase will be less than the star’s main sequence lifetime. What other factor shown on the diagram will also shorten the time spent as a red giant?
The luminosity of a red giant is more than the luminosity of the star when it was on the main sequence. Taken together these factors mean that a star the size of our Sun will spend no more than 1000 million years as a red giant – less than 10% of its main sequence lifetime. Stars considerably heavier than our Sun may have red giant lifetimes of no more than a few million years.
For low-mass stars such as the Sun, helium fusion is the last available source of energy. When the helium runs out gravity takes over once more and the star collapses, eventually forming a white dwarf, which is an extremely dense object approximately the same size as the Earth. Initially, the gravitational energy released as it collapses makes the white dwarf very hot, but with a low overall luminosity because of its small size, placing it on the lower left of the HR diagram. With no further nuclear reactions, this white dwarf will eventually cool and fade, although this can take a very long time.
More massive stars than our Sun have a more interesting fate, with some of them – for a short time – becoming periodic variable stars. You will learn more about these in the next section.