Astronomy with an online telescope
Astronomy with an online telescope

Start this free course now. Just create an account and sign in. Enrol and complete the course for a free statement of participation or digital badge if available.

Free course

Astronomy with an online telescope

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.

Described image
Figure 4 The Hertzsprung-Russell diagram showing the evolutionary path of a star such as our Sun after it leaves the main sequence.

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

Timing: Allow approximately 5 minutes

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.


Take your learning further

Making the decision to study can be a big step, which is why you'll want a trusted University. The Open University has 50 years’ experience delivering flexible learning and 170,000 students are studying with us right now. Take a look at all Open University courses.

If you are new to University-level study, we offer two introductory routes to our qualifications. You could either choose to start with an Access module, or a module which allows you to count your previous learning towards an Open University qualification. Read our guide on Where to take your learning next for more information.

Not ready for formal University study? Then browse over 1000 free courses on OpenLearn and sign up to our newsletter to hear about new free courses as they are released.

Every year, thousands of students decide to study with The Open University. With over 120 qualifications, we’ve got the right course for you.

Request an Open University prospectus371