6 Summary of Week 6

This week you have learned about the Hertzsprung-Russell diagram, which is a way of classifying stars according to their temperature and luminosity, and illustrates a powerful scientific technique of plotting one property of an object against another and looking for patterns.

In the case of stars, the HR diagram shows that most stars spend a substantial part of their lifetime in a stable state on the main sequence, in which gravity and nuclear energy are in equilibrium. The position of a star on the main sequence is largely determined by the mass of the star, with heavier stars being hotter and more luminous, and stars smaller than the Sun being cooler and fainter.

The balance between nuclear reactions and energy output and the amount of hydrogen available determines the lifetime of the star, and the combination of mass and luminosity on the diagram shows that heavier stars use up their fuel at a highly accelerated rate, giving them much shorter lifetimes than less massive stars.

The diagram also allows us to visualise what happens at the end of a star’s lifetime for a star of similar size to the Sun expanding first to a red giant and then collapsing to form a white dwarf. Heavier stars may have an even more dramatic fate, as you shall see next week when you explore the different types of variable stars.

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