2 Classifying stars – the Hertzsprung-Russell diagram

At the end of last week you saw that stars have different colours and levels of brightness. Comparing stars within the open cluster M29, all of which are at the same distance, allowed us to see the beginnings of a relationship between the colour and the brightness of the stars. In this cluster, the brightest stars were white with even a bluish tint, and the fainter stars were orange or red. As you learned in Week 5, the colour of a star is related to its temperature, with hot stars being white and cooler stars being orange and red. Average stars, such as our own Sun, have an intermediate temperature and brightness and a yellowish-white colour.

Taking all of this together suggests that there is a relationship between the temperature and the brightness of stars, and this gives us a way of organising and classifying stars – the Hertzsprung-Russell (HR) diagram. In this and the following sections you will explore this relationship in more detail and find out how the HR diagram helps us to understand the lifecycles of the stars.