1.4 The instability strip
When a star more massive than the Sun leaves the main sequence, its track on the HR diagram starts out approximately horizontal. During the course of its expansion to a red giant it can move back and forth on the diagram several times as different phases of core and shell fusion start up. At certain stages in this process, the star can undergo regular pulsations as it struggles to achieve equilibrium between gravity and radiation pressure. Instead of reaching a stable state, the star oscillates between contraction and heating – which speeds up the nuclear reactions – and expansion and cooling – which slows them down again. This results in periodic variations in the star’s luminosity. Figure 5 shows the track of a five solar mass star (i.e. five times the mass of the Sun) as it goes through this process.
The region on the HR diagram where these pulsations happen is called the instability strip. Stars passing through this strip are classified into different types, such as Cepheid and RR Lyrae stars, which result from stars of differing initial masses and which pulsate at different rates. However, all share the characteristic of a regular pattern to the variations in their luminosity.
Pulsating variables are one kind of variable star; in the next section you will learn about different types of variables, and this will help us to understand the eventual fate of stars.