1.2 Expansion and the red giant branch

Your answers to Activity 1 will have told you two things: that helium fusion requires more extreme conditions in the core of a star than hydrogen fusion, and that it can only provide a brief respite against the eventual gravitational collapse of the star.

This is clearly going to be a much less stable phase of the star’s life than its previous existence on the main sequence. Perhaps curiously, the star does not immediately start to contract under gravity, and in fact the outer layers of the star will continue to be supported for a while and will eventually expand.

To understand this, think back to the zigzag path that radiation takes on its way out from the core as shown in Figure 6 last week. The multiple collisions between the outgoing radiation and charged particles in the dense material in the star’s interior provide the radiation pressure that helps to support the star. These repeated collisions also slow the progress of the energy; in a star such as the Sun it can take up to a million years for the radiation to work its way from the core to the surface. This means that the outer layers can still be supported by radiation pressure for a significant time after the supply of energy in the core dies down. While the last gasps of this energy are still holding up the outer layers, the core of the star will be the first part to start collapsing, as shown in Figure 2.

Maximise

Figure 2 Helium fusion in a post main sequence star. On the left we see the core collapsing, and on the right the outer layers expanding as a result of energy released by helium fusion in the collapsed core.

Show description|Hide description

Diagram showing Helium fusion and expansion in a post-main sequence star. On the left, a cross section of a star is shown with an orange core labelled as containing Helium, surrounded by yellow outer layers containing Hydrogen. Six inward-pointing arrows around the core indicate inward collapse. On the right is a larger start with red outer layers, labelled 'Radiation pressure causes outer envelope to expand' (the red colour indicates that these outer layers cool during the expansion). Outward facing arrows depict this expansion. In the centre is a hot core labelled 'Helium fusion 3 – alpha' surrounded by a thin circular yellow zone labelled 'Hydrogen fusion shell'.

Figure 2 Helium fusion in a post main sequence star. On the left we see the core collapsing, and on the right the outer layers expanding ...

As the core starts to contract, gravitational energy is released. This causes the core temperature and pressure to increase as it becomes more compressed. The first consequence of this is that further hydrogen fusion will be triggered, both in the core – as any remaining hydrogen is subjected to these more extreme conditions – and in a thin shell of unused hydrogen around the core that is now heated enough to undergo fusion. The energy released by this new burst of fusion increases the radiation pressure on the outer layers, causing them to start expanding and taking the star from point A to D on the HR diagram in Figure 3.

Maximise

Figure 3 The post main sequence evolution of a star like the Sun. The right-hand diagram shows a sample of stars from a globular cluster such as M13. Because globular clusters are old, the more massive stars have already left the main sequence to become red giants. Only the lower portion of the main sequence, containing stars of similar mass to the Sun and smaller, is still intact. These less massive stars have longer lifetimes so have not yet left the main sequence.

Show description|Hide description

Two Hertzsprung – Russell diagrams showing the evolution of a star of similar mass to the Sun after leaving the Main Sequence. The left hand diagram is a schematic with a diagonal orange line from top left to bottom right depicting the Main Sequence. From the lower right of the Main Sequence is a red line leading at first upwards and to the right. This part of the track is labelled with letters A, B, C, and D, and 'red giant branch'. The track then heads to the left, labelled D – E – F and 'horizontal branch, before heading upward and to the right again, labelled 'asymptotic giant branch'. The track ends at the top right of the diagram in the red giant area. On the right is the HR diagram of a globular cluster, with individual stars indicated by dots. Only the lower right section of the Main Sequence contains stars. The pattern of stars then extends upwards and to the right, following the track of the red path indicated on the left hand plot. The upper parts of the plot are labelled HB (Horizontal Branch), RGB (Red Giant Branch) and AGB (Asymptotic Giant Branch) corresponding to the labels on the left hand diagram.

Figure 3 The post main sequence evolution of a star like the Sun. The right-hand diagram shows a sample of stars from a globular cluster ...

At point D, the core has become sufficiently compacted for helium fusion to start – particularly for smaller stars this is a very unstable process and can take place very rapidly in a runaway reaction known as the helium flash. This heats the star further, taking it from E to F on the diagram. After point F, helium fusion starts in a shell surrounding the core, much as it did for hydrogen, resulting in further expansion of the outer envelope, which cools as it expands, moving the track upward and to the right on the diagram. The star is well on its way to becoming a red giant.