1.06.01 Red Giants

Evolution of stars less than 8 solar masses

When all of the hydrogen has been depleted in the core of the star (after a total of about 10 billion years for our sun) the nuclear reactions cease. As the energy source supporting the star has now stopped gravity causes the star to contract. As the core contracts the temperature increases heating the surrounding layers of hydrogen which are falling inwards towards the core. The temperature increase is sufficient for nuclear fusion to take place in a shell of hydrogen surrounding the core.This burning shell of hydrogen produces enough pressure to support the outer layers of the star. However the pressure from the hydrogen shell is also pushing down on the core and so it increases the rate of core contraction. This produces greater core densities and temperatures. Eventually when the temperature reaches about 100 million Kelvin nuclear fusion of helium occurs within the core, (helium is converted into carbon and oxygen). The heat generated by the core increases the rate at which reactions take place in the hydrogen shell. As a result the pressure generated is great enough to overcome gravity and this causes the outer layers of the star to expand.


Red Giant

The star expands to tens or hundreds of times its original diameter. As the outer layers expand they cool to around 3500K. The enormous star now radiates with a cooler orange glow and is called a Red Giant. Despite the decrease in temperature the star will now be around 1000 times more luminous due to its increase in size. When the sun reaches this stage its core will have shrunk to about 1/50 of its original size and its outer layers will have expanded so far that its outer surface will reach the orbit of the Earth! The star will remain in the red giant phase for about 10% of its main sequence lifetime (about a billion years for our sun).


Eventually the helium in the core will be depleted and the core will consist of mainly carbon with some oxygen. As the nuclear reactions cease the core will again begin to contract. For stars of less than about 4 solar masses despite core contraction the temperature will not reach a sufficient level to produce further nuclear reactions in the core. However the heat from the core contraction and the infalling material will produce a burning helium shell around the core (from the helium produce by the previous hydrogen burning shell) and a new hydrogen burning shell outside of the helium shell. This extends the stars red giant phase.

For stars greater than about 4 solar masses the temperature does rise to the sufficient level of 600 million Kelvin at which carbon burning takes place. These stars therefore undergo a further period of carbon burning in which carbon in the core is converted into neon and magnesium. Helium and hydrogen burning take place in shells surrounding the core. Eventually the carbon in the core will also be consumed. The termination of nuclear reactions again causes the core to contract but the temperature does not increase sufficiently for anymore nuclear reactions to take place in the core.

Note each successive period of hydrogen, helium and carbon burning takes less time. This is because the energy released in a single reaction is less for helium than for hydrogen and less for carbon than for helium. The core contracts until the rate of reactions produces a pressure which is great enough to support the star. This means that the reactions occur an increased rate during each stage and therefore consume the fuel quicker.