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.
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.