The helium fl ash

7.2.3 The helium fl ash

The onset of helium burning in stars of this mass range is very dramatic. What happens is related to the properties of the electrons that also exist in the core. Under the extreme densities within the core the electrons cannot be regarded as an ideal gas, but begin to show quantum-mechanical aspects in their behaviour. They begin to act as what is termed a degenerate electron gas and behave differently. In an ideal gas, as the temperature increases, so will the pressure and thus the gas will tend to expand so reducing the temperature. This tends to prevent a rapid increase in temperature. However, in a degenerate gas the pressure does not rise, so the temperature in the centre of the star increases rapidly and, as the reaction rate is highly dependent on temperature, the reac- tion can ‘run away’ and produce an explosive release of energy in a very short period – perhaps as little as 30 s. This is called the helium fl ash . However, due to the overlying layers of the star this energy will take thousands of years to reach the surface.

For a given mass of gas, the 3α process only releases about 10% of the energy produced in forming helium nuclei from hydrogen. Hence, the length of the helium burning phase will be about 10% of the star’s life on the main sequence.

During its helium burning phase the core will be compressed to perhaps 1/50th of its original size and have a temperature of ∼100 million K with, in addition,

a shell of hydrogen burning surrounding the core. The energy so produced, in part by the shell of hydrogen burning, causes the outer parts of the star to also undergo signifi cant changes. The radius of the star as a whole increases by a fac- tor of ∼10, but at the same time the surface cools to (in the case of a 1 solar mass star) a temperature of ∼3500 K. The star will then have an orange colour and the star becomes what is called (perhaps perversely) a ‘red giant’. The result is that the star’s position moves up and to the right of the H–R diagram into the red giant region.

Stellar Evolution – The Life and Death of Stars

For midmass stars less in mass than our Sun, this is about as far as nuclear fusion can take the formation of elements as there is not enough overlying mass above the core to allow its temperature to rise suffi ciently for further nuclear fusion reactions to be carried out.

The stars in the upper part of this mass range are able to carry out one further nuclear reaction:

12 C⫹ 4 He → 16 O⫹γ

This reaction and the 3α process are thought to be the main source of carbon and oxygen in the universe today. However, we could not exist if these elements stayed within the stars. They must lose much of their material into space (this is the anthropic principle again); this is exactly what is observed.