The Helium Flash and Stages 10 and 11

M < 8 M

Electron degeneracy pressure: the pressure exerted by electrons caused by the confinement in the small core of a star that has left the main sequence.

The Helium Flash and Stage 10

M < 8 M

Important point: Degeneracy pressure is not directly related to gravitational effects, that is, it is not a thermal pressure. Under thermal pressure, as the core heats up, the resulting increase in pressure would expand the core. The expansion would cause cooling, and moderate the rate at which nuclear fusion occurs.

The Helium Flash and Stage 10

M < 8 M

Under degeneracy pressure, the core would heat up with no corresponding increase in pressure. Once fusion is triggered when the cores reaches about 108 K, it runs at a rate which is unchecked by the expansion and cooling of the core.

The Helium Flash and Stage 10

The Helium Flash

Once helium begins to fuse, it will run unchecked and rapidly. For a few hours, the helium will fuse rapidly and with an enormous release of energy.

The Helium Flash

Helium Flash: The rapid release of energy which occurs at the end of stage 9 that is generated in conjunction with degeneracy pressure in the core of the red giant.

The Helium Flash

Helium Flash: The rapid release of energy which occurs at the end of stage 9 that is generated in conjunction with degeneracy pressure in the core of the red giant.

The Helium Flash

The Tip: The point on the H-R diagram where the helium flash occurs.

Stage 10 - Helium Fusion

M < 8 M

After the rapid release of energy in the helium flash, the luminosity of the star will drop, while the surface temperature will grow in reaction to the enormous temperature of the core.

Stage 10 - Helium Fusion

M < 8 MThe rapid generation of energy and increase in temperature will expand the core, so that degeneracy pressure no longer dominates and thermal pressure dominates. The star will once again reach hydrostatic equilibrium, and fuse helium.

Helium fusion will start to occur when the core temperature reaches 108 K.

Stage 10 - Helium Fusion

M < 8 MHorizontal branch: the location on the H-R diagram occupied by the star as it fuses helium.

The star will remain on the horizontal branch while it fuses helium.

The horizontal branch is a “parallel” to the main sequence.

Stage 10 occurs on the horizontal

branch.

Stage 10 - Helium Fusion

M < 8 MHorizontal branch: the location on the H-R diagram occupied by the star as it fuses helium.

The star will remain on the horizontal branch, burning helium, for tens of millions of years.

Stage 11 – Asymptotic Giant Branch

As the helium depletes, the star will basically reproduce it’s the path that took it to stage 9. The helium will burn out at the center of the core, and helium burning as well as hydrogen burning will continue is the outer shells. The star will expand to a red giant once again, this time with shells of burning helium, and some shells still burning hydrogen. This path to red giant is called the asymptotic giant branch to distinguish this evolution from the hydrogen shell burning path to stage 9.

Stage 11 – Asymptotic Giant Branch

Because the carbon core is so compact and hot, the fusion rates increase in the outer shells, that the star increases its luminosity.

The non-burning core continues to contract, its temperature increases, which increases the helium and hydrogen fusion rates in the shells, continuing to increase the luminosity and radius.