Transcript The Helium Flash - Stages 10 to 11

The Helium Flash and Stages 10 and 11
M < 8 M
The Helium Flash and Stage 10
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
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
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 M
The 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 M
Horizontal 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 M
Horizontal 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.