Off the Main Sequence - The Evolution of a Sun-like Star

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Off the Main Sequence - The Evolution of a
Sun-like Star Stages 7 - 9
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Off the Main Sequence - The Evolution of a
Sun-like Star Stages 7 - 9
These stages have already been discussed Stage
1 Interstellar Cloud Stage 2 A Collapsing
Cloud Fragment Stage 3 Fragmentation
Ceases Stage 4 A Protostar Collapsing cloud
has ripped apart atoms, temperature at around
1,000,000 K. Stage 5 Protostellar
Evolution Central temperature around 5,000,000
K Protons still cannot overcome Coulomb
repulsion Stage 6 A Newborn Star Core
temperatures reach 10,000,000 K Fusion begins
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Off the Main Sequence - The Evolution of a
Sun-like Star Stages 7 - 9
These stages have already been discussed Stage
7 The Main Sequence Core temperatures around
15,000,000 K Hydrostatic equilibrium (radiation
pressure balances gravity) stable size
and burning of hydrogen
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Off the Main Sequence - The Evolution of a
Sun-like Star Stages 7 - 9
While on the main sequence, the star is burning
hydrogen. Its luminosity is determined by the
stars mass. The most intense fusion is occurring
at the center regions of the core (highest
pressure and temperature). A star like the sun
will remain on the Main Sequence for about 10
billion years
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Off the Main Sequence - The Evolution of a
Sun-like Star Stages 7 - 9
Initial depletion of hydrogen occurs in the
interior of the hydrogen burning core
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Off the Main Sequence - The Evolution of a
Sun-like Star Stages 7 - 9
Helium cannot fuse because a temperature of
around 108 K is required to overcome the coulomb
repulsion to trigger fusion (helium has a charge
of 2e).
Once the hydrogen in the central core is
depleted, hydrostatic equilibrium is lost and the
central core begins to collapse as a result of
strong gravity
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Off the Main Sequence - The Evolution of a
Sun-like Star The Red Subgiant Branch
Turn Off the location on the H-R diagram where
the star first leaves the main sequence as a
result of hydrogen depletion in the core.
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Off the Main Sequence - The Evolution of a
Sun-like Star The Red Subgiant Branch
Heating of the gaseous envelope causes it to
expand. Burning hydrogen in the expanding shell
increases the luminosity
Gravitational collapse of the core increases the
temperature of the core.
Hydrogen continues to burn outside of the central
core.
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Off the Main Sequence - The Evolution of a
Sun-like Star The Red Subgiant Branch
Hydrogen Shell Burning the fusion of hydrogen
in shells of a star outside of the hydrogen
depleted core. Because of the increase in
pressure of the core due to gravitational
collapse, the core temperatures will rise. The
increase in thermal energy increases the
temperature of the hydrogen burning (expanding)
shell, which increases the amount of hydrogen
fusion occurring in the shell. Therefore, the
temperature goes up, along with a slight increase
in luminosity. The star follows an almost
horizontal path on the H-R Diagram.
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Off the Main Sequence - The Evolution of a
Sun-like Star The Red Subgiant Branch
Heating of the gaseous envelope causes it to
expand. Burning hydrogen in the expanding shell
increases the luminosity somewhat.
Gravitational collapse of the core increases the
temperature of the core. Increase in core
temperature triggers more hydrogen fusion in the
shell.
Hydrogen continues to burn outside of the central
core.
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Off the Main Sequence - The Evolution of a
Sun-like Star The Red Subgiant Branch
Red Subgiant Branch
Gravitational collapse of the core increases the
temperature of the core. Hydrogen continues to
burn outside of the central core.
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Off the Main Sequence - The Evolution of a
Sun-like Star The Red Subgiant Branch
Red Subgiant Branch the portion of the H-R
Diagram that corresponds to the hydrogen shell
burning portion of the life of a star related to
the expansion of the radius and increase in
luminosity of the star before helium burning is
triggered in the core of the expanding star.
There is a decrease in the surface temperature of
the star as it expands.
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Off the Main Sequence - The Evolution of a
Sun-like Star Stage 8
Stage 8 the end of the Red Subgiant Branch. The
temperature of the core is very high, triggering
intense hydrogen fusion in the burning shell.
The huge pressures in the burning shell beings to
push the cooler outer shell outward, starting a
great change in radius.
R 3 Rsun
Gravitational collapse of the core increases the
temperature of the core. Hydrogen continues to
burn outside of the central core.
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Off the Main Sequence - The Evolution of a
Sun-like Star The Red Giant Branch
Outer envelop cools. Hydrogen burning still
occurs in the shells at a rapid rate producing
very high luminosity. Internal pressures cause a
big increase in radius.
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Off the Main Sequence - The Evolution of a
Sun-like Star Stage 9
Core density 108 kg/m3 Core temp 108 K R
100 Rsun