Transcript Stages 7 to 9 - Sun

Off the Main Sequence - The Evolution of a Sun-like Star
Stages 7 - 9
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
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
Off the Main Sequence - The Evolution of a Sun-like Star
Stages 7 - 9
While on the main sequence, the star is burning hydrogen. It’s 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
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
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
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.
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
Hydrogen continues
to burn outside of
the central core.
Gravitational
collapse of the core
increases the
temperature of the
core.
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 HR Diagram.
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.
Hydrogen continues
to burn outside of
the central core.
Gravitational
collapse of the core
increases the
temperature of the
core. Increase in
core temperature
triggers more
hydrogen fusion in
the shell.
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.
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.
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.
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.
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