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Lecture 16. Post-ms evolution. Overview: evolution. Subgiant branch. An inert, isothermal helium core grows, while H burns in a shell. When the Schönberg-Chandrasekhar limit is reached, the core begins to collapse on the Kelvin-Helmholtz timescale. . Subgiant Branch.

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Lecture 16


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lecture 16

Lecture 16

Post-ms evolution

subgiant branch
Subgiant branch
  • An inert, isothermal helium core grows, while H burns in a shell. When the Schönberg-Chandrasekhar limit is reached, the core begins to collapse on the Kelvin-Helmholtz timescale.
subgiant branch4
Subgiant Branch
  • Collapsing core releases gravitational energy on a short timescale, causing the envelope to expand and cool.
  • Hydrogen-burning shell narrows, and produces even more energy
  • This phase lasts about 2 million years
red giant branch
Red Giant Branch
  • Envelope cools, opacity increases
  • The star reaches the Hayashi track where efficient transport of energy by convection leads to increased luminosity, at constant T.
  • Lasts about 0.5 million years
first dredge up

5MSun

H →He burning

First dredge-up

He →C,O burning

MS

SGB

RGB

Convection

First Dredge-up
  • The energy generated by the shell increases as the core collapses
  • This energy is partially absorbed by the envelope, which expands and cools.
  • The increased opacity creates a surface convection zone, which reaches into the inner regions and brings processed material to the surface
helium ignition
Helium ignition
  • Once the central temperature and density have reached a high enough level, the triple-alpha process can occur.
  • Core expands, pushing the H-burning shell outward and decreasing the total luminosity
helium core flash
Helium Core Flash
  • Lower mass stars have strongly electron-degenerate cores
  • Energy produced by helium ignition goes into lifting the degeneracy, rather than expanding the core
    • The release of energy is explosive
    • Generates 1011 Lsun released in a few seconds
    • Absorbed by envelope, and may drive mass loss
horizontal branch
Horizontal branch
  • He → C → O fusion occurs in the core
  • Hydrogen burning occurs in a shell
  • Effective temperature increases
  • He-analogue of the main-sequence phase, but only lasts about 10 million years.
helium burning the horizontal branch

H →He burning

He →C,O burning

Convection

Helium burning: the Horizontal branch
  • The temperature-dependence of the triple-alpha process induces a convective core

HB

helium burning the horizontal branch12
Helium burning: the Horizontal branch
  • As the temperature increases, the star crosses instability strip
    • this leads to pulsations which allow a test of the theory.

Instability strip

second dredge up he shell burning
Second dredge-up: He-shell burning
  • A Helium-burning shell ignites around a C,O core.
    • Similar to the H-shell burning phase
  • Again, the envelope expands and cools, becoming convective and causing a second dredge-up.

Instability strip

helium burning the horizontal branch15

H →He burning

He →C,O burning

Convection

Helium burning: the Horizontal branch
  • Core helium is quickly exhausted; inert C-O core forms
  • Helium-burning shell established (like subgiant branch)
  • H-burning shell expands, cools and turns off.

End of HB

Start of HB

early asymptotic giant branch
Early Asymptotic Giant Branch
  • Helium-burning shell dominates the energy production
  • H-burning shell is almost inactive
second dredge up

H →He burning

He →C,O burning

Convection

Second Dredge-up
  • A Helium-burning shell ignites around a C,O core.
    • Similar to the H-shell burning phase
  • Again, the envelope expands and cools, becoming convective and causing a second dredge-up.

Start of AGB

asymptotic giant branch
Asymptotic giant branch
  • As the envelope cools it eventually reaches the Hayashi track and bends upward. This is the asymptotic giant branch.