6 - stellar evolution-i. the life history of a star is determined by its mass…
DESCRIPTION
From the mass-luminosity relation for MS stars (of solar mass and somewhat larger): First fuel in stellar cores is hydrogen “ burning ” via pp or CNO reactions. The rate of energy production is: If the fuel available for before the structure changes is, say, 0.1 M *, then the lifetime of the star (the time over which this can occur) is:TRANSCRIPT
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6 - Stellar Evolution-I
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The life history of a
star is determined by
its mass…..
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From the mass-luminosity relation for MS stars (of solar mass and somewhat larger):
First fuel in stellar cores is hydrogen “burning” via pp or CNO reactions. The rate of energy production is:
If the fuel available for before the structure changes is, say, 0.1 M*, then the lifetime of the star (the time over which this can occur) is:
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2 Important Basic Conclusions:
We should expect any very massive star to be young, and found near its place of formation.
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For a group of stars formed at the same approximate time, the more luminous ones should evolve faster than the less-luminous ones.
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Why Do Stars Evolve?
Nuclear fusion changes the chemical composition of the core, which in turn changes the mean molecular weight μ, which governs the equation of state.
For a star like the sun where the energy generation goes as
It can be shown that for a chemically uniform star the surface T and net L go approximately like:
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Homogeneously-mixed stars should evolve toward upper left. But real stars are stratified chemically…..
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Low-mass Stars:
•Radiative cores
•Convective envelopes
ZAMS
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“to flash, or not to flash….”
•Lower-mass stars - He-ignition begins in electron degenerate core
•Heating does not expand the gas in the core, merely raises T, increasing reaction rates
•Explosive reaction ensues - “He (core) Flash” - core briefly reaches ~1011 Lsun?? (NOTE: This is not visible to the outside observer)
•Degeneracy is lifted and “normal” He-burning follows.
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“Horizontal Branch”
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Instability Strip (Cepheids & RR Lyrae Stars)
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Asymptotic Giant Branch (AGB)
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Mass in H-rich envelope
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Massive Stars:
•Convective cores
•Radiative envelopes
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convective core
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1st dredge-up 2nd dredge-up
3rd dredge-up on its way!
Time (units of 107 years)