Assignments for Tues. 26 Mar

• Read sections 16 & 17 of DocOnotes-stars2.pdf

• Continue work on HW4.pdf; it’s due classtime Tuesday Mar. 26.

• Midterm exam Thursday March 28, in class

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Mass-Luminosity Relation

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Mass-Luminosity Relation

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Note alsos:1. Radius cancels!2. No mention of energy source (e.g fusion)

Requires nearly homogeneous stars with radiative diffusion

Pre-Main-Sequence:Contraction to main sequence

�4

Sources of opacity

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When Hydrogen recombines for T < 10kK, leads to strong Bound-Free opacity, so much steeper dT/dr

This makes the stellar envelope where T< 10 kK unstable to CONVECTION

Convective instability

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Pre-Main-Sequence:Contraction to main sequence

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Hydrogen fusion

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Hydrogen fusion

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Fusion works by QM tunneling

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Clicker QuestionBy the virial theorum, about what must the radius (in Rsun) of a

M=3Msun star be to sustain nuclear fusion in its core?

A. 1 Rsun

B. 3 Rsun

C. 1/3 Rsun

D. 31/2 Rsun

E. There’s not enough information to answer.

�13

Actually, for or Main-sequence stars, the scaling of Radius vs. Mass roughly follows:

This is partly because, since L ~ M3, higher mass stars must have hotter cores to generate the higher L.

By virial theorum, they thus must be more tightly bound, with M/R larger than the sun.

Also, their fusion is through CNO cycle, not PP, again requiring higher Tcore.s

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Lower mass limit for Hydrogen fusion:Brown Dwarf stars

Stars with mass below this cannot fuse Hydrogen.

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Upper mass limit for Stars:The Eddington Limit

Stars with mass near and above this very unstable.

Eddington Limit:

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