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Physics 160 Stellar Astrophysics Prof. Adam Burgasser
Lecture 7
Stellar Interiors: HydrostaAc Equilibrium & GravitaAonal ContracAon
17 October 2013
Announcements • HW #3 due tomorrow @ 5pm in box outside my office SERF 340
• Local lab => next Wednesday 6:30-‐8pm at Torrey Pines Glider Port – you will be given a worksheet to complete
– this is low stress! an opportunity to learn some practical knowledge about the sky
• Physics seminar today on string theory “Monstrous Moonshine”
• OBAFGKMLTY winners
OBAFGKMLTY winners
(3 Ae) Only Batman Avenges For Gotham's Killings, Murders, Lies (and) Treachery... Yay! (Aisha Iyer)
(3 Ae) One Big And Furry Gorilla Killed My LiWle Teeny Yorkie (Sean Riley)
(Runner-‐up): Oppenheimer Built Atomic Fusion Generators Kindling Many LasAng Tragedies Yonder (Myles Ishihara)
(Winner): Oh, Burgasser! A Failing Grade Kills My Luck To Yale (Adrian Wong)
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Lecture 7: Stellar Interiors: Hydrostatic Equilibrium & Gravitational Contraction
17 October 2013 PRELIMS • Announcements [5 min] • OBAFGKMLTY winners [5 min]
MATERIAL [70 min] • [10 min] Review • [5 min] Motivate stellar interiors • [20 min] Hydrostatic Eq. & Mass Conservation • [20 min] Equations of state • [15 min] Virial Theorem
DEMONSTRATIONS/EXERCISES • none
MATERIALS • awards for OBAFGKMLTY
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Radiation (Review) • Line emission and absorption depends on the population
of electrons in various states (Boltzmann) AND fraction of atoms that have those electrons (Saha)
• Both depend on temperature, but collisional processes can also seed electrons to high states
• Which lines form also depend on quantum selection rules o For electron dipole (E1) and magnetic dipole (M1) this
is ΔJ = 0,±1, no 0-‐>0 (s-‐>s) o Forbidden lines – break these rules, happen through
less probably mechanisms (electric quadrapole, etc.) – seen in very low density gas (emission nebulae) – origin of Nebulium [O III]
• There is a lot more on radiative transfer that we did not cover but you should know – be sure to read the book!
Stellar Interiors: Motivation • HR diagram – focused on the appearance of the star,
specifically T (horizontal axis) – line profiles and SED can constrain these
• Vertical axis: L = 4πR2σT4 – how do we predict R and hence L?
• Other atmospheric characteristics – local surface gravity g = GM/R2 ==> local pressure also depend on structure of star
• And where is this energy coming from? How does it get to the surface?
• For this we need to understand stellar structure Hydrostatic equilibrium • Consider average density ~ 103 kg/m3 (water);
photosphere ~ 10-‐4 kg/m3, core ~ 1.5x106 kg/m3
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• Also variations in T and P => some radial structure (radius is the parameter to describe star)
• How to solve: Force balance on thin slab –> derive hydrostatic equilibrium equation
• Estimate core pressure assume P ~ -‐r, compare for Sun Mass conservation • quick derivation
Equation of state • 3rd equation needed between P, rho and M • examples of EOSs • derivation of ideal gas law
o example of use of distributions – based on MB energy law and what pressure means
o definition of mean molecular mass – examples for neutral, ionized, XYZ
o estimate core T, ΔT/R and ΔP/R gradients • radiation pressure – compare to gas pressure in sun’s core • Now in principle we have enough information to solve for
the structure of a star’s interior Virial theorem • Where does all this thermal energy come from? How
about gravity? o Origin of stars – ISM o For ρISM = 10
-‐20 kg/m3, R(Msun) = R (ρsun/ρISM)
1/3 ≈ 5x106 R
≈ 0.1 pc.
o Gravitational energy release = -‐Ω = GM2[1/R –
1/0.1pc] = 4x1041 J o If entire star absorbed that, NkT = (M/mH)kT = -‐Ω => T
= ΩmH/Mk ≈ 2x107 K – hot enough! • Derive virial theorem
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o Show that kinetic energy gives just about the right answer
• Effect of radiating star losing energy => star shrinks and temperature increases (negative heat capacity)
• Derive Kelvin-‐Helmholtz timescale for sun – 20 Myr o Implications for spectral sequence o realization that solar system is much older – alternate
energy source needed