Stars: Our Favourite Plasma Balls Flashcards
What colour are stars on their surface?
Black (they absorb all light)
They’re hot enough to glow
Perfect black bodies
Star surface temperature range
2000-10^5K approx
Pretty small relatively
How is surface temp related to colour?
Red = cool
Blue = hot
Probably smth about the wavelengths of light emitted (weins law)
Surface luminosity of stars depends on what?
Temp ^4
Size ^2
Interior of stars
Hydrostatic equilibrium
Equation of state
Energy transport (radiative, convective)
To find temp, pressure, density
What keeps the sun shining?
Nuclear energy (only one with enough E to last this long)
What is a Blackbody?
Perfect absorber and perfect emitter
Any photon entering is absorbed via Compton scattering
What is plasma?
Electrically neutral gas of protons and electrons (ionized)
Ionized - thermal KE > 13.6 eV
What is temperature?
Average kinetic energy of particles
KE thermal
Ideal gas law (fundamentally)
PV = NkT P = pressure V = volume N = # of particles k = Boltzmann's constant T = temperature
Wien’s Law
λpeak•T = b b = 0.29cm•K
Stefan-Boltzmann Law
F = σT^4
Fun times man
σ = 5.67x10^-8 Nm^2/K^4
Luminosity of a star
Total power/unit area
4πR^2σT^4
Einstein luminosity
c^5/G (dimensional analysis)
Max luminosity of any physical process
Why wouldn’t the sun shine without quantum mechanics?
No protons would ever hit each other - they would repel
Quantum solution: tunnelling so protons are in many places at once! Then strong force affects them and they can stay together, creating helium and generating energy
Is plasma transparent?
Nah, it’s opaque
Remember Compton scattering? Light can’t get out
So for photons to get out, H atoms have to exist close to surface (not plasma)
So it has to be colder
Hydrostatic equilibrium
1) Fg with ro
2) Fp with dP/dr (F=AP)
3) Fg=Fp
How are M(r) and ρ(r) related
Change M/change r = 4(Pi)r^2(ro(r))
Equation of state + how do you get P from it?
1) P = NkT/V
2) N/V = mu(Np)/V = density/mu(mp)
3) sub into NkT/V
From this
4) integrate
5) exp(C) = P0
6) H = kT/gmump
Derivation for Ek=3/2kT
1) change in p
2) change in t
3) F = pt
4) P = F/A
5) v = vx/3
6) PV = NkT
