Stars: Our Favourite Plasma Balls

Question 1

What colour are stars on their surface?

Answer 1

Black (they absorb all light)
They’re hot enough to glow
Perfect black bodies

Question 2

Star surface temperature range

Answer 2

2000-10^5K approx

Pretty small relatively

Question 3

How is surface temp related to colour?

Answer 3

Red = cool
Blue = hot
Probably smth about the wavelengths of light emitted (weins law)

Question 4

Surface luminosity of stars depends on what?

Answer 4

Temp ^4

Size ^2

Question 5

Interior of stars

Answer 5

Hydrostatic equilibrium
Equation of state
Energy transport (radiative, convective)
To find temp, pressure, density

Question 6

What keeps the sun shining?

Answer 6

Nuclear energy (only one with enough E to last this long)

Question 7

What is a Blackbody?

Answer 7

Perfect absorber and perfect emitter

Any photon entering is absorbed via Compton scattering

Question 8

What is plasma?

Answer 8

Electrically neutral gas of protons and electrons (ionized)

Ionized - thermal KE > 13.6 eV

Question 9

What is temperature?

Answer 9

Average kinetic energy of particles

KE thermal

Question 10

Ideal gas law (fundamentally)

Answer 10

PV = NkT
P = pressure
V = volume
N = # of particles
k = Boltzmann's constant
T = temperature

Question 11

Wien’s Law

Answer 11

λpeak•T = b
b = 0.29cm•K

Question 12

Stefan-Boltzmann Law

Answer 12

F = σT^4
Fun times man
σ = 5.67x10^-8 Nm^2/K^4

Question 13

Luminosity of a star

Answer 13

Total power/unit area

4πR^2σT^4

Question 14

Einstein luminosity

Answer 14

c^5/G (dimensional analysis)

Max luminosity of any physical process

Question 15

Why wouldn’t the sun shine without quantum mechanics?

Answer 15

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

Question 16

Is plasma transparent?

Answer 16

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

Question 17

Hydrostatic equilibrium

Answer 17

1) Fg with ro
2) Fp with dP/dr (F=AP)
3) Fg=Fp

Question 18

How are M(r) and ρ(r) related

Answer 18

Change M/change r = 4(Pi)r^2(ro(r))

Question 19

Equation of state + how do you get P from it?

Answer 19

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

Question 20

Derivation for Ek=3/2kT

Answer 20

1) change in p
2) change in t
3) F = pt
4) P = F/A
5) v = vx/3
6) PV = NkT