Chapter 5 Flashcards

1
Q

What is lamda(v)?

A

It is the distance over which the intensity has dropped significantly (1/e) and is called the mean free path for photons

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
2
Q

What is the mean free path?

A

The average distance a photon of frequency v travels before being absorbed or scattered

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
3
Q

What is the main source of opacity?

A

Absorption processes

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
4
Q

What are absorption processes?

A

When a photon is absorbed(destroyed) although it is usually re-emitted a short time later but in a random direction

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
5
Q

How are absorption processes classified?

A

By the type of electron transition taking place in the particle doing the absorbing

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
6
Q

What is the absorption process known as BOUND-BOUND?

A

When a photon is absorbed when an electron undergoes transition between two bound energy levels in an ion, atom or molecule

hv=XeXe

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
7
Q

Where does high opacity occur with bound-bound?

A

Only at discrete frequencies (spectral lines)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
8
Q

What is the absorption process known as BOUND-FREE?

A

Transition from bound level into continuum and electron escapes (photoionisation)

hv=Xion + 1/2mv^2

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
9
Q

In Bound-free what is the opacity?

A

It has a continuous range of frequencies but rises as the ionisation threshold is approached

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
10
Q

What is the absorption process FREE-FREE?

A

Transition within the continuum and electron absorbs photon energy and is accelerated in vicinity of an ion “bremsstrahlung”

hv=1/2m(v1)^2 -1/2m(v2)^2

Continuous range of frequencies

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
11
Q

What kind of mean is Rosseland mean opacity?

A

A weighted mean which carries most weight where most of the stellar radiation emerges.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
12
Q

When is the Rosseland mean opacity in the optical?

A

From

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
13
Q

What of electrons can scatter photons?

A

Either bound to atoms or ions, or free flying

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
14
Q

What happens to the photon when it is scattered?

A

It is re-directed but frequency is unchanged (elastic collision)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
15
Q

What is Thomson Scattering?

A

Scattering by free electrons at constant cross section

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
16
Q

When does a star become convective?

A

If the temperature gradient is larger than the pressure gradient or if lamda approaches unity

17
Q

What is convection caused by?

A

Steep temperature gradients e.g core of high mass stars

Large increase in k e.g surface layers of cool stars

gamma is close to unity e.g change of state

18
Q

How do you decide whether there is convection?

A

Using hydrostatic equilibrium equation and radiative transfer equation.

19
Q

When is energy transported by radiation?

A

When the Schwarzchild criterion is not satisfied

20
Q

What leads to a large energy transport?

A

A relatively small change in T

21
Q

What is the mixing length?

A

The typical path length over which an individual convective gas element travels before exchanging its energy and merging with surroundings

mixing length is approximately H

22
Q

What is approximately constant over the path length?

A

g and T

this means that integrating gives the isothermal pressure scale height (H= RT/mui g)

23
Q

When does convection occur?

A

When the temperature gradient is too high or when the has is changing state e.g ionization

24
Q

What are the inner boundary conditions for the differential equations?

A

M=0, L=0 at r=0

25
What are the outer boundary conditions for the differential equations?
P=0, T=0 at r=Rs (and M(Rs)=Ms) After solving: P=0, T=Teff at r=Rs
26
What needs to be switched on and off according to the Schwarzschild criterion?
The convective transport
27
What does treatment of convection depend on?
Where the convection occurs in the star