Chapter 5

Question 1

What is lamda(v)?

Answer 1

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

Question 2

What is the mean free path?

Answer 2

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

Question 3

What is the main source of opacity?

Answer 3

Absorption processes

Question 4

What are absorption processes?

Answer 4

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

Question 5

How are absorption processes classified?

Answer 5

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

Question 6

What is the absorption process known as BOUND-BOUND?

Answer 6

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

hv=XeXe

Question 7

Where does high opacity occur with bound-bound?

Answer 7

Only at discrete frequencies (spectral lines)

Question 8

What is the absorption process known as BOUND-FREE?

Answer 8

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

hv=Xion + 1/2mv^2

Question 9

In Bound-free what is the opacity?

Answer 9

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

Question 10

What is the absorption process FREE-FREE?

Answer 10

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

Question 11

What kind of mean is Rosseland mean opacity?

Answer 11

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

Question 12

When is the Rosseland mean opacity in the optical?

Answer 12

From

Question 13

What of electrons can scatter photons?

Answer 13

Either bound to atoms or ions, or free flying

Question 14

What happens to the photon when it is scattered?

Answer 14

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

Question 15

What is Thomson Scattering?

Answer 15

Scattering by free electrons at constant cross section

Question 16

When does a star become convective?

Answer 16

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

Question 17

What is convection caused by?

Answer 17

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

Question 18

How do you decide whether there is convection?

Answer 18

Using hydrostatic equilibrium equation and radiative transfer equation.

Question 19

When is energy transported by radiation?

Answer 19

When the Schwarzchild criterion is not satisfied

Question 20

What leads to a large energy transport?

Answer 20

A relatively small change in T

Question 21

What is the mixing length?

Answer 21

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

Question 22

What is approximately constant over the path length?

Answer 22

g and T

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

Question 23

When does convection occur?

Answer 23

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

Question 24

What are the inner boundary conditions for the differential equations?

Answer 24

M=0, L=0 at r=0

Question 25

What are the outer boundary conditions for the differential equations?

Answer 25

P=0, T=0 at r=Rs (and M(Rs)=Ms)

After solving: P=0, T=Teff at r=Rs

Question 26

What needs to be switched on and off according to the Schwarzschild criterion?

Answer 26

The convective transport

Question 27

What does treatment of convection depend on?

Answer 27

Where the convection occurs in the star