Chapter 5 Flashcards
What is lamda(v)?
It is the distance over which the intensity has dropped significantly (1/e) and is called the mean free path for photons
What is the mean free path?
The average distance a photon of frequency v travels before being absorbed or scattered
What is the main source of opacity?
Absorption processes
What are absorption processes?
When a photon is absorbed(destroyed) although it is usually re-emitted a short time later but in a random direction
How are absorption processes classified?
By the type of electron transition taking place in the particle doing the absorbing
What is the absorption process known as BOUND-BOUND?
When a photon is absorbed when an electron undergoes transition between two bound energy levels in an ion, atom or molecule
hv=XeXe
Where does high opacity occur with bound-bound?
Only at discrete frequencies (spectral lines)
What is the absorption process known as BOUND-FREE?
Transition from bound level into continuum and electron escapes (photoionisation)
hv=Xion + 1/2mv^2
In Bound-free what is the opacity?
It has a continuous range of frequencies but rises as the ionisation threshold is approached
What is the absorption process FREE-FREE?
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
What kind of mean is Rosseland mean opacity?
A weighted mean which carries most weight where most of the stellar radiation emerges.
When is the Rosseland mean opacity in the optical?
From
What of electrons can scatter photons?
Either bound to atoms or ions, or free flying
What happens to the photon when it is scattered?
It is re-directed but frequency is unchanged (elastic collision)
What is Thomson Scattering?
Scattering by free electrons at constant cross section
When does a star become convective?
If the temperature gradient is larger than the pressure gradient or if lamda approaches unity
What is convection caused by?
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
How do you decide whether there is convection?
Using hydrostatic equilibrium equation and radiative transfer equation.
When is energy transported by radiation?
When the Schwarzchild criterion is not satisfied
What leads to a large energy transport?
A relatively small change in T
What is the mixing length?
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
What is approximately constant over the path length?
g and T
this means that integrating gives the isothermal pressure scale height (H= RT/mui g)
When does convection occur?
When the temperature gradient is too high or when the has is changing state e.g ionization
What are the inner boundary conditions for the differential equations?
M=0, L=0 at r=0
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
What needs to be switched on and off according to the Schwarzschild criterion?
The convective transport
What does treatment of convection depend on?
Where the convection occurs in the star
