Stellar Interiors
 It is nearly impossible to learn anything directly
about the interior of stars
 The optical depth increases very quickly
with distance from the surface, so
photons of all wavelengths originate near
the surface
 An important exception are neutrinos,
which interact very weakly with matter
and so can escape from the Sun unimpeded
 Also, solar oscillations give us a way to
probe the interior by measuring the sound
speed at various depths
 Therefore most of what we know is obtained by
constructing a theoretical model containing all
known physical laws, and consistent with
observable surface features.
 This generally requires detailed computer
programs that only became feasible to run
in the mid-60s (though most of the
physics was already understood).
Equilibrium
 The majority of stars are in a near-equilibrium
state, in that their structure changes only slowly
over many millions of years.
 Exceptions are very early and very late
stages of evolution, where changes happen
more rapidly
 Explosive events (such as supernovae)
occur on very short timescales
 Some stars are also unstable to pulsation
and their size varies in a regular way
 The Solar luminosity is 3.8x1026 J/s
 This is enough energy to melt a block of
ice 1AUx1kmx1km in only 0.1 s.
 Clearly the Sun is losing energy at an
enormous rate; since it does not have an
infinite reserve, its structure must be
changing with time.
Stellar Structure Equations
 The equation of mass conservation relates the
mass Mr enclosed within radius r, dM
2
r

4

r

to the density at that radius,
dr
(r).
 The equation of hydrostatic equilibrium
describes the balance between the pressure
gradient (dP/dr) and the
dP
GM r 
gravitational force required to

dr
r2
keep the star in equilibrium:
 The ideal gas law can be used in most cases to
relate the temperature (T), pressure (P) and
number density (n):
 or, defining the mean
m
kT
molecular mass

P
P  nkT
mH
mH