The energy produced by thermonuclear reactions in stars in produced in their deep
interior because only there are the pressures and temperatures high enough to
sustain thermonuclear reactions. However, most of the luminous energy of stars is
radiated from the thin region at the surface that we call the photosphere.
Methods of Energy Transport
Thus, a central issue for stars is how they transport the energy produced in the
core to the surface. There are 4 important categories of such transport:
Radiative transport of photons
Radiation of neutrinos
Radiative Transport of Photons
The most common method of energy transport in normal stars is by photons. In the
deep interior, the stellar material is very opaque, so light travels only a small
distance before it is absorbed. It is then re-emitted in a random direction,
absorbed after a small distance, remitted, and so on until it reaches the surface.
Physicists have a colorful name for such a transport process: it is called the
drunken sailor problem, because the path followed by the absorbed and
re-emitted photons is like that followed by someone too inebriated to stand up for
long. For example, in the case of the Sun the average distance traveled by a
photon between absorptions is about a centimeter, and it takes perhaps hundreds of
thousands of years for the energy released in the center to make its way to the
Conduction is the way in which metals transport heat. Conduction is not important
in most normal stars because the normal approximately ideal gas of a star is a good
thermal insulator (like a blanket rather than like a piece of metal). However,
under certain conditions involving very high densities
the matter of a star may become what is termed
degenerate. This can happen, for example, in white dwarfs or neutron
stars, or in the cores of massive stars.
We will see later that degenerate matter behaves like a metal and is a very good
conductor of heat. Thus, conduction is not important in normal stars, but must be
accounted for if stars contain degenerate matter.
If the rate at which energy is transported by radiative transport is too slow for
the amount of energy being produced, the stellar matter may "boil". This method of
energy transport, which is familiar from boiling water or from the rising air
associated with thunderstorms, is called convection. Convection is a very
efficient method of energy transport because it involves the vertical motion of
large packets of gas.
In most normal stars the energy transport is by radiation unless the rate at which
energy is being produced in the interior exceeds a critical value, in which case
the transport becomes convective. In many stars both may operate: some regions of
the interior may transport heat by convection and some by radiative transport.
Radiation of Neutrinos
In massive stars late in their lives the amount of energy that must be transported
is sometimes larger than either radiation of photons or convection can account for.
In these cases, significant amounts of energy may be transported from the center to
space by the radiation of neutrinos. This is the dominant method of cooling or
stars in advanced burning stages, and also plays a central role in events like
supernovae associated with the death of massive stars.