Energy Transport in Stars

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:

1. Radiative transport of photons
2. Conduction
3. Convection
4. 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 surface.

Conduction

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.

Convection

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.