The pressure force ,which opposes the self gravitation force ,takes the form of a gradient of pressure ; that is to say the pressure higher nearer the centre of the star than further out The thermal structure of a star is related to the heat flowing through it in a complicated way. It turns out that for typical main-sequence stars, the temperature, density and pressure all increase towards the centre. Now we ask: what happens to the heat energy the star contains ? The answer is that the heat must flow down the temperature gradient. This is the concept of energy transport in a star. The above argument is very simplified, but the same conclusion follows by saying that a star is hot inside and radiating away energy from its surface; energy must flow from inside to outside to replenish the loss. The energy must flow through the star by conduction, convection or radiation.

A simple argument shows that conduction is generally unimportant as a means of carrying energy in stars, compared with radiation. Conduction is a process in which electrons move the heat energy whereas in radiation photons carry the energy Generally the amount of energy carried by the electrons may be higher than that carried by the photons, but the average distance an electron can travel before interaction with another particle (its MEAN FREE PATH) is much smaller than the mean free path of a photon. This means that the photons find it easier to take energy away from the hotter parts of the Star, even though an individual photon carries less energy than a typical electron. We can fix our ideas at a point half-way between the surface and centre of the Sun in order to give an example. In this locality the mean free path of a photon, its typical unimpeded travelling distance, is about 1 cm, whereas it is only about 10 nm (one million times less) for an electron. The large difference in mean free path demonstrates that, generally, conduction can be neglected as a means of carrying energy in stars. The main exception comes in degenerate conditions: then the density is high and the radiation is absorbed after only a small mean free path. Furthermore the peculiar momentum distribution of the electrons m the very dense parts of stars permits some electrons to have a rather long mean free path; in such a case conduction may be an important energy transport mechanism.

When energy moves by convection, the carrier of energy is a blob of material which rises bodily upwards from a hotter part of the star and deposits its heat in a cooler layer above. Cooler blobs also descend and a circulation motion is therefore set up. The motion has some similarity to that of a heavier liquid on top of a lighter one. The relative importances of convection and radiation as energy transporters in different parts of a star can be estimated by a mean free path argument similar to that used above for conduction and radiation. In general, once convection gets started it is a highly efficient way of moving heat through a star. Consequently we need only know whether convection will start or not, since if conditions are right for the onset of convection it will dominate the energy transport. Convection starts when the resistance to radiation is very high (that is to say when the stellar material is very opaque to radiation) such as is the case in the outer layers of a cool star. It can also begin when the amount of energy to be carried is so large that radiation cannot do so and mass motions become inevitable as the heat builds up; this happens near the centre of a very luminous star. Of course, the exact picture of energy transport is more complicated than we have sketched above and it includes thermodynamic effects; the theory becomes especially problematical when the efficiency of convection is low, which occurs in the very outer layers of cool stars.

Except in the special conditions noted above, the main trans¬porter of energy in a star is radiation. The photons carrying this radiant energy are continually emitted and absorbed in the way described so aptly in the quotation from Eddington. While all the atomic processes are almost exactly in balance there is, nevertheless, a net diffusive outward flow of energy in response to the temperature gradient (about 10-2 degrees per metre) which alone dictates the direction along which the net flow of energy must occur.

Comments are closed.