Introduction

We have seen in the previous two chapters that the gravitational attraction inside a normal star is balanced by the thermal pressure caused by the thermonuclear reactions taking place in the stellar interior. Eventually, however, the nuclear fuel of the star is exhausted and there is no further source of thermal pressure to balance gravity. We have pointed out in §4.5 that such a star keeps on contracting – unless some kind of pressure other than thermal pressure is eventually able to balance gravity again. The aim of this chapter is to discuss the possible end configurations of stars which have nonuclear fuel left in them.

We have to make use of one very important property of Fermi particles. In a unit cell of volume h3 in the six-dimensional position-momentum phase space, there cannot be more than two Fermi particles (one with spin up and the other with spin down). The electrons inside the stellar matter make up a Fermi gas, and when the density inside the contracting star becomes sufficiently high, this electron gas becomes ‘degenerate’. This means that the theoretical limit of twoparticles per unit cell of phase space is almost reached. We shall show in §5.2 that such a degenerate Fermi gas exerts what is known as the degeneracy pressure. White dwarf stars discussed in §3.6 are believed to represent stellar configurations in which the inward pull of gravity is balanced by the degeneracy pressure of the electron gas.