165, 166 ] White Dwarfs 181 
the density and the star’s centre and the consequent deviations from the gas- 
laws. We have seen (§ 143) that when the deviations from the gas-laws become 
sufficiently great the barrier is no longer impenetrable. If the star presses 
sufficiently hard against the barrier a door suddenly opens, the star falls 
through to a still lower, and very much lower, terrace and finally regains 
stability as a white dwarf. 
In a white dwarf the great majority of the atoms are stripped bare down 
to their nuclei and so are immune from annihilation and have almost no 
capacity for generating radiation. As a consequence stars on the white 
dwarf terrace are only very feebly luminous, and the fall to the white dwarf 
branch is the end of a star’s career as a brilliant luminary. It is slightly 
disconcerting to find that our sun’s position in the temperature-luminosity 
diagram suggests that it is pressing with perilous force against the dangerous 
edge of the main sequence, so that its collapse into a feebly luminous white 
dwarf may commence at any moment. 
The white dwarf state represents the utmost limit of contraction which 
is observed. No further rings of electrons remain to be ionised, and even the 
central temperatures of the white dwarfs are insufficient to produce any 
appreciable nuclear disintegration. As regards further evolution, the white 
dwarf state is one of almost complete stagnation, changes of mass and 
luminosity being practically inappreciable within periods comparable with the 
whole life of ordinary stars. In the next 7 x 10 12 years or so, each component 
of Plaskett’s star is likely to change into a star very similar to our sun; in 
the same interval the only change in Sirius B will be a loss of one part in 
700 of its present mass, probably accompanied by a quite inappreciable change 
in its luminosity. 
Eddington* has pointed out that interesting and somewhat delicate 
questions arise as to the ultimate state of the white dwarfs, to which R. H. 
Fowlerf has given an answer in terms of the Fermi-Dirac mechanics. In 
brief, Fowler finds that in its last stage, all the nuclei and electrons of a 
white dwarf may be regarded as forming one gigantic molecule which is in 
its lowest quantum-state. In this state it emits no radiation and its energy 
cannot be further diminished. 
We can see in a general way what type of star is most likely to become 
a white dwarf. In brief, of course, it is the type of star which presses most 
forcibly against the not quite impenetrable barrier formed by the left-hand 
edge of the main sequence. This again is the type which would wander 
furthest over to the left-hand of the diagram if the barrier were not there— 
i.e. if the gas-laws were obeyed. 
In discussing stars in which the gas-laws were obeyed, we saw that the 
process of fission placed both components well over to the left of the diagram. 
In a young star in which the atomic ingredients are fairly well mixed, the 
* The Internal Constitution of the Stars, § 117. + M.N. lxxxvii. (1926), p. 114. 
J 
12