Constitution and Evolution of the Stars 95
This theory of stellar evolution was first propounded
by Sir Norman Lockyer who outlines clearly the physical
processes involved. His criteria for distinguishing be-
tween stars of rising and falling temperature were spectro-
scopic, and chosen in a rather arbitrary way, with little
explanation (though they were not very far from anticipat-
ing Adams’ later discovery), and his views failed of
general acceptance. It fell to the speaker’s lot, some years
later, to revive the theory, and point out the importance
of the absolute magnitudes, which, indeed, furnish the key
to the whole problem. This invaluable aid was not avail-
able when Lockyer began his work—for in those days
little indeed was known of stellar parallaxes—so that it is
not surprising that his individual assignments of stars to
the classes of rising and falling temperature are often
erroneous. With the wealth of material now available, it
is an easy matter to point out stars in every successive
stage of evolution, and to assign the large majority of
those for which we have data to their place in its sequence.
Mention should again be made, however, of the few, faint,
but perplexing white stars of low luminosity. These do
not fit into the scheme at all, and they present such an
extraordinary combination of high temperature, small
luminosity and considerable mass that it is very difficult to
form any consistent idea of the physical conditions which
exist on their surfaces. There are indeed more worlds for
theory to conquer—and some of them look as if it would
take hard fighting.
But there are other ways in which our knowledge of
the properties of matter may be applied to the stars. A
simple calculation shows that the gravitational pressure at
the centre of the Sun must be something like a hundred
million tons per square inch. The pressures in other dwarf