180 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1926 
A nebula condensed may thus become one or many giant red 
stars. May not the giant gaseous red stars, by further condensing 
grow hotter and hotter? They would thus become in turn light red, 
yellow, white, and blue. Having reached the acme of their effulgence, 
they might gradually cool again with increasing density until they 
should have passed through these stages in reverse order, and might 
end their starlike history as red dwarfs, still hot, but now com- 
pressed so small as to give little light. 
10. The gradations of spectrum, of temperature, and of density 
which have been traced among the stars support this view of stellar 
evolution. The question now comes, is it harmonious to the laws 
of physics as we know them? Here we have the results of the great 
English scientist, Professor Eddington. He finds that in accordance 
with the law announced many years ago by Lane, it is perfectly 
to be expected that a great globe of rare gas will become hotter and 
hotter as it shrinks, and its supply of radiation to space will keep 
up while its diameter shrinks, because its temperature rises. The 
change of spectrum, too, is well accounted for. Though the signs 
of the metals seem to disappear in the white and blue stars, this is 
only because the more violent excitation strips off from the metallic 
atoms certain of their orbital electrons, and causes the spectral lines 
to occur beyond the violet in a spectral region where the ozone of 
our upper atmosphere cuts off the light of all the stars. The metals 
still exist in the blue stars, but their signs do not reach us because 
our atmosphere prevents it. 
But Eddington found later on that even stars as dense as our sun, 
that is to say denser than water, behave as if they are perfect gases. 
This was at first a puzzle, until it occurred to him that in a gas it 
is the size of the separate entities, compared to their distances apart 
that determines whether they have the freedom of the perfect gas. 
In the interior of a star, as dense and hot as our sun, the atoms are 
well known to be broken up into their nuclei and their electrons. 
These objects are some thousandfold smaller in diameter than atoms 
or molecules. Hence, they have complete freedom of motion in 
stars so dense that ordinary gases (made up of molecules instead of 
electrons) would not at all obey the perfect gas law. 
11. This discovery for the moment threw discredit on the view 
that stars after reaching a certain density could no longer become 
hotter by contraction, and must inevitably thereafter begin to cool, 
and so go back down the descending branch in stellar evolution. It 
is now suggested that they keep on growing hotter within. Tem- 
peratures of tens and hundreds of millions of degrees occur, it is 
computed, inside such stars as our sun. 
At these unexampled degrees of effulgence, the rays sent out are 
even shorter in their wave lengths than X rays, and are so power- 
