414 
ASTRONOMY: H. N. RUSSELL 
solar corona as arising from hypothetical atoms of very simple structure 
— which has successfully met the test of prediction — and of the develop- 
ment of the theory of general relativity, which has already been used by 
deSitter^^ to set a superior limit to the whole quantity of matter in the 
universe, and may have important applications in future. 
16. Of more fundamental nature, and obvious importance, is the un- 
solved problem of the source of the energy which the stars are continually 
radiating at so rapid a rate. It is becoming increasingly plain that the 
gravitational energy liberated by contraction from infinity would not 
nearly sufiice to maintain the Sun^s radiation during geological time^° 
(according to even the more conservative estimates of the latter) ; yet 
the mere continuous existence of life on the Earth is evidence that the 
Sun has not merely kept on shining throughout this interval, but has 
not changed in brightness by more than one magnitude, at the outside. 
In the case of some giant stars, contraction from infinity would hardly 
suffice to furnish the energy which they have radiated during historic 
time.^^ There appear to be two ways out of the difiiculty; either the 
stars do not radiate heat in all directions to space at the same rate as 
they do towards the Earth, or else they have some unknown and ex- 
ceedingly great supplies of internal energy. The first alternative, 
however, seems to be excluded by the fact that the amount of heat 
which the Earth receives from the Sun, and loses again by radiation 
into space, is not greatly, and probably not at all, inferior to that 
which a black body of the same size and temperature as the Earth's 
effective radiating surface would radiate to an enclosure at the absolute 
zero. ^2 There seems therefore no escape from the conclusion that the 
heat radiated by a star can not be provided by contraction. What the 
source of the energy may be, how it is converted into heat in the body 
of the star, and where it goes after passing from the star's surface into 
the ether, are at present the greatest of all the unsolved problems of 
astronomy. 
^Newall, Baxandall and Butler, Monthly Not. Roy. Astron. Soc.y Londofi 76, 1916, (640)- 
2 Fowler, A., Froc. Royal Soc, London, A, 94, 1918, (470). 
3 Fowler, A., Ibid., A, 94, 1918, (472). 
4 Fowler and Brooksbank, Monthly Not. R. A. S., London, 77, 1917, (511-517). 
^Astrophy. J., Chicago, 40, 1914, (466-472). 
6 Curtiss, R. H., Popular Astronomy, Northfield, Minn., 25, 1917, (279-285). 
' Cannon, Miss A. J., Ibid., 24, 1916, (656). 
8 Adams, W. S., these Proceedings, 2, 1916, (143). 
9 Hertzsprung, E., Z. Wiss. Photog., Leipzig, 3, 1905, (429-435). 
10 Adams, W. S., these Proceedings, 2, 1916, (147-156). 
" See Ref. 6; also Rufus, W. C, Pub. Astron. Obs. Univ. Michigan, Ann Arbor, 2, 1916, 
(143). 
