PROBLEMS OF ASTROPHYSICS 449 



temperature of the sun's radiating surface is 6000 Centigrade, 1 and 

 the minimum value for the centre is perhaps five million degrees. In 

 view of these high temperatures, and the low average density of the 

 sun, the interior must be largely gaseous, and perhaps entirely so; 

 although, under the stupendous pressures, a great central core is 

 probably of a viscous consistency, 2 but ready to assume the usual 

 properties of a gas when the convection currents carry the viscous 

 masses up into regions of lower pressure. 



The surface strata are radiating heat into surrounding space. To 

 maintain the supply, it is imperative that convection currents should 

 carry the cooled masses down into the interior, and bring correspond- 

 ing hot masses up to the surface. These currents make the sun a very 

 tempestuous body. Further, the outrushing materials must acquire 

 the higher rotational speeds of the surface strata, and the inrushing 

 must lose their tangential momentum; and these can scarcely be 

 ineffective factors in the sun's circulatory system. 



The mechanical theory of the maintenance of at least a part of 

 the sun's radiation must be considered as a necessary consequence of 

 the law of gravitation as unavoidably a consequence of that law as 

 precession is. Helmholtz computed that a contraction of the solar 

 diameter of less than 400 feet per year 3 would suffice to maintain the 

 present rate of flow. Whether this is the sole source of supply is 

 uncertain, and very doubtful. The discovery of sub-atomic forces in 

 uranium, thorium, and radium is of interest in this connection. These 

 radioactive substances have revealed the existence of intense forces 

 within the atom, long dreamed of by students of physics and chem- 

 istry, but never before realized. The energy radiated by an atom of 

 these substances is thousands of times greater than that represented 

 by the ordinary chemical transformations of equal masses of any 

 known element. Whether these forces are working within the sun, 

 prolonging its life many fold, and incidentally diminishing the re- 

 quired rate of Helmholtzian contraction, we do not know; but we are 

 not justified in treating gravitation as the sole regulator of radiation. 4 

 We are encouraged to this view by the fact that the age of the earth, 

 as interpreted by geology and biology, is many times greater than the 

 superior limit set by the gravitational theory. 



The dazzlingly brilliant photospheric veil which limits the depth 

 of our solar view is due, with no room for doubt, to the condensation 

 of those metallic vapors which, by radiation to cold space, have cooled 

 below their critical temperatures. These clouds form and float in 



1 Young, Popular Astronomy, xn, 225. 



1 Young, The Sun, p. 331. 



8 Ibid, p. 315. 



* Young, Popular Astronomy, p. 225. This article, and the volume referred to 

 in footnote no. 2 are the best existing repositories of facts and theories relating 

 to the sun. 



