PROBLEMS OF ASTROPHYSICS 459 



elements. It is true that a collision may occur to transform a dark 

 body's energy of motion into heat, sufficient to convert it into a glow- 

 ing nebula, and start it once more over the long path of evolution. 

 This is a beautiful theory, but the facts of observation do not give 

 it satisfactory support. There is little doubt that the principal novae 

 of recent years have been the results of collisions, 1 either between two 

 massive dark bodies, or between a massive body and an invisible 

 nebula. The suddenness with which intense brilliancy is generated 

 would seem to call for the former, but the latter is much more prob- 

 able, in view of many facts. The nebular spectra of the novae are 

 generated in a few months : \ but in every case thus far observed the 

 bright nebular bands grow faint very rapidly, and in the course of 

 a few years leave a continuous spectrum, apparently that of an 

 ordinary star. Either the masses involved in the phenomena are 

 extremely small, or the disturbances are but skin-deep. In any case, 

 the novae afford little evidence as to the complete renebularization 

 of dark bodies. 



I spoke of the average temperature of a developing star as reaching 

 a maximum near the solar stage when the border-line between gaseous 

 and liquid constitution is reached. This refers to the entire mass. 

 The law of surface temperatures is quite a different one. The bright- 

 line and helium stars seem to have hotter surfaces than the solar and 

 red stars. The spectra which we observe are surface phenomena which 

 indicate the temperatures of the radiating and absorbing strata. The 

 maximum intensity of continuous radiations is higher up in the spec- 

 trum for the white stars than for the yellow and red, a safe indication 

 of higher temperatures. The lines in white-star spectra are distinctly 

 the enhanced lines thought to be produced by high temperatures. 

 These facts are not inharmonious. Surface temperature is a function 

 of the rapidity with which convection currents can carry heat from 

 the interior to the surface. The comparatively low internal heat of 

 white stars, delivered quickly at the surface by rapidly moving gases, 

 may readily maintain higher atmospheric temperatures than the 

 much hotter interiors of solar stars, whose circulation has the sluggish- 

 ness of viscosity. 



Sir William and Lady Huggins are inclined to assign greater im- 

 portance to mass and density, as factors in evolution, than to tem- 

 peratures. 3 Their view is that under the influence of great surface 

 gravity, the generation and radiation of heat is accelerated, and the 

 life of the star is lived more rapidly. They have been led to this view, 

 in part, by the apparent anomaly of double stars, in which the more 

 massive primary is generally yellower than the less massive com- 



1 Astronomy and Astro-Physics, xi, 907. 



2 Ibid, xi, 715. 



3 Clerke's Problems in Astro-Physics, p. 274. 



