AN ADDRESS ON ASTROPHYSICS. 3°9 



involved in the phenomena are extremely small, or the disturbances 

 are but skin-deep. In any case, the novas afford little evidence as to 

 the complete re-nebularization 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 

 spectrum for the white stars than for the yellow and red, a safe indi- 

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

 distinctly the enhanced lines thought to be produced by high tempera- 

 tures. 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 tempera- 

 tures. 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 companion. The 

 subject is one of great difficulty and importance, and, unfortunately, 

 laboratory methods are on too small a scale of mass and pressure to 

 solve the problem. 



Up to the year 1800 only twelve variable stars were known. 

 Chandler's catalogue dated 1888 contains 225 entries. The remark- 

 able progress made by astronomical science in the past fifteen years is 

 fairly indicated by the fact that in this interval the number of known 

 variable stars increased from 225 to more than 1400. To Harvard 

 College Observatory belongs the great credit of discovering nearly 

 900 of these objects. 



In many respects variable stars constitute the most interesting 

 class of objects in the heavens. The tens of millions of ordinary stars 

 are undoubtedly growing older; and the tens of thousands of nebulas, 

 from which stars will eventually be formed by processes of condensa- 

 tion, are undergoing transformation; but appreciable changes in the 

 ordinary stars and in the nebulas proceed with extreme deliberation, 



