STELLAR EVOH'TION. 15',) 



far al)ovc the wliitc-Jiot mass of condeiised vapors which (•onstitute 

 the iniclens of tlie star, is at this stage the predominant (denuMit, at 

 least so far as we may jvldge from a study of the light radiation. 



An interesting question has arisen regarding the period in a star's 

 life at which the highest temperature is attained. The appai'ently 

 paradoxical statement of Lane's law that the temperature of a cooling 

 mass of incandescent vapors, instead of falling, actually increases until 

 a certain stage has passed, applies in the present instance. We, indeed, 

 know that a condensing nel^ula, losing heat by radiation into space, will 

 continue to rise in temperature for thousands and even millions of 

 years. A question which has received some discussion of late is with 

 regard to the precise period at which the maximum ttMuperature occurs. 

 Shall we seek it in white stars like Sirius or in yellow stars like the 

 sun, which i-epresents the next well-dehned stage of stellar evolu- 

 tion 'i Witli an instrument of extraordinary' delicacy Professor Nichols 

 has recently measured at the Yerkes Observatory the amount of lieat 

 which we receive from Vega and Arctui'us. Tho distance of these 

 stars is so inconceivably great that the quantity of heat which they 

 send to the surface of the earth has hitherto been too small to be 

 detected by the most sensitive instruments. Professor Nichols's radi- 

 ometer, which, in coml)ination with a large concave mirror, renders it 

 easy to measure the heat radiated from a man's face 2,000 feet away, 

 pro\ed adequate for the task. He found that Arcturus sends us about 

 as nuich heat as we should get from a candle (3 miles away if there were 

 no intervening atmosphere to reduce the candle's intensity. Vega, which 

 to the eye is precisely equal to Arcturus in l)rightness, was found to 

 send us only half as nuicli heat. If the absorbing atmospheres of 

 Arcturus and Vega were similar in character, it would follow, from 

 Professor Nichols's results, that Vega, though it sends us less heat, is 

 really the hotter of the two stars. For we know from laboratory 

 experiments that the proportion of long (heat) waves to short (light) 

 waves is greater in the radiation of the cooler of two bodies heated to 

 incandescence. In this case the fact that Arcturus sends the greater 

 amount of heat would be ascribed rather to greater size than to lesser 

 distance, as there is good reason to l)elieve that it is farther from us 

 than Vega. 



But unfortunatidy the dissimilarity of the atmospheres of th(^ two 

 stars renders it unc(M'tain whether such conclusions can safely be drawn. 

 This is particularly true in view of the fact that Sir William Huggins 

 concludes from his spectroscopic studies that the highest stage of stellar 

 temperature is reached in stars likc^ Arcturus aud the sun, while stars 

 like Vega are still in an earlicn- and less highly h(>ated condition. 



While some uncertainty nuist therefore pi-evail until furtluM- investi- 

 gations have been conqileted I'egarding the (^xact stage at which the 

 highest stellar temperatur(>s are attained, there can be little doubt as 



