3o6 POPULAR SCIENCE MONTHLY. 



casual examination of the spectrum. The period for their greatest 

 development has not yet arrived. The light gas hydrogen, reaching 

 far above the white-hot mass of condensed vapors which constitutes 

 the nucleus of the star, is at this stage the predominant element, at 

 least so far as we may judge 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 apparently 

 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 

 Icnow that a condensing nebula 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 temperature occurs. 

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

 sun, which represents the next well-defined stage of stellar evolu- 

 tion? With an instrument of extraordinary delicacy Professor Nichols 

 has recently measured at the Yerkes Observatory the amount of heat 

 v.'hich we receive from Vega and Arcturus. The 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 de- 

 tected by the most sensitive instruments. Professor Nichols' radiometer, 

 which in combination with a large concave mirror renders it easy to 

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

 adequate for the task. He found that Arcturus sends us about as much 

 heat as we should get from a candle six 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 brightness, was found to send 

 us only half as much heat. If the absorbing atmospheres of Arcturus 

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

 Nichols' 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 believe that it is farther from us than Vega. 



But unfortunately tlie dissimilarity of the atmospheres of the two 

 stars renders it uncertain 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 like Vega, while stars like Arcturus and 

 the Sun have passed the stage of highest temperature and are already 

 well advanced in their decline. 



