NO. lO 



SMITHSONIAN EXPLORATIONS, I923 



29 



intensity of solar heat. From rough preHminary computations it 

 appears, for instance, that the radiation sent by the bright star Alde- 

 baran, if collected over a square mile, would produce i calory of heat 

 per minute, whereas the sun's radiation collected over a surface of i 

 square centimeter, that is to say about three-eighths of an inch on a 

 side, amounts to i .94 calories per square centimeter per minute. 



It is also possible, in this manner, to determine the diameters of 

 some of the stars, providing their distance from the earth is known. 

 In the case of the star Aldebaran, preliminary computations give the 

 diameter as 58,000,000 miles. 



Still more interesting are the opportunities offered by the method 

 for estimating the temperatures of the stars. In the case of Aldebaran. 



Fig. 31. — The energy spectrum of Aldebaran, as reduced to wave-length 



scale, and compared to the perfect radiator at 



3000° Absolute Centigrade. 



the distribution of heat in the spectrum between wave-lengths 0.4 

 and 2.0 microns, that is to say between a point in the violet and a 

 point far beyond the end of the visible red, fits almost precisely vipon 

 the curve of the radiation of the perfect radiator or " absolutely 

 black body " of 3,000° Absolute Centigrade. The fit is. indeed, start- 

 lingly close, so that one has no hesitation in assigning to the star 

 Aldebaran the temperature 3.000° Absolute Centigrade. In the case 

 of other stars, including our sun. the fit is less exact, so that one can 

 only give moderately approximate estimates of their temperatures, 

 but the accurate determination of the distribution of the stellar heat 

 in the spectrum cannot but lead to advances in our knowledge of the 

 physical constitution of the stars. 



The accompanying figure 30 shows the results as originally ob- 

 served on the prismatic spectrum of the sun and the stars Rigel, 



