146 POPULAR SCIENCE MONTHLY. 



red toward the violet end. It would seem, therefore, that the hotter 

 stars should be the white ones and the cooler the yellow or red ones. 



There are, however, two circumstances to be considered in connec- 

 tion with the contracting star. In the first place, the light which we 

 receive from a star does not emanate from its hottest interior, but from 

 a region either upon or, in most cases, near its surface. It is, there- 

 fore, the temperature of this region which determines the color of the 

 light. In the next place, part of the light is absorbed by passing 

 through the cooler atmosphere surrounding the star. It is only the 

 light which escapes through this atmosphere that we actually see. 



In the case of the Sun all the light which it sends forth comes from 

 an extreme outer surface, the photosphere. The most careful tele- 

 scopic examination shows no depth to this surface. It sends light to 

 us, as if it were an opaque body like a globe of iron. This surface 

 would rapidly cool off were it not for convection currents bringing up 

 heated matter from the interior. It might be supposed that such a 

 current would result in the surface being kept at nearly as high a tem- 

 perature as the interior; but, as a matter of fact, the opposite is the case. 

 As the volume of gas rises, it expands from the diminished pressure and 

 it is thus cooled in the very act of coming to the surface. 



In the case of younger stars, there is probably no photosphere 

 properly so called. The light which they emit comes from a consider- 

 able distance in the interior. Here the effect of gravity comes into 

 play. The more the star condenses, the greater is gravity at its sur- 

 face; hence the more rapidly does the density of the gas increase from 

 the surface toward the interior. In the case of the Sun, the density of 

 any gas which may immediately surround the photosphere must be 

 doubled every mile or two of its depth until we reach the photosphere. 

 But if the Sun were many times its present diameter, this increase 

 would be less in a still larger proportion. Hence, when the volume is 

 very great the increase of density is comparatively slow; there being no 

 well-defined photosphere, the light reaches us from a much greater 

 depth from the interior than it does at a later stage. 



The gradual passing of a white star into one of the solar type is 

 marked by alterations in its spectrum. These alterations are especially 

 seen in the behavior of the lines of hydrogen, calcium, magnesium and 

 iron. The lines of hydrogen change from broad to thin; those of 

 calcium constantly become stronger. 



Of the greatest interest is the question — at what stage does the 

 temperature of the star reach its maximum and the body begin to cool? 

 Has our Sun reached this stage? This is a question to which, owing 

 to the complexity of the conditions, it is impossible to give a precise 

 answer. It seems probable, however, that the highest temperature is 

 reached in about the stage of our Sun. 



