64 CARNEGIE INSTITUTION 



removed the intensity of the sun's light would increase by as much 

 as I )^ times for the red rays to 2 J^ times for the violet rays. Under 

 such circumstances the color of the sun would appear blue. Later 

 results indicate that the remov^al of the sun's atmosphere would in- 

 crease its radiation of heat about 1.7 times. It is evident that if 

 there were any considerable variation in the absorptive power of the 

 sun's atmosphere, due to such changes as might easily take place 

 during the passage from minimum to maximum solar activity, the 

 total radiation of solar heat might be very appreciably increased or 

 diminished. Indeed, such a variation in the solar absorption is con- 

 sidered by Halm, in his recent memoir on the solar constitution, to 

 be the cause of the eleven year sun spot period. It is therefore evi- 

 dent that the plan of observations should include an investigation 

 of the heat radiation of various parts of the sun's disk, carried on 

 during a term of years. 



EvohUion of the Sun and Stars. 



We have already- called attention to the fact that while telescopic 

 observations may give us an intimate acquaintance with the sun's 

 surface phenomena, from which conclusions may be drawn as to 

 its physical constitution, they can give us no direct information as 

 to the past or the future of the sun. We must seek such informa- 

 tion in the stars and nebulae, Keeler's work with the Crossley re- 

 flector of the Ivick Observatory brought out the extraordinary fact 

 that there are in the heavens at least 100,000 nebulae, the majority 

 of which are of spiral form. It is the belief of many astronomers 

 that this spiral form is indicative of a process of development, and 

 that the teachings of the nebular hypothesis must be modified so as 

 to accord with this new point of view. Advantage should be taken 

 of every possible increase in the scale of the photographs by which 

 the forms of spiral nebulae are recorded, in order that further data 

 may be obtained that may be of service in future attempts to modify 

 the nebular hypothesis. Photographs taken with all possible in- 

 strumental refinements might well be expected to show changes in 

 the nebulae after the lapse of a comparatively short period of time. 



While we may thus seek in the nebulae for evidences of the sun's 

 origin, its early life and subsequent development may be traced in 

 the stars ; for, in spite of their enormous distance from the earth 

 and the consequent impossibility of observing their surface phe- 

 nomena as we do those of the sun, the spectroscope is competent to 



