70 ILLINOIS STATE ACADEMY OF SCIENCE 



however delicate the means of detection might be we 

 could only hope to detect the relatively brightest regions. 

 In addition to the nebulosity that is luminescent we 

 recognize a large amount of nebulosity that is not lum- 

 inescent by its accidental projection upon a bright back- 

 ground. In addition to the many examples of this given 

 by the astronomical photographs, there is a splendid 

 naked eye example lying across the Milky Way near the 

 star Deneb, which under brilliant skies may easily be 

 mistaken for a cloud in our own atmosphere. Indeed, 

 the more the subject of nebulosity is studied the clearer 

 it becomes that perceptibly nebulous regions are very 

 common, and it seems not at all improbable that all of 

 space actually is more or less nebulous. 



In addition to the correlation of the spectral types of 

 classes of stars and their masses and densities Avhich has 

 already been discussed, a second correlation is known 

 which is worthy of note. It is known that the average 

 speeds of classes of stars increases steadily from the 

 slowly moving massive type B stars to the relatively 

 rapid type M stars, which as a class have the smallest 

 masses. This correlation, which was very puzzling at the 

 time of its discovery some ten years ago, is a natural 

 consequence of the present hypotheses. If a star plunges 

 into a richly nebulous region and thereby adds consider- 

 ably to its mass, its spectrum will be pushed towards the 

 B type for the reasons which have already been ex- 

 plained, while its speed will be decreased, the matter 

 which is gathered in acting somewhat like friction. On 

 the other hand a star which is consuming its mass more 

 rapidly than it is adding thereto will increase in speed 

 if it be granted, as seems natural, that its momentum re- 

 mains constant. There is difficulty in this correlation 

 only with the giant stars, which instead of moving slower 

 with increasing mass seem possibly to move faster.* The 

 exceptional masses of the giant stars and their relatively 

 small numbers imply that they have penetrated into re- 

 gions which are exceptionally nebulous. If these were 

 regions of high potential energy, high velocities would of 

 course be the natural result. It is useless however to 



The recent investigations of Seares indicates that this is not so. 



