194 ANNUAL KKPOET SMITHSONIAN INSTITUTION, 192 8 



Sir William Hiiggins gave the decisive negative ansAver to this 

 question — some of the nebiilse may be great clusters of stars, but 

 there are others truly named nebulse, for they are masses of glowing 

 gas having the type of spectrum typical of gas whose atoms are 

 radiating their characteristic quanta of energy. 



Such is the nature of the great nebula in Orion and of many 

 other nebulse where vast regions of space are sparsely filled with 

 gaseous matter. Where these gases are sufficiently excited they 

 radiate the distinctive wave lengths of light associated with the 

 atoms and molecules of which they are composed. Thus the spec- 

 troscopist identifies in these nebular spectra the unmistakable radia- 

 tions of hydrogen and helium, often also nitrogen and carbon, and 

 in addition to these he finds intense radiations which until recently 

 were attributed to an unknown substance called nebulium. The 

 mystery is now solved by Doctor Bowen, California, who attributes 

 these radiations to oxygen and nitrogen atoms, radiating in an 

 unusual manner as a result of their ionized condition and the low 

 density of the nebulae. 



The gaseous nebula3 are not all sufficiently hot to radiate; some 

 of them glow only because of the proximity of very bright hot stars ; 

 others are so cool that they absorb all the starlight that falls on them, 

 thus forming great black patches in the sky. A famous dai-k nebula 

 in the region of the Southern Cross is known as the "coal sack." 

 A great American observer, the late Professor Barnard, made a 

 systematic study of these opaque clouds, of which he listed over 180, 

 varying from very small patches with sharp outlines to the long, 

 irregular " dark lanes " so striking a feature of the constellation of 

 Ophiuchus. 



We know, then, that of the celestial objects originally called 

 nebulai some are vast clouds of gas occupying regions of space com- 

 pared with which our solar system is absolutely insignificant; others 

 are clusters of stars; while yet others, in particular the spiral 

 nebulae, are composed of both stars and gaseous nebulosity. 



For a long time no one had any conception of the immensity of 

 spiral nebulas. They were thought of as comparatively small aggre- 

 gations of stars within the great asj^'emblage of stars surrounding 

 our sun in all directions. The authors of the planetesimal theory 

 drew an analogy between the arms of a spiral nebula and the arms 

 of gaseous matter which they assumed to have been drawn out from 

 the surface layers of our sun by the tidal forces produced by a 

 passing star, these disrupted arms giving rise to the several con- 

 densations of matter which eventualh^ became the planets of the 

 solar system. Gradually, however, it became apparent that a spiral 

 nebula was not to be compared with the solar system, but rather 



