THE STARS AND NEBULAE 275 



tern; but the hypothesis has recently been revived so far as re- 

 gards the spiral nebulae. Although the same term "nebula" is 

 used to denote the three classes irregular, planetary and spiral 

 we must not be misled into supposing that there is any close re- 

 lation between these objects. All the evidence points to a wide 

 distinction between them. We have no reason to believe that the 

 arguments which convince us that the irregular and planetary 

 nebulae are within the stellar system apply to the spirals. 



It must be admitted that direct evidence is entirely lacking 

 as to whether these bodies are within or without the stellar sys- 

 tem. Their distribution, so different from that of all other ob- 

 jects, may be considered to show that they have no unity with 

 the rest; but there are other bodies, the stars of Type M for in- 

 stance, which remain indifferent to galactic influence. Indeed, 

 the mere fact that spiral nebulae shun the galaxy may indicate 

 that they are influenced by it. The alternative view is that, lying 

 altogether outside our system, those that happen to be in low 

 galactic latitudes are blotted out by great tracts of absorbing mat- 

 ter similar to those which form the dark spaces of the Milky Way. 



If the spiral nebulae are within the stellar system, we have no 

 notion what their nature may be. That hypothesis leads to a 

 full stop. It is true that according to one theory the solar system 

 was evolved from a spiral nebula, but the term is here used only 

 by a remote analogy with such objects as those depicted in the 

 Plate. The spirals to which we are referring are, at any rate, too 

 vast to give birth to a solar system, nor could they arise from the 

 disruptive approach of two stars; we must at least credit them 

 as capable of generating a star cluster. * * * 



The two arms of the spiral have an interesting meaning for 

 us in connection with stellar movements. The form of the 

 arms a logarithmic spiral has not as yet given any clue to the 

 dynamics of the spiral nebulae. But though we do not understand 

 the cause, we see that there is a widespread law compelling matter 

 to flow in these forms. 



NEBULA- AND STAR SPECTRA 



The manner of the formation of nebulae, as just de- 

 scribed, seems to me to supply the key to the peculiarities 

 of thviir spectra. For the freshly liberated gases will 

 be expelled to unequal distances, and the lighter elements 

 the farther. The outer envelopes of the nebula should 

 therefore consist largely of hydrogen and similar gases; 

 and if these envelopes be sufficiently dense so as to be 

 opaque to the light of the other incandescent elements 



