312 FKOM NEBULA TO NEBULA 



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 thoir 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 

 ranged behind them, the predominance of such lines in 

 celestial spectra should cease to surprise. Given suf- 

 ficient allowance of time in which to act, gravitation 

 would of itself dispose the elements in the same order, 

 that is, the lighter on the outside. 



The so-called " green " nebulas are therefore younger 

 than the " white", and, by the same token, naturally 

 larger, from not having been condensed by long exposure 

 to the cold of space. Their youthfulness is further at- 

 tested by the fact that less than five per cent of nebulae 

 are green, a circumstance which opens interesting ave- 

 nues to the determination of the relative ages of nebulae 

 in general. The Milky Way, being the densest portion 

 of the sidereal system, is naturally richest in all sorts 

 of nebulae, but particularly in the percentage of those 

 of this green, or ultra-gaseous, type. The difference be- 

 tween the various classes of nebulae is therefore acci- 

 dental rather than fundamental or generic. The manner 

 of their formation (that is, by explosion) naturally ar- 

 rests and masks such proper motions as their parent 

 star may have had just prior to the catastrophe, so that 

 the new-born nebula is obliged to "find itself", as it 

 were, and to acquire its new motions by a protracted pro- 

 cess of gravitational acceleration. As the "dust from 

 the explosion" settles and clears, the nuclei, which were 

 always there, but only hidden from our sight, loom 

 gradually into view, and then we perceive a spiral nebu- 

 la, evenly poised like a giant pinwheel on a center, re- 

 crudescing into a new cycle of existence. 



