456 ASTROPHYSICS 



dicated by several considerations the planets in the solar system, 

 the spectroscopic binaries, the eclipsing variable stars, and the gravi- 

 tational power of the universe should outnumber the bright ones 

 several fold. 1 It is the thesis of astrophysics that all these objects 

 the nebulae, the bright stars, and the invisible bodies-- are related 

 products of a system of sidereal evolution. The general course of the 

 evolutionary process, as applied to the principal classes of celestial 

 objects, is already known. We are able to group these classes, with 

 little chance of serious error, in the order of their effective ages. 



The earliest form of material life known to us is that of the gaseous 

 nebulae. In accordance with the simplest of physical laws, a nebula 

 must radiate its heat to surrounding space. In accordance with 

 another law, equally simple, it must contract in volume, toward 

 a centre, or toward several nuclei, and generate additional heat in 

 the process. Eventually a form of considerable regularity will result. 

 Whether this form is that of a typical planetary nebula, of a spiral 

 nebula, or of some other type, is a matter of detail. It is quite possible 

 that nature uses several moulds in shaping the contracting masses, 

 according as they lie on one side or the other of critical conditions. The 

 variety of existing forms is extensive. One can see very little resem- 

 blance in the Trifid Nebula, 2 which is apparently breaking up into 

 irregular masses; the Dumb-Bell Nebula, 3 from whose nearly circular 

 form rings of matter seem to be separating; the great spiral nebulae; 3 

 the Ring Nebula in Lyra, 4 with a central star; the compact planetary 

 nebula G. C. 4390, 5 containing a dense, well-defined nucleus; and 

 many others of distinct types. 



The condensed globular forms occupying the positions of nebular 

 nuclei have almost reached the first stage of stellar life. 



It is not difficult to select a long list of well-known stars which 

 cannot be far removed from nebular conditions. These are the stars 

 containing both the Huggins and the Pickering series of bright 

 hydrogen lines, the bright lines of helium, and a few others not yet 

 identified. Gamma Argus 6 and Zeta Puppis 7 are of this class. An- 

 other is DM. +30.3639, 8 which is actually surrounded with a spher- 

 ical atmosphere of hydrogen, some five seconds of arc in diameter. 

 A little further removed from the nebular state are the stars contain- 

 ing both bright and dark hydrogen lines; 9 caught, so to speak, in 

 the act of changing from bright-line to dark-line stars. Gamma 



1 Report, British Association for the Advancement of Science, 1901, 563. 



2 Newcomb, The Stars, Frontispiece. 

 8 Roberts's Celestial Photographs. 



4 Astrophysical Journal, x, 193. 



5 Publications, Lick Observatory, in, following p. 229. 



6 Astronomy and Astro-Physics, xm, 456. 



7 Astrophysical Journal, v, 92. 



8 Astronomy and Astro-Physics, xm, 461. 

 8 Astrophysical Journal, n, 177. 



