3 o6 POPULAR SCIENCE MONTHLY. 



number discoverable with this instrument, with exposures of ordinary 

 length, would be half a million. This estimate would be too large, 

 in case the smaller nebulae have a tendency to cluster around the 

 prominent nebulae, which to some extent is probably true. The num- 

 ber of stars visible in our great telescopes is of the order of one hun- 

 dred millions. The dark or invisible bodies indicated by several 

 considerations — the planets in the solar system, the spectroscopic 

 binaries, the eclipsing variable stars, and the gravitational power of 

 the universe — should outnumber the bright ones several fold. 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 center, 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 molds 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 

 resemblance in the Trifid Nebula, which is apparently breaking up 

 into irregular masses; the Dumb Bell Nebula, from whose nearly 

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

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

 planetary nebula G. C. 4390, 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 

 can not 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 and Zeta Puppis are of this class. An- 

 other is DM. -f- 30.° 3639, which is actually surrounded with a spherical 

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

 little further removed from the nebular state are the stars containing 

 both bright and dark hydrogen lines; — caught, so to speak, in the 

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

 Cassiopeiae, Pleione and Mu Centauri are examples. Closely related 



