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 nebulæ have a tendency to cluster around the prominent nebulæ, which to some extent is probably true. The number of stars visible in our great telescopes is of the order of one hundred 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 nebulæ, 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 nebulæ. 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 nebulæ; 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. Another is DM. + 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
