Fig. 60.—Comparison of spectra of stars of classes 2, 3, 4. After photographs taken at the Yerkes Observatory. Green and yellow portions of spectrum. Wave-lengths in millionths of a millimetre] “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 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 Cassiopeiæ, Pleione, and My Centauri are examples. Closely related to the foregoing are the helium stars. Their absorption lines include the Huggins hydrogen series complete, a score or more of the conspicuous helium lines, frequently a few of the Pickering series, and usually some inconspicuous metallic lines. The white stars in Orion and in the Pleiades are typical of this age. “The assignment of the foregoing types to an early place in stellar life was first made upon the evidence of the spectroscope. The photographic discovery of nebulous masses in the regions of a large proportion of the bright-line and helium stars affords extremely strong confirmation of their youth. Who that has seen the nebulous background of Orion (Fig. 51) or the remnants of nebulosity in which the individual stars of the Pleiades (Fig. 52) are immersed can doubt that the stars in these groups are of recent formation?