"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?
"With the lapse of time, stellar heat radiates into space, and, so far as the individual star is concerned, is lost. On the other hand, the force of gravity on the surface strata increases. The inevitable contraction is accompanied by increasing average temperature. Changes in the spectrum are the necessary consequence. The second hydrogen series vanishes, the ordinary hydrogen absorption is intensified, the helium lines become indistinct, and calcium and iron absorptions begin to assert themselves. Vega and Sirius are conspicuous examples of this period. Increasing age gradually robs the hydrogen lines of their importance, the H and K lines broaden, the metallic lines develop, the bluish-white color fades in the direction of the yellow, and, after passing through types exemplified by many well-known stars, the solar stage is reached. The reversing layer in solar stars represents but four or five hydrogen lines of moderate intensity; the calcium lines are commandingly permanent, and some twenty thousand metallic lines are visible. The solar type seems to be near the summit of stellar life. The average temperature of the mass must be nearly a maximum; for the low density indicates a constitution that is still gaseous [compare Chapter VII.].
"Passing time brings a lowering of the average temperature. The color passes from yellow to red, in consequence of lower radiation, temperature, and increasing general absorption by the atmosphere. The hydrogen lines become indistinct, metallic absorption remains permanent, and broad absorption bands are introduced. In one type (Secchi’s Type III.), of which Alpha Herculis is an example, these bands are of unknown origin. In another class (Secchi’s Type IV.), illustrated by the star 19 Piscium, they have been definitely identified as of carbon origin.
"There is scarcely room for doubt that these types of stars (Type IV.) are approaching the last stages of stellar development. Surface temperatures have been lowered to the point of permitting more complex chemical combinations than those in the sun.
"Secchi’s Type III. includes the several numbered long-period variable stars of the Mira Ceti class, whose spectra at maximum brilliancy show several bright lines of hydrogen and other chemical elements.[17] It is significant that the dull-red stars are all very faint; there are none brighter than magnitude 5.5. Their effective radiatory power is undoubtedly very low."
The state of evolution, which succeeds that characterized as the Secchi Type IV., may be elucidated with the aid of the examples of Jupiter and the earth, with which we are more familiar. These planets would be invisible if they were not shining in borrowed light.
Jupiter has not advanced so far as the earth. The specific gravity of Jupiter is somewhat lower than that of the sun (1.27 against 1.38), and, apart from the clouds in its atmosphere, this planet is probably altogether in a gaseous condition, while the earth, with its mean density of 5.52, possesses a solid cold crust, enclosing its incandescent interior. This state of the earth corresponds to the last stage in the evolution of the stars.
Of the streams of gaseous matter which are ejected when stars collide with one another, the metallic vapors are rapidly condensed by cooling; only helium and hydrogen will remain in the gaseous condition and form nebular masses about the central body. These nebulæ yield bright lights. Their luminosity is due to the negative particles which are sent to them by the radiation pressure of near stars, and especially by the central bodies of the nebula.
With the new stars which have so far been observed, this pressure of radiation soon diminishes, and the nebular light likewise decreases in such cases. In other instances, as with the stars characterized by bright hydrogen and helium lines, the radiation of the central body or stars in their vicinity seems to be maintained at full force for long periods.