Chapter XV: The Evolution of the Stars

The suns of space are subject to the _sic transit_ of mortality. The time has been when they were not, and in the time to come they will surely cease to be. The “incorruptibility of the heavens” is no longer a postulate of science; it has been in a measure superseded by the still more antique notion of the “perpetual flux” of things. Creation is a process; it has a history; and the records of its history are not wholly illegible to science.

Those inscribed in the heavens more particularly invite attempts at decipherment. Inquiries into the physical constitution of the stars inevitably lead to them, nay, insensibly merge into them. In the celestial regions, more than elsewhere, we are impelled to read the past and future between the lines of the present. There, by a wonderful course of development, the designs of the Maker are being unfolded, but with such majestic leisureliness that each step represents the lapse of millions of years. To trace even its broad features is, then, a task to be undertaken only with extreme diffidence; yet some few safe principles are available, guided by which we hope not to wander far from the truth.

Long ago it became evident to observation that nebulæ were the matrices of stars. Stars visibly nebulous are then in the earliest stage of growth. So much, at any rate, may be assumed without sensible risk of error. Again, radiating globes necessarily condense with the efflux of time. As heat, the source of their expansive vigour, is dissipated, their particles succumb to gravity, which suffers no waste. They contract; the same quantity of matter occupies in them a continually diminishing space, and acquires a proportionately more substantial consistence.

The application of these tests gives a concordant result. Both point to helium stars as being at the start of the cosmical procession. They have often nebulous appurtenances; they congregate in nebulous regions; they show with nebulæ spectral relationships of a subordinate, but significant kind. Their mean density, moreover, is known to be extremely small. The conditions of their eclipses, where they form occulting couples, gives the means of assigning to it a fairly definite value; and it appears to be about one-seventh that of the sun. This result is, of course, only preliminary, and cannot legitimately be generalised. It serves, however, to confirm what evidence of a different kind more vaguely indicates.

Helium stars are, then, the most primitive class of suns; and the point of outset being once established, the advance takes a prescribed and inevitable line. We have seen that helium stars pass by the finest gradations into Sirian, Sirian into solar stars, and these again into stars giving fluted spectra. So far there is no breach of continuity. Individual varieties must unquestionably arise, varieties due to minor diversities of chemical constitution, to systemic conditions, to physical influences exerted, possibly, in certain tracts of space; but the great wave of change sweeps on independently of these ripples on its surface.

The order of succession of the four chief stellar families leaves, accordingly, little room for doubt. Our next inquiry relates to the causes of their progressive transformation. We know of two which must be operative—dissipation of heat and augmentation of gravity. The function of a sun is to dispense energy; its distinctive organ, the photosphere, is precisely an apparatus for discharging this function rapidly and effectively; every year of a star’s radiation involves, then, a corresponding subtraction from its not unlimited thermal store. Yet this is not necessarily accompanied by a fall in temperature. Gaseous bodies, on the contrary, grow hotter as they cool. This seeming paradox was enunciated by Homer Lane of Washington in 1870. It is now a universally admitted principle of science. What is signified by it is that the contraction of masses in the gaseous state more than supplies their loss of heat by radiation. It ceases to apply when liquefaction sets in, but we are entirely unable to fix the stage of evolution at which this point is reached. We are only certain that the youngest stars, being unquestionably gaseous to the core, are rising in temperature; their acme is still to come.

But average temperature is not the same thing as surface temperature. The former, in two radiating globes, may be the same, while the latter is very different. For it depends essentially upon the rapidity with which heat can be conveyed outward and upward, and this again is prescribed by interior conditions varying with mass, density, and radiative facilities. Now Lane’s law has to do only with average temperature, while spectral indications relate to purely superficial heat-conditions. If we can learn something definite even as to these it will be much; but at present the utmost uncertainty prevails as to how the recorded facts should be interpreted. It seems indeed pretty clear, from the frequent occurrence of “enhanced” lines in the spectra of white stars, both of the Orion and the Sirian kinds, that the state of things in their reversing strata approximates to that in the electric spark, while vapours glowing in the arc represent better the layers absorbing sunbeams and the rays of Antarian stars. But in regard to the essential nature of the difference, authorities are not unanimous. According to Sir Norman Lockyer, Dr. Scheiner, and others, temperature alone is concerned; the spark is hotter than the arc. Intense molecular excitement, due to the disruptive discharge, gives rise to altered modes of vibration, betrayed by substitutions of new spectral lines for those previously visible; and these substitutions, reiterated in the stars, tell emphatically of their enormous temperatures. On the other hand, Sir William and Lady Huggins relegate temperature to a position of secondary importance, and count density the main factor in spectral change, their contention being supported by impressive experimental arguments. Their photographs, too, show some unexpected signs of superior strength of ultra-violet radiation in solar as compared with white stars; and this, if substantiated, would assuredly imply their higher temperature. For the radiative _centre of gravity_ shifts upward with increase of heat, a relation familiarly illustrated by the whitening of red-hot iron before the melting-point is reached.

All of which is exceedingly perplexing; and there is more behind. Gravity is of potent influence in determining temperature. The physical condition of bodies cannot be compared without reference to the scale of their construction, and their spectra vary to correspond. A score of years ago Ritter enunciated the theorem that “the surface temperatures of two stars of equal densities are to each other nearly as the square roots of their masses.”[526] And Professor Perry reached quite lately the analogous conclusion that the temperature of a star varies as the product of its age and mass so long as it behaves after the manner of a body gaseous throughout.[527] Further, the superficial heat of stars obviously depends upon the activity of convection-currents in their interiors, and these of course slacken as viscosity increases. This adds greatly to the complexity of the problem, since the transcendental temperature and pressure reigning in the depth of stellar globes must affect in unforeseen ways the viscosity of the materials placed under circumstances outside experience.

Clearly, then, the stars can be arranged in order of temperature only with hesitation and tentatively. If we might accept Ritter’s inference that the sun’s radiating layer was never in the past, and can never be in the future, at a much higher temperature than that now belonging to it, some difficulties would be removed. For it involves the consequences that the solar type of spectrum marks the culminating point of superficial heat, and that no star can be hotter than the sun unless it contains a larger quantity of matter; and these, if valid, would provide solid ground for classification. But they are highly disputable, and we can only conclude that it is safest not to dogmatise about relative stellar temperatures.

Sir William and Lady Huggins regard as of primary importance in the development of stars the gain of surface-gravity which inevitably accompanies their contraction. They are unquestionably right. Atmospheric pressure varies with gravity, and the spectral characteristics of incandescent vapours are affected to an incalculable degree by their density. Every addition to gravitational power, moreover, serves to quicken atmospheric circulation. The tendency to sorting out by the formation of concentric shells of substances differing in atomic weight, is overborne by the uprushing of convection-currents. The strata become mixed, and the heat-gradient becomes steep. These atmospheric modifications are reasonably numbered among the concurrent causes of development from the Sirian to the solar spectral type. They must, at any rate, be concomitants of stellar condensation, unless the path of progress is deflected by unknown agencies. It is well to remember that electro-magnetic forces play a part in cosmical evolution—a part deprived of none of its importance by our inability to define its nature. We can only see that they may not be excluded, and await patiently the outcome of future research.

All this refers to the individual history of cooling globes. How, we may ask, does it apply to the relative histories of various globes differing very greatly in mass? The customary answer is that massiveness retards development. That it retards cooling is quite certain, since the larger of two unequal spheres has, relatively, the smaller radiating surface. Hence the old view that change of temperature and spectrum proceeds evenly together had as a corollary that the quicker pace belonged to the lesser star. Spectra of the Orion and Sirian patterns should, accordingly, distinguish orbs on the whole of far more imposing proportions than those giving light of the solar and Antarian qualities. Just the reverse, however, appears to be the case. All practicable modes of comparison agree to indicate that the “mean” solar star sends out a larger sum-total of light from a considerably smaller luminous surface than the “mean” Sirian star.[528] The solar star is, moreover, the denser body, and therefore the more massive in a ratio very much beyond that of its superiority in luminous power.

But the most cogent proof that giant suns develop quickly is derived from the spectra of double stars. The members of binary systems may fairly be regarded as contemporaneous. Their origin was in common; their destinies are indissoluble; they are identically circumstanced; they must be similarly composed. They should then be exceptionally trustworthy guides to the unravelment of evolutionary time-relations. Now they inform us, in distinct terms, that in contrasted pairs the earlier type of spectrum characterises the minor body. The primary being solar or Antarian, the satellite is of the Sirian class. Further, the inequality of mass in such cases is certainly greater than the inequality of light. The small blue star is more tenuous than the reddish luminary it attends. These phenomena enforce the conclusion—the inverse of Ritter’s—that stars of the first type are greatly less massive than coeval stars of the second.

Here resides the crux of the evolutionary problem. We have no choice but to believe that the four ages of stellar life succeed each other with relative promptitude in globes built on a great scale. But what looks like an insurmountable difficulty may, on closer inspection, prove a most valuable help towards the establishment of sound doctrine. Sir William and Lady Huggins threw out the suggestion in 1897[529] that “the effect of great mass on surface density, together with the working of Lane’s law, will favour the coming in of a solar type of spectrum at a somewhat earlier relative time.” They indeed finally rejected the idea;[530] yet it is strongly confirmatory of their own views as to the importance of the gravitational factor in the unfolding of stellar life-history. Rapid atmospheric circulation, indispensable, as they hold, to the production of a solar spectrum, would be set up earlier in _heavy_ than in _light_ globes; and the requisite adjustment between temperature and pressure should be similarly anticipated. That this is what really happens, we are assured by the prismatic observation of jewel-tinted star couples.

It does not, however, follow that large stars are short-lived. The explanation of the facts just offered involves no such paradox. For it is amply possible that the lesser order of stars may not survive to reach the Antarian stage. They may perish on the way. Extinction perhaps overtakes them while still in mid-career. They may lapse into the ranks of “dark stars” before time has been allowed them to put on any recognisable badge of decadence. If this be so, stars with fluted spectra are the outcome of a kind of natural selection. They are bodies endowed with sufficient heat to keep them luminous to the end, while others, having squandered less ample supplies by quicker cooling, sink prematurely into invisibility. This is no idle speculation. The sidereal system is known to include countless non-luminous globes, the origin of which is largely enigmatical. Their obscurity, most likely, dates from various epochs in stellar life. And the smallest masses should, under similar circumstances, cease first from sun-like existence.

So far, account has been taken of only four stellar families, selected as the basis of the evolutionary argument because their mutual relations seem unmistakable. Helium stars are the direct progeny of nebulæ. The formation of a photosphere definitely marks the transition. By the gradual effacement of “Orion” lines they merge into hydrogen suns, these, through the creeping into prominence of innumerable metallic absorption rays, into solar orbs, which finally pass, by successive minute changes, into the fluted stage. But what, we cannot refrain from asking ourselves, lies beyond? Through what phases of decline do great red stars of the Antarian order subside into extinction? No confident pronouncement on the subject is possible, but the conjecture may be hazarded that a stadium of variability precedes the end. Periodic light-spasms perhaps indicate failing vitality. They may eventually die out, and be succeeded by a permanent minimum. Already one such example seems to be afforded by T Ophiuchi, which has for some time ceased from its annual brightenings. Recurrent maxima may, after all, be only flickerings in the socket. This possibility lends a particular interest to research into the causes of these extraordinary outbursts.

Now about the same proportion of carbon stars as of Antarians are markedly variable. Hence, if radiative instability betoken decrepitude in one class, it must do so in the other. It would, indeed, on many grounds, be unreasonable to set the two families far apart in the chronology of the heavens. Professor Vogel regards them as collaterals. They represent, in his scheme, alternative lines of descent towards the final quenching, there being a total absence of evidence that either has sprung from the other. The pedigree of carbon stars is, in truth, highly obscure. Besides them only one celestial body shows recognisable traces of carbon absorption, and that body is our sun. As effete suns, accordingly, Sir Norman Lockyer ranks these remarkable objects. But the transitional spectra we should expect to meet with, if this were the case, are missing. Solar stars with incipient carbon flutings are unknown. No road runs between the designated stations. A line of communication is wanting. Nor is the development of Antarian into carbon stars easy to admit. A few instances of nondescript banded spectra have, to be sure, been recorded, and might conceivably serve to bridge the gap; nothing, however, resembling an intermediary series can be made out. Now stars without obvious relationships presumably developed quickly under abnormal conditions. And carbon stars seem to be in this case. They must indeed have had progenitors, although none openly claim them. With three stocks, nevertheless, they show distant affinities, and from one or other they must have sprung. Their banded spectrum can be traced in embryo in the sun; their dark-line spectrum is analogous to that associated with Antarian flutings; their bright-line spectrum partially matches Wolf-Rayet emissions. But these are no more than hints towards a genealogy, of which nature still keeps the secret.

There can be no hesitation in placing the Wolf-Rayet and the bright-line helium groups at an early stage of cosmic growth. The Pickering and Rydberg hydrogen lines, which commonly go together, are, for some unknown reason, characteristic of a primitive condition, and they are essential elements of the Wolf-Rayet spectrum. The absence from it of metallic rays is an indication of the same purport; for they are similarly suppressed in nebulæ, while gaining strength and depth in the successive stellar generations. Yet Wolf-Rayet stars are not visibly nebulous. Must we then suppose that they have sprung from stars that are? This is scarcely possible, in view of the peculiarities just adverted to; nor is the admission necessary. Small, compact nebulæ, without hazy appendages, are quite likely, by their condensation, to have given rise to this class of stars. If so, their telescopic sharpness is a necessary consequence of their mode of origin. But the connecting links have still to be detected. Until they are, the suggested parentage remains an unverified conjecture.

Nebulous attachments, on the other hand, plainly seen or photographed, not unfrequently declare the affinities of bright-line helium stars. They are accordingly at the outset of their careers as suns—that is to say, they have given since the time when they were first formed into powerfully radiating globes the same kind of spectrum now exhibited by them. It will, however, eventually become modified; and the most probable modification to which, so far as our limited view extends, it can be subject, is by the disappearance of its specific rays of emission. Their progressive effacement might plausibly be represented by a series of objects, in which linear radiation grows less and less, from γ Cassiopeiæ, with its full complement of bright lines, down to Alcyone, showing a solitary C. Yet this would not compel the admission that every dark-line helium star has traversed a bright-line phase. Such an episode, on the contrary, can be inferred from many indications to occur by exception in stellar history as a consequence, perhaps of peculiarities of internal constitution, perhaps of unusual influences exerted from without, possibly of the mutually reactive effects of both classes of cause.

We must be prepared to meet with side-tracks in evolution. Nature does not run in a groove. Her operations are free and various; they defy the restrictions of feasibility which a rigid methodism of thought would seek to impose. The order of the universe has a wider scope than is imaginable by us. Creative Wisdom disposes of superabundant resources, and, if we may dare say so, takes delight in bringing them into play. Our best attitude of mind, then, in attempting to speculate on the course of things, is that of the utmost possible flexibility to the teaching of well-ascertained facts.