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postulate of science; it has been in a measure superseded by the still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.