THE PROCESSION OF SUNS
Phenomena are functions of time, and the form of the function has to
be determined in each particular case. That is what the historical
method comes to, and its use is prevalent and almost compulsory. We can
no longer be satisfied with a simple bird's-eye view of the universe;
our thoughts are irresistibly driven to grope into its past, and to
divine its future. Statical conceptions sufficed for our intellectual
forefathers. They aimed at establishing the equilibrium of things,
while we see them in a never-ending flux. One aspect of them calls up
the next, and that another, and so on _ad infinitum_; we cannot, if we
would, balance our ideas on the pivot of the transient present.
The immutable heavens of the ancients strike us to-day as the
invention of a strange race of beings. We see them, on the contrary,
with Shelley as a 'frail and fading sphere,' a 'brief expanse,' the
seat and scene of change. The 'fixed' stars long ago broke away from
their moorings, and began to flit at large through space. Of late a
less obvious, more intimate kind of mobility has been attributed to
them. Grooves of individual development have been assigned to them,
along which they appear to shift as the tardy ages go by; and since
everything that grows must decay, the orbs of heaven, too, incur
the doom of mortality. But modern science has done much more than
extend to them the dismal philosophy of the phrase, 'Tout passe,
tout casse, tout lasse.' The grandiose enterprise has been not
unsuccessfully essayed of tracing in detail the progress of sidereal
evolution, and of marshalling the vast stellar battalions in order of
seniority. This has been rendered feasible by the disclosures of the
spectroscope. Apart from their guidance, the track might have been
seen by elusive glimpses, but could never have been laid down with any
approach to definiteness. Herschel found for it a _terminus à quo_
in nebulæ of various forms, but attempted to pursue it no further.
We do not hesitate to run it on, from station to station, right down
to the _terminus ad quem_. Not, it is true, without the perception
of outstanding difficulties and insecurities, which yet seem to be
outweighed by a certain inevitableness of self-arrangement in the
related facts.
The argument from continuity is that mainly relied upon. An unbroken
succession of instances is strongly persuasive of actual transition,
provided only that a principle of development (so to call it) may
reasonably be assumed as influential. A series of mineralogical
specimens, however finely differenced, does not suggest the progressive
enrichment of one original mass of ore. In the stars, on the other
hand, a species of vitality may be said to reside. They are not
finished-off products, but spontaneously-acting machines. They are
centres of energy, which they dispense gratis, supplying the cost out
of their own funds. And the process is not only obviously terminable,
but must be accompanied by constitutional alterations, which might be
traceable by subtle methods of inquiry. They _are_ traceable, unless
we are deceived by illusory appearances.
Secchi's classification of the stars was unwarped by any speculative
fancy. It was purely formal; it aimed only at providing distinct
compartments for the convenient arrangement of a multitude of
differently characterized items of information. Then, by degrees,
the closeness of the gradations between one class and the next came
to be noticed; partitions melted away; the methodized array showed
itself to be in movement; and the bare framework took shape, under the
auspices of Zöllner and Vogel, as a cosmic pedigree. The white stars
were set forth as the progenitors of yellow, yellow of red stars; and
the insensibly progressive reinforcement of the traits of relationship
between the successive types went far towards demonstrating some
partial, if not a complete, correspondence of the indicated order
with the truth of things. It has since been found necessary to divide
the first stellar class into helium and Sirian stars; and here,
too, essential diversity shades off imperceptibly into likeness
approximating to identity. All the groups hang together; the entire
scheme is on an inclined plane of change. Helium stars, as they
condense, pass into Sirian, these into solar stars, which finally,
reddening through the increase of absorption, exhibit the badge of
post-meridional existence in fluted spectra. The finality of the red
stage is, indeed, very far from being absolute, but what lies beyond is
matter of conjecture.
There are several good reasons for taking helium stars to be the
'youngest' or most primitive of the amazing assemblage that sparkle in
the vault of heaven. The first is their affinity with nebulæ. Every
star, perceived to be involved in folds or effusions of shining haze,
has yielded--if bright enough for profitable examination--a spectrum
of helium quality. Further, they are remarkably tenuous bodies. It has
been ascertained with approximate certainty, from the investigation of
stellar eclipses, that helium stars are commonly, perhaps invariably,
of far slighter consistence than the sun. Radiation, however, is
maintained by contraction; hence, orbs at the outset of their course
must be, on the whole, the most diffuse. A third note of youth is
membership of embryo systems, and this is affixed very markedly to
helium stars. One-third certainly, probably one-half of those lately
submitted to trial by Professors Frost and Adams proved to have
spectroscopic companions. They are pairs believed to have been recently
divided by the fission of a single parent-globe. And this is an
operation which must, we should suppose, be undergone early, or not at
all.
The spectra of helium stars are peculiar and suggestive. Those
belonging to Miss Maury's earliest groups--many of them visibly
nebulous--bear next to no traces of metallic absorption, showing
instead lines of oxygen, of nitrogen, and of hydrogen in all its
three series. The conditions, accordingly, needed to produce the
'cosmic' modification of hydrogen are realized in these inchoate
bodies. What those conditions actually are we cannot tell, yet it may
be confidently surmised that they will prove to be of an electrical
nature. Hydrogen resembles the metals in being electro-positive; it
collects at the negative pole during the electrolytic decomposition
of water. There is, however, an unmistakable tendency in primitive
sidereal objects to display absorption rays of electro-negative
rather than of electro-positive elements. It is conceivable that
hydrogen may be capable of altering its behaviour in this respect,
and that the molecules radiating the Pickering and Rydberg series, in
addition to the more familiar Huggins series, have, in fact, through
some corpuscular re-arrangement, assumed the electro-negative quality
properly characterizing a non-metallic substance. The association of
this form of hydrogen with oxygen and nitrogen in early helium stars
would thus be naturally related to the simultaneous quasi-disappearance
from them of the spectral badges of metals.
The helium-line most distinctive of this stellar family is situated
well up in the blue. It appertains to the same vibrational sequence
with D3, which is also represented in Rigel, one of the more
'advanced' Orion stars. In Rigel, too, we meet a fairly prominent
magnesium ray, lying below the blue helium emanation, while as yet iron
is unapparent. Numerous fine, faint streaks, due to its absorption,
only emerge when the Sirian type is fully reached, and they are mostly
of the 'enhanced' kind. When the spark discharge is substituted for
the arc as the source of illumination, certain lines in the resulting
spectrum brighten relatively to the others, and these have been
distinguished by Sir Norman Lockyer as 'enhanced.' Now, the rule is
strikingly prevalent that the absorption rays in white stars are of
this class; yet it can no longer be interpreted as indicating for them
an excessively high temperature. Rather, it would seem that electrical
conditions still imperfectly defined are in question, and their
gradual removal or subsidence is, beyond doubt, largely instrumental
in bringing about the transition to the solar stage. The effacement of
helium-absorption is even more perplexing. No sooner does iron begin to
show than it vanishes. There is still a faint trace of its 'blue' line
in Vega; none survives in Sirius.
In spectra of the solar type two great bars of violet light are stopped
out by calcium; otherwise metallic arc-lines predominate, while those
of hydrogen are no longer so powerfully emphasized as in white stars.
Moreover, the whiteness of the unveiled Sirian photospheres has become
tinged with yellow owing to the development of a shallow envelope
partly impermeable to blue rays. For this reason the comparative
extension of their ultra-violet spectra affords, for stars of different
types, no secure criterion of relative temperature. Sound in principle,
it becomes inapplicable when the unknown factor of general absorption
comes into play. The energy-curve of the solar spectrum, as it is, can
be determined; the energy-curve of the solar spectrum, as it would
be if unaffected by general absorption, has to be constructed from
inference. But only photospheres bare to space give comparable results.
Hence, there are no valid grounds for asserting that Sirius is hotter
than the sun, or the sun than Betelgeux. It may be so, but the evidence
at present available is inconclusive. The appearances expounded in this
sense may bear quite different meanings.
The reasons for holding that solar mature into Antarian stars are of
the same character, and of equal cogency with those tending to prove
their own development from luminaries of Sirian type. There is a
similar continuity of specimens. They can be ranged one after another
in an unbroken series, in which, as we run down the line, primrose
shades into orange, and orange into red; general absorption arrests an
increasing percentage of the blue radiations, while specific absorption
becomes strengthened by dusky channellings of titanium. Carbon stars
are less easily located. Dr. Vogel regards them as co-ordinate with
the Antarian class. The two varieties of red stars with banded spectra
descend, in his opinion, from the common stock exemplified by our sun.
Professor Hale also favours this view, some attendant difficulties
notwithstanding. His photographs have certainly established for carbon
stars links of relationship both with the Antarian and the solar
families; yet the fact remains indisputable that the carbon type is,
to a great extent, isolated from all the rest. Tokens of a genuine
migration towards it are few and obscure.
The ultimate fate of both tribes of red stars can only be conjectured.
Most of the objects constituting them vary in brightness, some to
the verge of periodical extinction; and variability may be a symptom
of interior dilapidation. But the organization of such bodies is
profoundly enigmatical. They are exceptionally remote, and offer slight
holding-ground for inquiry. No indications have been gathered as to
their density or intrinsic light-power. Very little is known about
their movements. They rarely form binary combinations, and those that
they do form are almost always relatively fixed. No red star travels
in a computed orbit; only one, η Geminorum, occurs on the long list of
spectroscopic binaries. The revolutions of this curious system ought
to prove, when thoroughly investigated, replete with interest and
instruction.
Coupled stars present special opportunities to students of cosmogony.
They are obviously contemporaries; they have started fair in the
evolutionary race; identical influences have acted upon them; hence,
differences in their standing can only result from dissimilarities
in mass or composition. It is commonly taken for granted that a body
containing less matter than its fellow must develop faster, and incur
the final quenching sooner. But Sir William and Lady Huggins have
adverted to the probability of the very opposite being the case.
Powerful surface-gravity may, they consider, serve to hasten the
transition from a Sirian to a solar spectrum; and we should then
have giant suns like Capella, advanced in type while at a very early
stage of condensation. This perhaps explains the remarkable spectral
relations of contrasted stellar pairs. Always, so far as we yet know,
the Sirian spectrum is yielded by the lesser star, the mass of which,
judging by analogy, must be even smaller than would be indicated by the
proportion of its faintness. It is true that the distribution of mass
in binary systems is often widely different from what might have been
anticipated. Certain purplish satellites, for instance, of undetermined
spectral quality exercise a gravitative sway of surprising force. Some
results of this kind lately obtained by Mr. Lewis and others are likely
to prove of fundamental importance to theories of stellar evolution.
What we know of 'dark stars' has been mainly derived from the
observation of stellar systems. They are assumed to be the denizens of
a stellar Hades, dim wanderers amid the shades, who 'have had their
day, and ceased to be' as suns. In the 'cold obstruction' of these
viewless orbs the grand cosmical procession is held to terminate.
Their presence attests the downward progress of decay, and gives
logical completeness to the argument for development. Yet there are
circumstances warning us against too full an assurance that their
status is really that of skeletons at the feast of light. They are very
frequently found to be in close attendance upon brilliant white stars.
Thus intimately, if incongruously, coupled, they circulate and compel
circulation in brief periods, as members of systems just, it might be
said, out of the shell. What are we to think, for instance, of the
obscure body spectroscopically discovered to control the revolutions of
the chief star in the Orion trapezium? It is evidently comparable in
mass with that imperfectly condensed luminary. Is it credible that it
has already traversed all the stages of stellar existence, and cooled
down to planetary rank? So violent an assumption should at least not
be made without due consideration; and we may more prudently hold our
judgment in suspense as to whether globes so circumstanced--and they
abound--should be regarded as effete, or as abortive suns.[100]
Speculations on the exhaustion of stellar vitality have lately become
inextricably involved with the complex problem of elemental evolution.
A dim inkling has been acquired of the activity in the universe of
obscure forces, availing, we can just see, to falsify many forecasts.
The theory, among others, of the dissipation of energy needs to be
revised or qualified. Nor was it propounded by Lord Kelvin with
dogmatic certainty. He carefully noted the possibility that in 'the
great storehouses of creation' reserves of energy might be provided
by which the losses incurred through radiation could be, wholly or in
part, made good.[101] The anticipated possibility is perhaps realized
in the phenomena of radio-activity. But if we inquire how, we are met
at the threshold by difficulties connected with the origin of helium.
Helium appears to result from the disintegration of radium, its
generation being accompanied by the setting free of enormous quantities
of energy. Its copious presence, then, argues long-continued and lavish
expenditure of heat and light. Yet it is as a constituent of highly
primitive orbs that it is chiefly conspicuous. Gaseous nebulæ, too,
include immeasurable supplies of it, while it is incompatible with
whatever we seem to know about them to suppose that radium at any time
entered into their composition.
The genesis of the elements has, in truth, not yet been made the
subject of coherent speculation. Current ideas regarding it imply a
double course of change, by aggregation first, and subsequently by
disintegration. And this should give us a twofold series of elements.
On one side there should be fixed survivals from the advancing process,
on the other, products of decomposition, continuously evolved, and
even now accumulating. If the claim of helium to take rank among these
last should be finally established, our conceptions of the nature and
history of nebulæ might have to undergo a strange inversion; but the
outcome of the researches in progress is still uncertain, and may be
far off.
It is, nevertheless, quite clear that the electronic theory of matter
supplies no genuine explanation of the source of energy in the
universe. What is given out when the atoms go to pieces must have been
stored up when they were put together. Whence was it derived? This is
the fundamental question which underlies every discussion concerning
the maintenance of the life of suns. It is unanswered, and probably
unanswerable.
FOOTNOTES:
[Footnote 100: It must at the same time be borne in mind that their
total darkness is not proved. All that is certain is that their spectra
are not bright enough to leave any impression on the exposed plates.]
[Footnote 101: Thomson and Tait, _Natural Philosophy_, Appendix E, p.
494, edition 1890.]