Chapter IV: Introduction: Rise and Scope of Astrophysics

The astronomy of the ancients was purely formal. It did not profess to look beyond appearances. Its aim was reached, provided that phenomena were—in the old Greek phrase—mathematically “saved,” at whatever cost of material impossibility. Kepler first speculated on the causes of celestial movements, and introduced the term “physical astronomy” with a full sense of what it implied. Its establishment as an effective branch of knowledge was a prime desideratum with Francis Bacon. While rejecting the Copernican system and ignoring Kepler’s laws, he aspired towards a science of the skies that should be no simulacrum, like the “stuffed ox of Prometheus,” but should have in it the breath of life and the instinct of progress.[1] Anticipating with imaginative insight the prosecution of researches which Comte, two centuries later, declared to be, in the nature of things, futile, he broadly laid down the lines of a new astronomy, indistinguishable from modern astrophysics. As the province of this “philosophy” of the heavenly bodies, he assigned inquiries into the nature of their substance, of their qualities, properties, and influences, as well as into the source of the motive power acting upon them. Vitalised, as it were, by contact with motherearth, it was destined in his prevision to a community of advance with terrestrial science, one imparting to the other novel truths tending to mutual profit and simultaneous development. Thus the long-divorced sublunary and translunary worlds were conjoined, and their material unity—the essential principle of astrophysics—was definitively proclaimed. This daring forecast transcended the scope even of Newton’s discovery, and left Kepler’s prescience far behind. For Kepler, as an astronomer _ex professo_, took full account of apparent impossibilities, while Bacon’s was “the golden guess”—

That’s morning star to the full round of truth.

Yet it might have been derided as that of an uninformed amateur. It rose indeed out of sight of ordinary minds.

The establishment of the law of gravity was the first step towards its realisation. Thereby the terrestrial regimen was, in part, extended to the skies. The moon assumed the aspect of a projectile in perpetual flight, tangentially launched _ab initio_, and hence keeping its distance from the earth, while the planets, including our own globe, proved to be similarly related to the sun. Thus celestial movements lost the mystical character long ignorantly attributed to them, and were found to own a common cause with movements at the surface of the earth. They became predictable, since the cause acts uniformly and simply; theoretical astronomy, with practical astronomy at hand to provide its raw materials and test its results, took rank as the most perfect of the sciences; the idea of a definable force put to the rout the old vague notions of “tendencies,” “appetites,” “passions,” or “potencies,” and a dynamical was substituted for a merely kinematical system.

Gravity, however, is a force of the utmost generality in the way it affects matter. It takes no notice of distinctions of kind or quality. The substances acted upon may be hot or cold, dense or rare, elementary or compound; they may be of any imaginable chemical or mineralogical constitution; they may be in any state of aggregation; they may be organic or inorganic; no difference is perceptible; gravity is concerned solely with mass, and is measured strictly by movement; and from gravitational inquiries, accordingly, mass and movement can alone be learned. So far, then, only one principle of unification was introduced. One fundamental property of matter was known from 1687 to belong equally to the earth and planets; and Herschel’s discovery in 1802 of mutually revolving stars virtually made the “writ to run” throughout the sidereal world as well. The universality of an apparent mass-attraction was a great fact, but seemed destined to remain isolated; for Olbers’s “electrical theory” of comets amounted to no more than a suggestive speculation. Then in 1852 the triple identification by Sabine, Wolf, and Gautier of the sun-spot and terrestrial magnetic periods showed the reality of solar influences exercised in a manner not easy to apprehend, but capable of being brought to the test of experimental investigation. Cosmical physics began to separate out and take recognisable shape. The spring of its most rapid growth, however, lay in another direction.

The discovery (in Professor Keeler’s words[2]) “that the light which reveals to us the existence of the heavenly bodies also bears the secret of their constitution and physical condition,” afforded a solid basis for a science of far-reaching import. “The spectroscope placed new and hitherto undreamtof powers in the hands of men. It is to the astrophysicist what the graduated circle and the telescope are to the astronomer.” Observations of the heavenly bodies by means of their _analysed_ light came to the aid of observations through their _integrated_ light. Their radiations, visible and invisible, were brought within the range of detailed study.

Of study, not only visual, but photographic. The sensitive plate has three leading prerogatives. It _sees_ where the eye is blind; its impressions are cumulative to an indefinite extent; they are permanent; they constitute documentary evidence of incontestable validity, which can be produced or referred to at pleasure. Spectroscopic photography, or “spectrography,” dates from Sir William Huggins’s adoption of the dry gelatine process in 1876; and his discovery, three years later, of the ultra-violet series of hydrogen-lines in stellar spectra started it on its career amid acclamations. Nor has the promise been belied. The efficiency of the camera is of so high an order that direct visual observations of prismatic light are now only by exception made. This is perhaps unfortunate, since the two kinds of results are, to some extent, supplementary, and can often be most usefully compared and correlated. The superiority of the chemical method, however, is nowhere more conspicuous than in the motion department of the new astronomy. Its powers in this direction were tested with striking success by Vogel in 1888, and their development, rapid as it has been, does not seem to be near its term. The determination of radial velocities through changes in the refrangibility of light emanating from the bodies actuated by them, has made astrophysicists free of a territory which belongs equally to the domain of traditionally equipped astronomers. Here we get back to elementary facts of mass and motion, ascertained, however, not _frontally_ by measures of position, but _strategically_ by inference from radiative modifications. They are, indeed, of a nature unaffected by position, and hence undiscernible with the micrometer. A body shown spectroscopically to be in swift movement might be absolutely immobile telescopically; or the conditions might be inverted, each method taking cognisance of only one component of the total velocity. An immense extension was accordingly given to the field of research in sidereal dynamics by the application, through Sir William Huggins’s initiative, of “Doppler’s principle.” It supplied not alone the means of completing investigations which could otherwise be pursued only in a one-sided manner, but of setting on foot entirely new ones of incalculable significance. Thus the rate of the solar translation through space, valued little better than conjecturally from the _proper_, or thwartwise motions of the stars, can be derived securely and at once from their _radial_ motions. Of the movements of nebulæ nothing is known—and very little is likely to become known for some centuries yet to come—except through spectroscopic measurements; for they are so remote that their positions change with extreme slowness, while the evidence of radial speed is tendered immediately, without regard either to time or distance. But the most curious discoveries afforded by it are of double stars revolving in such close contiguity as to be permanently inaccessible to telescopic observation. And these are precisely the systems of highest cosmogonic interest, as being, most likely, at the outset of their evolutionary careers. They are surprisingly numerous, and will doubtless prove to be linked on to telescopic binaries by an uninterrupted succession of couples farther and farther apart.

This common ground of the two astronomies, where motion in the line of sight is the object of research, has already proved fruitful of varied novelties, and its yield is not within view of being exhausted. It could, however, never have been worked to advantage but for the timely assistance of photography. The living retina is here conspicuously inferior to the chemical retina; for aerial disturbances are eminently baffling to eyeestimates of line-shiftings, while the sensitive plate, ignoring momentary fluctuations, records true mean positions. Visual measures are hence rarely trustworthy; advantageous occasions for securing them are few; so that they must always be either poor in quality or scant in quantity; and accumulated data are needed as the bases of systematic inquiries. The use of the camera is accordingly indispensable, and has become all but exclusive.

Astrophysics widens in scope year by year, and as it wins _extension_ it gains _intension_, each advance carrying it deeper into the secrets of nature. Towards this result the alliance with photography has contributed with signal effectiveness. The impersonal method confers a certainty and power in dealing with obscure phenomena which can only in special cases be claimed for the eye. Moreover, it is of larger application. It can be employed on an expanded scale both of time and space. It is thus the fittest means for collecting statistics of the heavens; and statistics are urgently in demand for the ultimate purposes of celestial science. The whole future of astronomy has indeed come to depend upon the validity of photographic evidence, and specialties of manipulation and development, the idiosyncrasies of variously prepared plates, the shrinkage of gelatine films, the effects of graduated exposures, have to be studied no less diligently than the theories and errors of brass and glass instruments where immediate determinations of celestial situations are in question. Astrophotography is an art, and has a technique of its own needing labour for its mastery.

The ramifications of astrophysics are numerous and intricate. To trace them out in detail would be to unroll an elaborate chart of the sciences. Celestial chemistry is in itself an all but limitless department. It includes terrestrial chemistry, thermotics, thermo-electricity, and slides inevitably into the wonderland of molecular physics and ethereal powers and qualities. For the interpretation of spectra demands acquaintance with the nature of the vibrating systems originating them, with their relations to “imponderable” agencies, with their perturbations, modifications, and disruptions under the stress of circumstances at present scarcely definable. Here there are worlds to conquer. One phase of these inquiries is marked by the recognition of harmonic line-series in the spectra of the chemical elements. Another by the discovery that wave-length is a function of density, that an increase of pressure slightly shifts the rays emitted by a glowing vapour downward towards the red. A third, still more significantly, by the “Zeeman effect,” with its barely conjectured implications. It consists in the distension and subdivision of lines normally slender and single, when the radiation takes place in a strong magnetic field; and the specification of the laws of its production, whether close at hand under controlled conditions, or far out of reach at the surface of celestial spheres, allures the imagination with possibilities of far-reaching consequence. Above all, there seems to be a reasonable chance of learning from it something about the electrical state of the stars. The relative strength and brilliancy, moreover, of spectral lines afford criteria of temperature, density, and modes of electrical action, but not with satisfactory explicitness. There is much difficulty in duly apportioning the effects. Thermal and electrical conditions are rarely separable; degrees of density and of temperature again need very careful discrimination. Electricity is the indispensable agent for exciting luminosity; precisely, however, what part it reserves for itself in the matter—whether heat, as generally assumed, is its plenipotentiary, or merely a delegate with limited powers—is, so far, unknown. Nor is it easy to define what takes place in the path of the discharge, yet it is from the carrying molecules only that the light examined is derived, and it is their state only that is indicated by its peculiarities. Still, beginnings have been made in the experimental disentanglement of this web of interdependent operations, and specific inferences of value regarding the heavenly bodies have already been drawn from some preliminary ordering of the various classes of facts.

The rotation of the heavenly bodies is a department annexed, while their chemistry has been created by the new astronomy. No longer treated as a simple geometrical datum, it is studied as an index to their physical constitution. Spectroscopic observations of axial movements in the sun and planets are among the most delicate and curious that have been made. They may possibly be extended to stars, nebulæ, and comets, but the prospects here are dubious. Nor has the old direct mode of determining rotation been superseded by the novel method. Its employment, in some cases supplementary, is rendered in others, by the force of circumstances, exclusive.

Moreover, nearly the whole “descriptive” section of astronomy is embraced by astrophysics. It is now extensively yet not altogether worked by photographic means. The camera has so far succeeded very imperfectly in depicting planetary surfaces; but the required special conditions are being carefully studied, and will perhaps before long be realised. The difficulties attending lunar photography have of late been, in the main, overcome, as the magnificent Paris and Lick Atlases of the moon testify. They nevertheless record essentially what was known before; they elucidate no perplexity; selenology has been adorned and illustrated, but not greatly promoted by their compilation. The self-portrayal of recent comets, on the other hand, has been accompanied by remarkable disclosures. They need, however, skilled interpretation, and experts in this branch are to seek. The pictures are there, full of rapidly changing and significant detail; yet patience must be exercised before we can read in explicit terms what they implicitly convey regarding the constitution of the bodies they represent.

The photographic study of the Milky Way—pursued systematically by Professor Barnard—has been more definitely and distinctly communicative. For his plates not only bring clearly to view the mixed stellar and nebulous nature of that gigantic assemblage, but also afford grounds for inferences of great moment as to the general distribution of the stars. This indeed is a subject which might seem expressly reserved for treatment by the older astronomy. Yet the all-pervasive physics of the skies has a lien upon it. Spectroscopic considerations come into play. The modes of stellar scattering in space are different for the various stellar types, and the connection suggests queries, not readily answered, regarding the origin of those types, and the very genesis of the sidereal system itself. Abysses of speculation open before us as we contemplate the surging galactic cloud-forms depicted through the simple instrumentality of a portrait-lens and a sensitive-plate.

In the photometric branch of astronomy there is a similar concurrence of claims. The arrangement of the stars in light-ranks serves primarily as a test of their arrangement in space; the test, however, is illusory unless the nature of their spectra be taken into account. Again, while measurements of the brightness of individual stars are essentially physical in their import, they are also carried out for the geometrical purpose of determining occultation-phases. The photometric observation of the eclipses of Jupiter’s satellites is a corresponding example in the solar system. Otherwise, in its varied applications to the sun and moon, to planets, asteroids, and comets, photometry may be said to have purely physical aims. These have to do, not only with integral, but also with analysed light. The “spectrometric” division of photometry consists in the comparative estimation of ray-intensities, in balancing one against another the differently refrangible beams from a given source of luminosity, in constructing, that is to say, its spectral energy-curve. In both departments the camera proves an invaluable ally. Photographic photometry occupies, indeed, a place apart among the arts and crafts of astronomy. It has its own laws, its own problems, its own difficulties, and it furnishes data which can be interpreted on principles valid for them alone.

The specialties of solar physics are too numerous to be particularised. Among stars, perhaps an insignificant star, the sun is nevertheless by its comparative vicinity to ourselves brought within range of a whole series of observations impracticable elsewhere. In solar research, accordingly, novel devices abound; such as the “double-slit method,” so happily availed of by Hale and Deslandres for the spectrographic portrayal of “flames,” facular and chromospheric. The complex operations conducted under shelter of eclipse are equally peculiar in their objects and in their system; by them only is the unique problem of the corona at present accessible to attack; that of the “reversing layer” is even more elusive in its momentary presentations. Sun-spots, on the contrary, are open to leisurely daylight inspection; yet the perplexities connected with their structure and spectra grow rather more than less acute as facilities for their scrutiny are increased. But this is no uncommon experience in the arduous walks of science.

The pliancy and generality of astrophysics contrasts singularly with the austere exclusiveness of gravitational astronomy. The new mode of celestial inquiry follows every indication, lays hold of every clue; it promises much, it often performs more; yet its advance is at times hampered by the very circumstances which make it brilliant and surprising. For it “deals,” as Professor Mendenhall said in 1892,[3] “with a matter of many properties, some of which are but little understood. While its conclusions are of vital importance and of intense interest, they result from deductions in which the premisses are insufficient, and are proportionately uncertain. The new astronomy must for a long time abound in contradictions and controversies, until, and largely through its development, we shall possess a knowledge of the properties of matter when subjected to conditions differing enormously from those with which we are now quite familiar.”

Here indeed lies the fundamental peril, and at the same time the essential prerogative of astrophysics. Its concern is with phenomena falling partly within, partly without the range of ordinary experience. It has to do with matter in transcendental states. Hence the necessity for having recourse to the risky expedient of “extrapolation”—that is, of applying unrestrictedly to the unknown, rules gathered from observation over a comparatively narrow area. The indefinite continuity of natural laws is assumed by it, but certainly on no sufficient warrant. There is indeed no help; no other means are available; the line and plummet that have proved serviceable for sounding the estuary must be used likewise for the ocean. The upshot, however, is merely a “first approximation,” to be subsequently corrected and controlled. And it may be of immense importance as an index to consequences or possibilities which could not have been foretold, and defy even imaginative realisation. But just here resides the exploring faculty of astrophysics. It often acts as the pioneer of terrestrial science. “The discovery of unknown laws” (in Professor Keeler’s words), “as well as the explanation of phenomena by laws already known, is one of its most important objects.”

A great future is reserved for it. It postulates a law of order, the same always and everywhere, and its primary function is to verify that postulate, step by step, point by point, under continually widening horizons of knowledge. There is no such thing as chaos, it tacitly asserts, in the sidereal world or outside of it. For chaos is the negation of law, and law is the expression of the Will of God.