A History of Science - v.3 (Williams)

All this seems wonderful enough, but even greater things were in store. In 1859 the spectroscope came upon the scene, perfected by Kirchhoff and Bunsen, along lines pointed out by Fraunhofer almost half a century before. That marvellous instrument, by

revealing the telltale lines sprinkled across a prismatic spectrum, discloses the chemical nature and physical condition of any substance whose light is submitted to it, telling its story equally well, provided the light be strong enough, whether the luminous substance be near

or far--in the same room or at the confines of space. Clearly such an instrument must prove a veritable magic wand in the hands of the astronomer.

Very soon eager astronomers all over the world were putting the spectroscope to the test. Kirchhoff himself led the way, and Donati and Father Secchi in Italy, Huggins and Miller in England, and Rutherfurd in America, were the chief of his immediate followers.

The results exceeded the dreams of the most visionary. At the very outset, in 1860, it was shown that such common terrestrial substances as sodium, iron, calcium, magnesium, nickel, barium, copper, and zinc exist

in the form of glowing vapors in the sun, and very soon the stars gave up a corresponding secret. Since then the work of solar and sidereal analysis has gone on

steadily in the hands of a multitude of workers (prominent

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among whom, in this country, are Professor

Young of Princeton, Professor Langley of Washington, and Professor Pickering of Harvard), and more

than half the known terrestrial elements have been definitely located in the sun, while fresh discoveries are in prospect.

It is true the sun also contains some seeming elements that are unknown on the earth, but this is no

matter for surprise. The modern chemist makes no claim for his elements except that they have thus far

resisted all human efforts to dissociate them; it would be nothing strange if some of them, when subjected to the crucible of the sun, which is seen to vaporize iron, nickel, silicon, should fail to withstand the test. But again, chemistry has by no means exhausted the resources of the earth's supply of raw material, and the

substance which sends its message from a star may exist undiscovered in the dust we tread or in the air

we breathe. In the year 1895 two new terrestrial elements were discovered; but one of these had for years

been known to the astronomer as a solar and suspected as a stellar element, and named helium because of its abundance in the sun. The spectroscope had reached

out millions of miles into space and brought back this new element, and it took the chemist a score of years to discover that he had all along had samples of the

same substance unrecognized in his sublunary laboratory. There is hardly a more picturesque fact than

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that in the entire history of science.

But the identity in substance of earth and sun and stars was not more clearly shown than the diversity of

their existing physical conditions. It was seen that sun and stars, far from being the cool, earthlike, habitable bodies that Herschel thought them (surrounded by

glowing clouds, and protected from undue heat by other clouds), are in truth seething caldrons of fiery liquid, or gas made viscid by condensation, with lurid envelopes

of belching flames. It was soon made clear, also, particularly by the studies of Rutherfurd and of Secchi, that stars differ among themselves in exact constitution or condition. There are white or Sirian stars, whose spectrum revels in the lines of hydrogen; yellow or solar stars (our sun being the type), showing various metallic vapors; and sundry red stars, with banded spectra indicative of carbon compounds; besides the purely gaseous stars of more recent discovery, which Professor Pickering had specially studied. Zollner's

famous interpretation of these diversities, as indicative of varying stages of cooling, has been called in question as to the exact sequence it postulates, but the general

proposition that stars exist under widely varying conditions of temperature is hardly in dispute.

The assumption that different star types mark varying stages of cooling has the further support of modern

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been shown to be--under existing conditions, at any rate--inadequate. In accordance with the theory of Helmholtz, the chief supply of solar energy is held to

be contraction of the solar mass itself; and plainly this must have its limits. Therefore, unless some means as

yet unrecognized is restoring the lost energy to the

stellar bodies, each of them must gradually lose its lustre, and come to a condition of solidification, seeming sterility, and frigid darkness. In the case of our own particular star, according to the estimate of Lord

Kelvin, such a culmination appears likely to occur within a period of five or six million years.

The Astronomy of the Invisible

But by far the strongest support of such a forecast as this is furnished by those stellar bodies which even now

appear to have cooled to the final stage of star development and ceased to shine. Of this class examples in

miniature are furnished by the earth and the smaller of its companion planets. But there are larger bodies of the same type out in stellar space--veritable "dark

stars"--invisible, of course, yet nowadays clearly recognized.

The opening up of this "astronomy of the invisible"

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is another of the great achievements of the nineteenth century, and again it is Bessel to whom the honor of discovery is due. While testing his stars for parallax; that astute observer was led to infer, from certain unexplained aberrations of motion, that various stars, Sirius himself among the number, are accompanied by invisible companions, and in 1840 he definitely predicated the existence of such "dark stars." The correctness

of the inference was shown twenty years

later, when Alvan Clark, Jr., the American optician, while testing a new lens, discovered the companion of Sirius, which proved thus to be faintly luminous. Since then the existence of other and quite invisible star companions has been proved incontestably, not merely by renewed telescopic observations, but by the curious testimony of the ubiquitous spectroscope.

One of the most surprising accomplishments of that instrument is the power to record the flight of a luminous object directly in the line of vision. If the luminous body approaches swiftly, its Fraunhofer lines are

shifted from their normal position towards the violet

end of the spectrum; if it recedes, the lines shift in the opposite direction. The actual motion of stars whose distance is unknown may be measured in this way.

But in certain cases the light lines are seen to oscillate on the spectrum at regular intervals. Obviously the

star sending such light is alternately approaching and

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receding, and the inference that it is revolving about a companion is unavoidable. From this extraordinary

test the orbital distance, relative mass, and actual speed of revolution of the absolutely invisible body may be determined. Thus the spectroscope, which deals only with light, makes paradoxical excursions

into the realm of the invisible. What secrets may the stars hope to conceal when questioned by an instrument of such necromantic power?

But the spectroscope is not alone in this audacious assault upon the strongholds of nature. It has a worthy companion and assistant in the photographic film,

whose efficient aid has been invoked by the astronomer even more recently. Pioneer work in celestial photography was, indeed, done by Arago in France and

by the elder Draper in America in 1839, but the results then achieved were only tentative, and it was not till forty years later that the method assumed really important proportions. In 1880, Dr. Henry Draper, at Hastings-on-the-Hudson, made the first successful photograph of a nebula. Soon after, Dr. David Gill,

at the Cape observatory, made fine photographs of a comet, and the flecks of starlight on his plates first suggested the possibilities of this method in charting the heavens.

Since then star-charting with the film has come virtually to supersede the old method. A concerted effort

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is being made by astronomers in various parts of the world to make a complete chart of the heavens, and

before the close of our century this work will be accomplished, some fifty or sixty millions of visible stars being

placed on record with a degree of accuracy hitherto unapproachable. Moreover, other millions of stars

are brought to light by the negative, which are too distant or dim to be visible with any telescopic powers

yet attained--a fact which wholly discredits all previous inferences as to the limits of our sidereal system. Hence, notwithstanding the wonderful instrumental advances of the nineteenth century, knowledge of the exact form and extent of our universe seems more unattainable than it seemed a century ago.

The Structure of Nebulae

Yet the new instruments, while leaving so much

untold, have revealed some vastly important secrets of cosmic structure. In particular, they have set at rest the long-standing doubts as to the real structure and position of the mysterious nebulae--those lazy masses, only two or three of them visible to the unaided eye,

which the telescope reveals in almost limitless abundance, scattered everywhere among the stars, but

grouped in particular about the poles of the stellar stream or disk which we call the Milky Way.

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Herschel's later view, which held that some at least of the nebulae are composed of a "shining fluid," in process of condensation to form stars, was generally accepted for almost half a century. But in 1844, when

Lord Rosse's great six-foot reflector--the largest telescope ever yet constructed--was turned on the nebulae,

it made this hypothesis seem very doubtful. Just as Galileo's first lens had resolved the Milky Way into stars, just as Herschel had resolved nebulae that resisted all instruments but his own, so Lord Rosse's even

greater reflector resolved others that would not yield to Herschel's largest mirror. It seemed a fair inference

that with sufficient power, perhaps some day to be attained, all nebulae would yield, hence that all are in

reality what Herschel had at first thought them--

vastly distant "island universes," composed of aggregations of stars, comparable to our own galactic system.

But the inference was wrong; for when the spectroscope was first applied to a nebula in 1864, by Dr. Huggins, it clearly showed the spectrum not of discrete

stars, but of a great mass of glowing gases, hydrogen among others. More extended studies showed, it is true, that some nebulae give the continuous spectrum

of solids or liquids, but the different types intermingle and grade into one another. Also, the closest affinity

is shown between nebulae and stars. Some nebulae are found to contain stars, singly or in groups, in their

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actual midst; certain condensed "planetary" nebulae

are scarcely to be distinguished from stars of the gaseous type; and recently the photographic film has

shown the presence of nebulous matter about stars

that to telescopic vision differ in no respect from the generality of their fellows in the galaxy. The familiar stars of the Pleiades cluster, for example, appear on the negative immersed in a hazy blur of light. All in all, the accumulated impressions of the photographic film reveal a prodigality of nebulous matter in the stellar system not hitherto even conjectured.

And so, of course, all question of "island universes" vanishes, and the nebulae are relegated to their true position as component parts of the one stellar system--the

one universe--that is open to present human inspection. And these vast clouds of world-stuff have been found

by Professor Keeler, of the Lick observatory, to be floating through space at the starlike speed of from ten to thirty-eight miles per second.

The linking of nebulae with stars, so clearly evidenced by all these modern observations, is, after all,

only the scientific corroboration of what the elder Herschel's later theories affirmed. But the nebulae have

other affinities not until recently suspected; for the spectra of some of them are practically identical with the spectra of certain comets. The conclusion seems

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warranted that comets are in point of fact minor nebulae that are drawn into our system; or, putting it otherwise, that the telescopic nebulae are simply gigantic

distant comets.

Lockyer's Meteoric Hypothesis

Following up the surprising clews thus suggested, Sir Norman Lockyer, of London, has in recent years elaborated what is perhaps the most comprehensive cosmogonic guess that has ever been attempted. His theory, known as the "meteoric hypothesis," probably

bears the same relation to the speculative thought of our time that the nebular hypothesis of Laplace bore to that of the eighteenth century. Outlined in a few

words, it is an attempt to explain all the major phenomena of the universe as due, directly or indirectly, to

the gravitational impact of such meteoric particles, or specks of cosmic dust, as comets are composed of. Nebulae are vast cometary clouds, with particles more or

less widely separated, giving off gases through meteoric collisions, internal or external, and perhaps glowing also with electrical or phosphorescent light. Gravity eventually brings the nebular particles into closer aggregations,

and increased collisions finally vaporize the entire mass, forming planetary nebulae and gaseous stars. Continued condensation may make the stellar mass hotter and more luminous for a time, but eventually

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