The 100-Inch Hooker Telescope, Largest Reflector in the World, on Mt. Wilson.(Photo, Mt. Wilson Solar Observatory.)
The 100-Inch Hooker Telescope, Largest Reflector in the World, on Mt. Wilson.(Photo, Mt. Wilson Solar Observatory.)
The Largest Refractor, the 40-Inch Telescope at Yerkes Observatory. Dome 90 Ft. in Diameter.(Photo, Yerkes Observatory.)
The 150-ft. Tower at the Mt. Wilson Solar Observatory.At the left is a diagram of tower, telescope and pit. At the upper right is an exterior view of the tower; below a view looking down into the pit, 75 ft. deep. (Photo, Mt. Wilson Solar Observatory.)
The focal lengths of object glass and eyepiece will determine just what distance apart the lenses must be in order to give perfect vision. But it is quite as important that the axes of all the lenses be adjusted into one and the same straight line, and then held there rigidly and permanently. Otherwise vision with the telescope will be very imperfect and wholly unsatisfactory. The distance from the objective, or object glass to its focal point is called its focal length; and if we divide this by the focal length of the eyepiece, we shall have the magnifying power of the telescope. The eyepiece will usually be made of two lenses, or more, and we use its focal length considered as a single lens, in getting the magnifying power. A telescope will generally have many eyepieces of different focal lengths, so that it will have a corresponding range of magnifying powers. The lowest magnifying power will be not less than four or five diameters for each inch of aperture of the objective; otherwise the eye will fail to receive all the light which falls upon the glass. A 4-inch telescope will therefore have no eyepiece with a lower magnifying power than about 20 diameters. The highest magnifying power advantageous for a glass of this size will be about 250 to 300, the working rule being about 70 diameters to each inch of aperture, although the theoretical limit is regarded as 100.
The reason for a variety of eyepieces with different magnifying powers soon becomes apparent on using the telescope. Comets and nebulæ call for very low powers, while double stars and the planetary surfaces require the higher powers, providedthe state of the atmosphere at the moment will allow it. If there is much quivering and unsteadiness, nothing is gained by trying the higher powers, because all the waves of unsteadiness are magnified also in the same proportion, and sharpness of vision, or fine definition, or "good seeing," as it is called, becomes impossible. The vibrations and tremors of the atmosphere are the greatest of all obstacles to astronomical observation, and the search is always in order for regions of the world, in deserts or on high mountains, where the quietest atmosphere is to be found.
Quite another power of the telescope is dependent on its objective solely: its light-gathering power. Light by which we see a star or planet is admitted to the retina of the eye through an adjustable aperture called the pupil. In the dark or at night, the pupil expands to an average diameter of one-fourth of an inch. But the object-glass of a telescope, by focusing the rays from a star, pours into the eye, almost as a funnel acts with water, all the light which falls on its larger surface. And as geometry has settled it for us that areas of surfaces are proportioned to the squares of their diameters, a two-inch object glass focuses upon the retina of the eye 64 times as much light as the unassisted eye would receive. And the great 40-inch objective of the Yerkes telescope would, theoretically, yield 25,600 times as much light as the eye alone. But there would be a noticeable percentage of this lost through absorption by the glasses of the telescope and scattering by their surfaces.
The first makers of telescopes soon encountered a most discouraging difficulty, because it seemed to them absolutely insuperable. This is known aschromatic aberration, or the scattering of light in a telescope due simply to its color or wave length. When light passes through a prism, red is refracted the least and violet the most. Through a lens it is the same, because a lens may be regarded as an indefinite system of prisms. The image of a star or planet, then, formed by a single lens cannot be optically perfect; instead it will be a confused intermingling of images of various colors. With low powers this will not be very troublesome, but great indistinctness results from the use of high magnifying powers.
The early makers and users of telescopes in the latter part of the seventeenth century found that the troublesome effects of chromatic aberration could be much reduced by increasing the focal length of the objective. This led to what we term engineering difficulties of a very serious nature, because the tubes of great length were very awkward in pointing toward celestial objects, especially near the zenith, where the air is quietest. And it was next to impossible to hold an object steadily in the field, even after all the troubles of getting it there had been successfully overcome.
Bianchini and Cassini, Hevelius and Huygens were among the active observers of that epoch who built telescopes of extraordinary length, a hundred feet and upward. One tube is said to have been built 600 feet in length, but quite certainly it could never have been used. So-called aerial telescopes were also constructed, in which the objective was mounted on top of a tower or a pole, and the eyepiece moved along near the ground. But it is difficult to see how anything but fleeting glimpses of the heavenly bodies could have been obtained with suchcontrivances, even if the lenses had been perfect. Newton indeed, who was expert in optics, gave up the problem of improving the refracting telescope, and turned his energies toward the reflector.
In 1733, half a century after Newton and a century and a quarter after Galileo, Chester More Hall, an Englishman, found by experiment that chromatic aberration could be nearly eliminated by making the objective of two lenses instead of one, and the same invention was made independently by Dollond, an English optician, who took out letters patent about 1760. So the size of telescopes seemed to be limited only by the skill of the glassmaker and the size of disks that he might find it practicable to produce.
What Hall and Dollond did was to make the outer or crown lens of the objective as before, and place behind it a plano-concave lens of dense flint glass. This had the effect of neutralizing the chromatic effect, or color aberration, while at the same time only part of the refractive effect of the crown lens was destroyed. This ingenious but costly combination prepared the way for the great refracting telescopes of the present day, because it solved, or seemed to solve, the important problem of getting the necessary refraction of light rays without harmful dispersion or decomposition of them.
Through the 18th century and the first years of the 19th many telescopes of a size very great for that day were built, and their success seemed complete. With large increase in the size of the disks, however, a new trouble arose, quite inherent in the glass itself. The two kinds of glass, flint and crown, do not decompose white light with uniformity, so that when the so-called achromatic objectivewas composed of flint and crown, there was an effect known as irrationality of dispersion, or secondary spectrum, which produced a very troublesome residuum of blue light surrounding the images of bright objects. This is the most serious defect of all the great refractors of the day, and effectively it limits their size to about 60 inches of aperture, with present types of flint and crown. It is expected by present experimenters, however, that further improvements in optical glass will do much to extend this limit; so that a refracting telescope of much greater size than any now in existence will be practicable.
Improvements in mounting telescopes, too, are still possible. Within recent years, Hartness, of Springfield, Vermont, has erected a new and ingenious type of turret telescope which protects the observer from wind and cold while his instrument is outside. It affords exceptional facilities for rapid and convenient observing, as for variable stars, and is adaptable to both refractors and reflectors.
The captivating study of the heavens can of course be begun with the naked eye alone, but very moderate optical assistance is a great help and stimulates. An opera-glass affords such assistance; a field-glass does still better, and best of all, for certain purposes, is a modern prism-binocular.
Cherished with the utmost care in the rooms of the Royal Society of London is a world-famous telescope, a diminutive reflector made by the hands of Sir Isaac Newton. We have already mentioned his connection with the refractor; and how he abandoned that type of telescope in favor of the reflecting mirror, or reflector in which the obstacles to great size appeared to be purely mechanical. By many, indeed, Newton is regarded as the inventor of the reflector.
By the principles of optics, all the rays from a star that strike a concave mirror will be reflected to the geometric focal point, provided a section of that mirror is a parabola. Such a mirror is called a speculum, and is an alloy of tin, copper, and bismuth. Its surface takes a very high polish, reflecting when newly polished nearly 90 per cent of the light that falls upon it.
But the focus where the eyepiece must be used is in front of the mirror, and if the eye were placed there, the observer's head would intercept all or much of the light that would otherwise reach the mirror. Gregory, probably the real inventor of the reflector, was the first to dodge this difficulty by perforating the mirror at the center and applying the eyepiece there, at the back of the speculum; but it was necessary to first send the rays to thatpoint by reflection from a second or smaller mirror, in the optical axis of the speculum. This reflects the rays backward down the tube to the eyepiece, or spectroscope, or camera.
Another English optician, Cassegrain, improved on this design somewhat by placing the secondary mirror inside the focus of the speculum, or nearer to it, so that the tube is shorter. This form is preferable for many kinds of astronomical work, especially photography. Herschel sought to do away with the secondary reflector entirely and save the loss of light by tilting the speculum slightly, so as to throw the image at one side of the tube; but this modification introduces bad definition of the image and has never been much used.
A better plan is that of Newton, who placed a small plane speculum at an angle of 45 degrees in the optical axis where the secondary mirror of the Gregory-Cassegrainian type is placed. The rays are then received by the eyepiece at the side of the upper end of the tube, the observer looking in at right angles to the axis. And a modern improvement first used by Draper is a small rectangular prism in place of the little plane speculum, effecting a saving of five to ten per cent of the light.
It is not easy to say which type of telescope, the refractor or the reflector, is the more famous. Nor which is the better or more useful, or the more likely to lead in the astronomy of the future. When the successors of Dollond had carried the achromatic refractor to the limit enforced by the size of the glass disks they were able to secure, they found these instruments not so great an improvement after all. The single-lens telescopes of great focal length were nearly as good optically, though muchmore awkward to handle. But the quality of the glass obtainable in that day appeared to set an arbitrary limit to that great amplification of size and power which progress in observational astronomy demanded.
Then came the elder Herschel, best known and perhaps the greatest of all astronomers. At Bath, England, music was his profession, especially the organ. But he was dissatisfied with his little Gregorian reflector, and being a very clever mechanician he set out to build a reflector for himself. It is said that he cast and polished nearly 200 mirrors, in the course of experiments on the most highly reflective type of alloys, and the sort of mechanism that would enable him to give them the highest polish. In all his work he was ably and enthusiastically aided by his sister, Caroline Herschel, most famous of all women astronomers.
Upward in size of his mirrors he advanced, till he had a speculum of two feet diameter with a tube 20 feet long. Twelve to fifteen years had elapsed when in 1781, while testing one of these reflectors on stars in the constellation Gemini, he made the first discovery of a planet since the invention of the telescope—the great planet now known as Uranus.
Under the patronage of King George, he advanced to telescopes of still greater size, his largest being no less than forty feet in length, with a speculum of four feet in diameter. Two new satellites of Saturn were discovered with this giant reflector, which was dismantled by Sir John Herschel with appropriate ceremonies, including the singing of an ode by the Herschel family assembled inside of the tube, on New Year's Eve, 1839-40.
We have record of but few attempts to improve the size and definition of great reflectors by the continental astronomers during this era. In England and Ireland, however, great progress was made. About 1860 Lassell built a two-foot reflector, with which he discovered two new satellites of Uranus, and which he subsequently set up in the island of Malta. Ten years later Thomas Grubb and Son of Dublin constructed a four-foot reflector, now at the Observatory in Melbourne, Australia. Calver in conjunction with Common of Ealing, London, about 1880-95 built several large reflectors, the largest of five feet diameter, now owned by Harvard College Observatory; and, rather earlier, Martin of Paris completed a four-foot reflector.
The mirrors of these latter instruments were not made of speculum metal, but of solid glass, which must be very thick (one-seventh their diameter) in order to prevent flexure or bending by their own weight. So sensitive is the optical surface to distortion that unless a complicated series of levers and counterpoises is supplied, to support the under surface of the mirror, the perfection of its optical figure disappears when the telescope is directed to objects at different altitudes in the sky. The upper or outer surface of the glass is the one which receives the optical polish on a heavy coat of silver chemically deposited on the polished glass after its figure has been tested and found satisfactory.
But far and away the most famous reflecting telescope of all is the "Leviathan" of Lord Rosse, built at Birr Castle, Parsonstown, Ireland, about the middle of the last century. His Lordship made many ingenious improvements in grinding the mirror, which was of speculum metal, six feet indiameter and weighed seven tons. It was ground to a focal length of fifty-four feet and mounted between heavy walls of masonry, so that the motion of the great tube was restricted to a few degrees on both sides of the meridian. The huge mechanism was very cumbersome in operation, and photography was not available in those days; nevertheless Lord Rosse's telescope made the epochal discovery of the spiral nebulæ, which no other telescope of that day could have done.
In America the reflector has always kept at least even pace with the refractor. As early as 1830, Mason and Smith, two students at Yale College, enthused by Denison Olmsted, built a 12-inch speculum with which they made unsurpassed observations of the nebulæ. Dr. Henry Draper, returning from a visit to Lord Rosse, began about 1865 the construction of two silver-on-glass reflectors, one of 15 inches diameter, the other of 28 inches, with which he did important work for many years in photography and spectroscopy, and his mirrors are now the property of Harvard College Observatory. Alvan Clark and Sons have in later years built a 40-inch mirror for the Lowell Observatory in Arizona, and very recently a 6-foot silver-on-glass mirror has been set up in the Dominion of Canada Astrophysical Observatory at Victoria, British Columbia, where it is doing excellent work in the hands of Plaskett, its designer.
The huge glass disk for the reflector weighs two tons, and it must be cast so that there are no internal strains; otherwise it is liable to burst in fragments in the process of grinding. It should be free from air-bubbles, too; so the glass is cast in one melting, if possible. This disk was made bythe St. Gobain Plate Glass Company, whose works have been ruthlessly destroyed by the enemy during the war; but fortunately the great disk had been shipped from Antwerp only a week before declaration of hostilities.
Brashear of Allegheny was intrusted with the optical parts, which occupied many months of critical work. The finished mirror is 73 inches in diameter, its focal length is 30 feet, and its thickness 12 inches. A central hole 10 inches in diameter makes possible its use as a Gregorian or Cassegrainian type, as well as Newtonian. The mechanical parts of this great telescope are by Warner and Swasey of Cleveland, after the well-known equatorial mounting of the Melbourne reflector by Grubb of Dublin. Friction of the polar and declination axes is reduced by ball bearings. The 66-foot dome has an opening 15 feet wide and extending six feet beyond the zenith. All motions of the telescope, dome shutters, and observing platform are under complete control by electric motors. Spectroscopic binaries form one of the special fields of research with this powerful instrument, and many new binaries have already been detected.
The great reflectors designed and constructed by Ritchey, formerly of Chicago and now of Pasadena, deserve especial mention. While connected with the Yerkes Observatory he constructed a two-foot reflector for that institution, with which he had exceptional success in photography of the stars and nebulæ. Later he built a 5-foot reflector, now at the Carnegie Observatory on Mount Wilson, California, with which the spiral nebulæ and many other celestial objects have been especially well photographed. Ritchey's later years have beenspent on the construction of an even greater mirror, no less than 100 inches in diameter, which was completed in 1919, and has already yielded photographic results dealt with farther on, and far surpassing anything previously obtained. Theoretically this huge mirror, if its surface were perfectly reflective so that it would transmit all the rays falling upon it, would gather 160,000 times as much light as the unaided eye alone.
Whether a 72-inch refractor, should it ever be constructed, would surpass the 100-inch reflector as an all-round engine for astronomical research, is a question that can only be fully answered by building it and trying the two instruments alongside.
Probably three-quarters of all the really great astronomical work in the past has been done by refractors. They are always ready and convenient for use, and the optical surfaces rarely require cleaning and readjustment. With increase of size, however, the secondary spectrum becomes very bothersome in the great lenses; and the larger they are, the more light is lost by absorption on account of the increasing thickness of the lenses. With the reflector on the other hand, while there is clearly a greater range of size, the reflective surface retains its high polish only a brief period, so that mere tarnish effectively reduces the aperture; and the great mirror is more or less ineffective in consequence of flexure uncompensated by the lever system that supports the back of the mirror.
Both types of telescope still have their enthusiastic devotees; and the next great reflector would doubtless be a gratifying success, if mounted in some elevated region of the world, like the Andes of northern Chile, where the air is exceptionally steadyand the sky very clear a large part of the year. The highest magnifying powers suitable for work with such a telescope could then be employed, and new discoveries added as well as important work done in extension of lines already begun on the universe of stars.
On the authority of Clark, even a six-foot objective would not necessitate a combined thickness of its glasses in excess of six inches. Present disks are vastly superior to the early ones in transparency, and there is reason to expect still greater improvement. The engineering troubles incident to execution of the mechanical side of the scheme need not stand in the way; they never have, indeed the astronomer has but just begun to invoke the fertile resources of the modern engineer. Not long before his death the younger Clark who had just finished the great lenses of the 40-inch Yerkes telescope, ventured this prevision, already in part come true: "The new astronomy, as well as the old, demands more power. Problems wait for their solution, and theories to be substantiated or disproved. The horizon of science has been greatly broadened within the last few years, but out upon the borderland I see the glimmer of new lights that await for their interpretation, and the great telescopes of the future must be their interpreters."
Practically all the great telescopes of the world have in turn signalized the new accession of power by some significant astronomical discovery: to specify, one of Herschel's reflectors first revealed the planet Uranus; Lord Rosse's "Leviathan" the spiral nebulæ; the 15-inch Cambridge lens the crape, or dusky ring of Saturn; the 18½-inch Chicago refractor the companion of Sirius; the Washington26-inch telescope the satellites of Mars; the 30-inch Pulkowa glass the nebulosities of the Pleiades; and the 36-inch Lick telescope brought to light a fifth satellite of Jupiter. At the time these discoveries were made, each of these great telescopes was the only instrument then in existence with power enough to have made the discovery possible. So we may advance to still farther accessions of power with the expectation that greater discoveries will continue to gratify our confidence.
Sir Isaac Newton ought really to have been the inventor of the spectroscope, because he began by analyzing light in the rough with prisms, was very expert in optics, and was certainly enough of a philosopher to have laid the foundations of the science.
What Newton did was to admit sunlight into a darkened room through a small round aperture, then pass the rays through a glass prism and receive the band of color on a screen. He noticed the succession of colors correctly—violet, indigo, blue, green, yellow, orange, red; also that they were not pure colors, but overlapping bands of color. Apparently neither he nor any other experimenter for more than a century went any further, when the next essential step was taken by Wollaston about 1802 in England. He saw that by receiving the light through a narrow slit instead of a round hole, he got a purer spectrum, spectrum being the name given to the succession of colors into which the prism splits up or decomposes the original beam of white sunlight. This seemingly insignificant change, a narrow slit replacing the round hole, made Wollaston and not Newton the discoverer of the dark lines crossing the spectrum at various irregular intervals, and these singularly neglected lines meant the basis of a new and most important science.
Even Wollaston, however, passed them by, and it was Fraunhofer who in 1814-1815 first made a chart of them. Consequently they are known as Fraunhofer lines, or dark absorption lines. Sending the beam of light through a succession of prisms gives greater dispersion and increases the power of the spectroscope. The greater the dispersion the greater the number of absorption lines; and it is the number and intensity of these lines, with their accurate position throughout the range of the spectrum which becomes the basis of spectrum analysis.
The half century that saw the invention of the steam engine, photography, the railroad and the telegraph elapsed without any farther developments than mere mapping of the fundamental lines, A, B, C, D, E, F, G, H of the solar spectrum. The moon, too, was examined and its spectrum found the same, as was to be expected from sunlight simply reflected.
Sir John Herschel and other experimenters came near guessing the significance of the dark lines, but the problem of unraveling their mystery was finally solved by Bunsen and Kirchhoff who ascertained that an incandescent gas emits rays of exactly the same degree of refrangibility which it absorbs when white light is passed through it. This great discovery was at once received as the secure basis of spectrum analysis, and Kirchhoff in 1858 put in compact and comprehensive form the three following principles underlying the theory of the science:
(1) Solid and liquid bodies, also gases under high pressure, give when incandescent a continuous spectrum, that is one with a mere succession of colors, and neither bright nor dark lines;
(2) Gases under low pressure give a discontinuous spectrum, crossed by bright lines whose number andposition in the spectrum differ according to the substances vaporized;
(3) When white light passes through a gas, this medium absorbs or quenches rays of identical wave-length with those composing its own bright-line spectrum.
Clearly then it makes no difference where the light originates whether it comes from sun or star. Only it must be bright enough so that we can analyze it with the spectroscope. But our analysis of sun and star could not proceed until the chemist had vaporized in the laboratory all the elements, and charted their spectra with accuracy. When this had been done, every substance became at once recognizable by the number and position of its lines, with practical certainty.
How then can we be sure of the chemical and physical composition of sun and stars? Only by detailed and critical comparison of their spectra with the laboratory spectra of elements which chemical and physical research have supplied. As in the sun, so in the stars, each of which is encircled by a gaseous absorptive layer or atmosphere, the light rays from the self-luminous inner sphere must pass through this reversing layer, which absorbs light of exactly the same wave-length as the lines that make up its own bright line spectrum. Whatever substances are here found in gaseous condition, the same will be evident by dark lines in the spectrum of sun or star, and the position of these dark lines will show, by coincidence with the position of the laboratory bright lines, all the substances that are vaporized in the atmospheres of the self-luminous bodies of the sky.
Here then originated the science of the new astronomy: the old astronomy had concerned itselfmainly with positions of the heavenly bodies,wherethey are; the new astronomy deals with their chemical composition and physical constitution, andwhatthey are. Between 1865 and 1875 the fundamental application of the basic principles was well advanced by the researches of Sir William Huggins in England, of Father Angelo Secchi in Rome, of Jules Janssen in Paris, and of Dr. Henry Draper in New York.
In analyzing the spectrum of the sun, many thousands of dark absorption lines are found, and their coincidences with the bright lines of terrestrial elements show that iron, for instance, is most prominently identified, with rather more than 2,000 coincidences of bright and dark lines. Calcium, too, is indicated by peculiar intensity of its lines, as well as their great number. Next in order are hydrogen, nickel and sodium. By prolonged and minute comparison of the solar spectrum with spectra of terrestrial elements, something like forty elemental substances are now known to exist in the sun. Rowland's splendid photographs of the solar spectrum have contributed most effectively. About half of these elements, though not in order of certainty, are aluminum, cadmium, calcium, carbon, chromium, cobalt, copper, hydrogen, iron, magnesium, manganese, nickel, scandium, silicon, silver, sodium, titanium, vanadium, yttrium, zinc, and zirconium. Oxygen, too, is pretty surely indicated; but certain elements abundant on earth, as nitrogen and chlorine, together with gold, mercury, phosphorus, and sulphur, are not found in the sun.
The two brilliant red stars, Aldebaran in Taurus, and Betelgeuse in Orion, were the first stars whose chemical constitution was revealed to the eye of man,and Sir William Huggins of London was the astronomer who achieved this epoch-making result. Father Secchi of the Vatican Observatory proceeded at once with the visual examination of the spectra of hundreds of the brighter stars, and he was the first to provide a classification of stellar spectra. There were four types.
Secchi's type I is characterized chiefly by the breadth and intensity of dark hydrogen lines, together with a faintness or entire absence of metallic lines. These are bluish or white stars and they are very abundant, nearly half of all the stars. Vega, Altair, and numerous other bright stars belong to this type, and especially Sirius, which gives to the type the name "Sirians."
Type II is characterized by a multitude of fine dark metallic lines, closely resembling the lines of the solar spectrum. These stars are somewhat yellowish in tinge like the sun, and from this similarity of spectra they are called "solars." Arcturus and Capella are "solars," and on the whole the solars are rather less numerous than the Sirians. Stars nearest to the solar system are mostly of this type, and, according to Kapteyn of Groningen, the absolute luminous power of first type stars exceeds that of second type stars seven-fold.
Secchi's type III is characterized by many dark bands, well defined on the side toward the blue end of the spectrum, but shading off toward the red—a "colonnaded spectrum", as Miss Clerke aptly terms it. Alpha Herculis, Antares, and Mira, together with orange and reddish stars and most of the variable stars, belong in type III.
Type IV is also characterized by dark bands, often called "flutings," similar to those of type III, butreversed as to shading, that is, well defined on the side toward the red, but fading out toward the blue. Their atmospheres contain carbon; but they are not abundant, besides being faint and nearly all blood-red in tint.
Following up the brilliant researches of Draper, who in 1872 obtained the first successful photograph of a star's spectrum, that of Vega, Pickering of Harvard supplemented Secchi's classification by Type V, a spectrum characterized by bright lines. They, too, are not abundant and are all found near the middle of the Galaxy. These are usually known as Wolf-Rayet stars, from the two Paris astronomers who first investigated their spectra. Type V stars are a class of objects seemingly apart from the rest of the stellar universe, and many of the planetary nebulæ yield the same sort of a spectrum.
The late Mrs. Anna Palmer Draper, widow of Dr. Henry Draper, established the Henry Draper Memorial at Harvard, and investigation of the photographic spectra of all the brighter stars of the entire heavens has been prosecuted on a comprehensive scale, those of the northern hemisphere at Cambridge, and of the southern at Arequipa, Peru. These researches have led to a broad reclassification of the stars into eight distinct groups, a work of exceptional magnitude begun by the late Mrs. Fleming and recently completed by Miss Annie Cannon, who classified the photographic spectra of more than 230,000 stars on the new system, as follows:—
The letters O, B, A, F, G, K, M, N represent a continuous gradation in the supposed order of stellar evolution, and farther subdivision is indicated by tenths, G5K meaning a type half way between G andK, and usually written G5 simply. B2 would indicate a type between B and A, but nearer to B than A, and so on. On this system, the spectrum of a star in the earliest stages of its evolution is made up of diffuse bright bands on a faint continuous background. As these bands become fewer and narrower, very faint absorption lines begin to appear, first the helium lines, followed by several series of hydrogen lines. On the disappearance of the bright bands, the spectrum becomes wholly absorptive bands and lines. Then comes a very great increase in intensity of the true hydrogen spectrum, with wide and much diffused lines, and few if any other lines. Then the H and K calcium lines and other lines peculiar to the sun become more and more intense. Then the hydrogen lines go through their long decline. The calcium spectrum becomes intense, and later the spectrum becomes quite like that of the sun with a great wealth of lines. Following this stage the spectrum shortens from the ultra violet, the hydrogen lines fade out still farther, and bands due to metallic compounds make their appearance, the entire spectrum finally resembling that of sun spots. To designate these types rather more categorically:—
Type O—bright bands on a faint continuous background, with five subdivisions, Oa, Ob, Oc, Od, Oe, according to the varying width and intensity of the bands.
Type B—the Orion type, or helium type, with additional lines of origin unknown as yet, but without any of the bright bands of type O.
Type A—the Sirian type, the regular Balmer series of hydrogen lines being very intense, with a few other lines not conspicuously marked.
Type F—the calcium type, hydrogen lines less strongly marked, but with the narrow calcium lines H and K very intense.
Type G—the solar type, with multitudes of metallic lines.
Type K—in some respects similar to G, but with the hydrogen lines fading out, and the metallic lines relatively more prominent.
Type M—spectrum with peculiar flutings due to titanium oxide, with subdivisions Ma and Mb, and the variable stars of long period, with a few bright hydrogen lines additional, in a separate class Md.
Type N—similar to M, in that both are pronouncedly reddish, but with characteristic flutings probably indicating carbon compounds.
The Draper classification being based on photographic spectra, and the original Secchi classification being visual, the relation of the two systems is approximately as follows:
Pickering's marked success in organization and execution of this great programme was due to his adoption of the "slitless spectroscope," which made it possible to photograph stellar spectra in vast numbers on a single plate. The first observers of stellar spectra placed the spectroscope beyond the focus of the telescope with which it was used, thereby limiting the examination to but one star at a time. In the slitless spectroscope, a large prism is mounted in front of the objective (of short focus), so that the star's rays pass through it first, and then are brought to the same focus on the photographicplate, for all the stars within the field of view, sometimes many thousand in number. This arrangement provides great advantages in the comparison and classification of stellar spectra.
When spectroscopic methods were first introduced into astronomy, there was no expectation that the field of the old or so-called exact astronomy would be invaded. Physicists were sometimes jocularly greeted among astronomers as "ribbon men," and no one even dreamed that their researches were one day to advance to equal recognition with results derived from micrometer, meridian circle, and heliometer.
The first step in this direction was taken in 1868 by Sir William Huggins of London, who noticed small displacements in the lines of spectra of very bright stars. In fact the whole spectrum appeared to be shifted; in the case of Sirius it was shifted toward the red, while the whole spectrum of Arcturus was shifted by three times this amount toward the violet end of the spectrum. The reason was not difficult to assign.
As early as 1842 Doppler had enunciated the principle that when we are approaching or are approached by a body which is emitting regular vibrations, then the number of waves we receive in a second is increased, and their wave-length correspondingly diminished; and just the reverse of this occurs when the distance of the vibrating body is increasing. It is the same with light as with sound, and everyone has noticed how the pitch of a locomotive whistle suddenly rises as it passes, and falls as suddenly on retreating from us. So Huggins drew the immediate inference that the distance between the earth and Sirius was increasing at therate of nearly twenty miles per second, while Arcturus was nearing us with a velocity of sixty miles per second.
These pioneer observations of motions in the line of sight, or radial velocities as they are now called, led directly to the acceptance of the high value of spectroscopic work as an adjunct of exact astronomy in stellar research. Nor has it been found wanting in application to a great variety of exact problems in the solar system which would have been wholly impossible to solve without it.
Foremost is the sun, of course, because of the overplus of light. Young early measured the displacement of lines in the spectra of the prominences, and found velocities sometimes exceeding 250 miles per second. Many astronomers, Dunér among them, investigated the rotation of the sun by the spectroscopic method. The sun's east limb is coming toward us, while the west is going from us; and by measuring the sum of the displacements, the rate of rotation has been calculated, not only at the sun's equator but at many solar latitudes also, both north and south. As was to be expected, these results agree well with the sun's rotation as found by the transits of sun spots in the lower latitudes where they make their appearance.
Bélopolsky has applied the same method to the rotation of the planet Venus, and Keeler, by measuring the displacement of lines in the spectrum of Saturn, on opposite sides of the ring, provided a brilliant observational proof of the physical constitution of the rings; because he showed that the inner ring traveled round more swiftly than the outer one, thus demonstrating that the ring could not be solid, but must be composed of multitudes ofsmall particles traveling around the ball of Saturn, much as if they were satellites. Indeed, Keeler ascertained the velocity of their orbital motion and found that in each case it agreed exactly with that required by the Keplerian law.
Even the filmy corona of the sun was investigated in similar fashion by Deslandres at the total eclipse of 1893, and he found that it rotates bodily with the sun. But the complete vindication of the spectroscopic method as an adjunct of the old astronomy came with its application to measurement of the distance of the sun. The method is very interesting and was first suggested by Campbell in 1892. Spectrum-line measurements have become very accurate with the introduction of dry-plate photography, and ecliptic stars were spectrographed, toward and from which the earth is traveling by its orbital motion round the sun. By accurate measurement of these displacements, the orbital velocity of the earth is calculated; and as we know the exact length of the year, or a complete period, the length of the orbit itself in miles becomes known, and thus, by simple mensuration, the length of the radius of the orbit—which is the distance of the sun.
If we pass from sun to star, the triumph of the spectroscope has been everywhere complete and significant. As the spectroscopic survey of the stars grew toward completeness, it became evident that the swarming hosts of the stellar universe are in constant motion through space, not only athwart the line of vision as their proper motions had long disclosed, but some stars are swiftly moving toward our solar system and others as swiftly from it.
Fixed stars, strictly speaking—there are no such. All are in relative motion. Exact astronomy by discussionof the proper motions had assigned a region of the sky toward which the sun and planets are moving. Spectrography soon verified this direction not only, but gave a determination of the velocity of our motion of twelve miles per second in a direction approximately that of the constellation Lyra. From corresponding radial velocities, we draw the ready conclusion that certain groups or clusters of stars are actually connected in space and moving as related systems, as in the Pleiades and Ursa Major.
Rather more than a quarter century ago, the spectroscope came to the assistance of the telescope in helping to solve the intricate problem of stellar distribution. Kapteyn, by combining the proper motions of certain stars with their classification in the Draper catalogue of stellar spectra, drew the conclusion that, as stars having very small proper motions show a condensation toward the Galaxy, the stars composing this girdle are mostly of the Sirian type, and are at vast distances from the solar system. The proper motion of a star near to us will ordinarily be large, and, in the case of solar stars, the larger their proper motion the greater their number. So it would appear that the solar stars are aggregated round the sun himself, and this conclusion is greatly strengthened by the fact that of stars whose distances and spectral type are both ascertained, seven of the eight nearest to us are solar stars.
In 1889 the spectroscope achieved an unexpected triumph by enabling the late Professor Pickering to make the first discovery of a spectroscopic double, or binary star, a type of object now quite abundant. Unlike the visual binary systems whose periods areyears in length, the spectroscopic binaries have short periods, reckoned in some cases in days, or hours even. If the orbit of a very close binary is seen edge on, the light of the two stars will coalesce twice in every revolution. Halfway between these points there are two times when the two stars will be moving, one toward the earth and the other from it. At all times the light of the star, in so far as the telescope shows it, proceeds from a single object.
Now photograph the star's spectrum at each of the four critical points above indicated: in the first pair the lines are sharply defined and single, because at conjunction the stars are simply moving athwart the line of sight, while at the intermediate points the lines are double. Doppler's principle completely accounts for this: the light from the receding companion is giving lines displaced toward the red, while the approaching companion yields lines displaced toward the violet. Mizar, the double star at the bend of the handle in the Great Dipper was the first star to yield this peculiar type of spectrum, and the period of its invisible companion is about 52 days. The relative velocity of the components is 100 miles a second, and applying Newton's law we find its mass exceeds that of the sun forty-fold. Capella has been found to be a spectroscopic binary; also the pole star. Spectroscopic binaries have relatively short periods, one of the shortest known being only 35 hours in length. It is in the constellation Scorpio. Beta Aurigæ is another whose lines double on alternate nights, giving a period of four days; and the combined mass of both stars is more than twice that of the sun. The catalogue of spectroscopic binaries is constantly enlarging; but thousands doubtless exist that can never be discovered by thismethod, as is evident if their orbits are perpendicular to the line of sight or nearly so. The history of the spectroscopic binaries is one of the most interesting chapters in astronomy, and affords a marvelous confirmation of the prediction of Bessel who first wrote of "the astronomy of the invisible."
Find a star's distance by the spectroscope? Impossible, everyone would have said, even a very few years ago. Now, however, the thing is done, and with increasing accuracy.
Adams of Mount Wilson has found, after protracted investigation, that the relative intensity of certain spectral lines varies according to the absolute brightness of a star; indeed, so close is the correspondence that the spectroscopic observations are employed to provide in certain cases a good determination of the absolute magnitude, and therefore of the distance. To test this relation, the spectroscopic parallaxes have been compared with the measured parallaxes in numerous instances, and an excellent agreement is shown. This new method is adding extensively to our knowledge of stellar luminosities and distances, and even the vast distances of globular clusters and spiral nebulæ are becoming known.
In fact, but few departments of the old astronomy are left which the new astronomy has not invaded, and this latest triumph of the spectroscope in determining accurately the distances of even the remotest stars is enthusiastically welcomed by advocates of the old and new astronomy alike.
The most powerful ally of both telescope and spectroscope is photography. Without it the marvelous researches carried on with both these types of instrument would have been essentially impossible. Even the great telescopes of Herschel and Lord Rosse, notwithstanding their splendid record as optical instruments, might have achieved vastly more had photography been developed in their time to the point where the astronomer could have employed its wonderful capabilities as he does to-day. And, with the spectroscope, it is hardly too much to say that no investigator ever observes visually with that instrument any more: practically every spectrum is made a matter of photographic record first. The observing, or nowadays the measuring, is all done afterward.
All telescopes and cameras are alike, in that each must form or have formed within it an image by means of a lens or mirror. In the telescope the eye sees the fleeting image, in the camera the process of registering the image on a plate or film is known as photography. Daguerre first invented the process (silver film on a copper plate) in 1839. The year following it was first employed on the moon, in 1850 the first star was photographed, in 1851 the first total eclipse of the sun; all by the primitive daguerreotypeprocess, which, notwithstanding its awkwardness and the great length of exposure required, was found to possess many advantages for astronomical work.
About the middle of the last century the wet plate process, so called because the sensitized collodion film must be kept moist during exposure, came into general use, and the astronomers of that period were not slow to avail themselves of the advantages of a more sensitive process, which in 1872, in the skillful hands of Henry Draper, produced the first spectrum of a star. In 1880 a nebula was first photographed, and in 1881 a comet.
Before this time, however, the new dry-plate process had been developed to the point where astronomers began to avail of its greater convenience and increased sensitiveness, even in spite of the coarseness of grain of the film. Forty years of dry-plate service have brought a wealth of advantages scarcely dreamed of in the beginning, and nearly every department of astronomical research has been enhanced thereby, while many entirely new photographic methods of investigation have been worked out.
Continued improvement in photographic processes has provided the possibility of pictures of fainter and fainter celestial objects, and all the larger telescopes have photographed stars and nebulæ of such exceeding faintness that the human eye, even if applied to the same instrument, would never be able to see them. This is because the eye, in ten or twelve seconds of keen watching, becomes fatigued and must be rested, whereas the action of very faint light rays is cumulative on the highly sensitive film; so that a continuous exposure of manyhours' duration becomes readily visible to the eye on development. So a supersensitive dry plate will often record many thousand stars in a region where the naked eye can see but one.
Perhaps the greatest amplification of photography has taken place at the Harvard Observatory under Pickering, where a library of many hundred thousand plates has accumulated; and at Groningen, Holland, where Kapteyn has established an astronomical laboratory without instruments except such as are necessary to measure photographic plates, whenever and wherever taken. So it is possible to select the clearest of skies, all over the world, for exposure of the plates, and bring back the photographs for expert discussion.
Of course the sun was the celestial body first photographed, and its surpassing brilliance necessitates reduction of exposure to a minimum. In moments of exceptional steadiness of the atmosphere, a very high degree of magnification of the solar surface on the photographic plate is permitted, and the details in formation, development, and ending of sun spots are faithfully registered. Nevertheless, it cannot be said that photography has yet entirely replaced the eye in this work, and careful drawings of sun spots at critical stages in their life are capable of registering fine detail which the plate has so far been unable to record. Janssen of Paris took photographs of the solar photosphere so highly magnified that the granulation or willow-leaf structure of the surface was clearly visible, and its variations traceable from hour to hour.
The advantages of sun spot photography in ascertaining the sun's rotation, keeping count of the spots, and in a permanent record for measurementof position of the sun's axis and the spot zones, are obvious. In direct portrayal of the sun's corona during total eclipses, photography has offered superior advantages over visual sketching, in the form and exact location of the coronal streamers; but the extraordinary differences of intensity between the inner corona and its outlying extensions are such that halation renders a complete picture on a single plate practically impossible. The filamentous detail of the inner corona, and the faintest outlying extensions or streamers, the eye must still reveal directly.
In solar spectrum photography, research has been especially benefited; indeed, exact registry of the multitudinous lines was quite impossible without it. Photographic maps of the spectrum by Thollon, McClean and Rowland are so complete and accurate that no visual charts can approach them. Rowland's great photographic map of the solar spectrum spread out into a band about forty feet in length; and in the infra-red, Langley's spectrobolometer extended the invisible heat spectrum photographically to many times that length. At the other end of the spectrum, special photographic processes have extended the ultra-violet spectrum far beyond the ocular limit, to a point where it is abruptly cut off by absorption of the earth's atmosphere. On the same plate with certain regions of the sun's spectrum, the spectra of terrestrial metals are photographed side by side, and exact coincidences of lines show that about forty elemental substances known to terrestrial chemistry are vaporized in the sun.