Photography promises to a.s.sist in the measurement of parallax and proper motions. Herr Pulfrich, of the firm of Carl Zeiss, has vastly extended the applications of stereoscopic vision to astronomy--a subject which De la Rue took up in the early days of photography. He has made a stereo-comparator of great beauty and convenience for comparing stereoscopically two star photographs taken at different dates. Wolf of Heidelberg has used this for many purposes. His investigations depending on the solar motion in s.p.a.ce are remarkable.
He photographs stars in a direction at right angles to the line of the sun"s motion. He has taken photographs of the same region fourteen years apart, the two positions of his camera being at the two ends of a base-line over 5,000,000,000 miles apart, or fifty-six astronomical units. On examining these stereoscopically, some of the stars rise out of the general plane of the stars, and seem to be much nearer. Many of the stars are thus seen to be suspended in s.p.a.ce at different distances corresponding exactly to their real distances from our solar system, except when their proper motion interferes. The effect is most striking; the accuracy of measurement exceeds that of any other method of measuring such displacements, and it seems that with a long interval of time the advantage of the method increases.
_Double Stars._--The large cla.s.s of double stars has always been much studied by amateurs, partly for their beauty and colour, and partly as a test for telescopic definition. Among the many unexplained stellar problems there is one noticed in double stars that is thought by some to be likely to throw light on stellar evolution. It is this: There are many instances where one star of the pair is comparatively faint, and the two stars are contrasted in colour; and in every single case the general colour of the faint companion is invariably to be cla.s.sed with colours more near to the blue end of the spectrum than that of the princ.i.p.al star.
_Binary Stars._--Sir William Herschel began his observations of double stars in the hope of discovering an annual parallax of the stars. In this he was following a suggestion of Galileo"s. The presumption is that, if there be no physical connection between the stars of a pair, the largest is the nearest, and has the greatest parallax. So, by noting the distance between the pair at different times of the year, a delicate test of parallax is provided, unaffected by major instrumental errors.
Herschel did, indeed, discover changes of distance, but not of the character to indicate parallax. Following this by further observation, he found that the motions were not uniform nor rectilinear, and by a clear a.n.a.lysis of the movements he established the remarkable and wholly unexpected fact that in all these cases the motion is due to a revolution about their common centre of gravity.[11] He gave the approximate period of revolution of some of these: Castor, 342 years; Serpentis, 375 years; Leonis, 1,200 years; Bootis, 1,681 years.
Twenty years later Sir John Herschel and Sir James South, after re-examination of these stars, confirmed[12] and extended the results, one pair of Coronae having in the interval completed more than a whole revolution.
It is, then, to Sir William Herschel that we owe the extension of the law of gravitation, beyond the limits of the solar system, to the whole universe. His observations were confirmed by F.G.W. Struve (born 1793, died 1864), who carried on the work at Dorpat. But it was first to Savary,[13] and later to Encke and Sir John Herschel, that we owe the computation of the elliptic elements of these stars; also the resulting identification of their law of force with Newton"s force of gravitation applied to the solar system, and the force that makes an apple fall to the ground. As Grant well says in his _History_: "This may be justly a.s.serted to be one of the most sublime truths which astronomical science has. .h.i.therto disclosed to the researches of the human mind."
Latterly the best work on double stars has been done by S. W. Burnham,[14] at the Lick Observatory. The shortest period he found was eleven years ( Pegasi). In the case of some of these binaries the parallax has been measured, from which it appears that in four of the surest cases the orbits are about the size of the orbit of Ura.n.u.s, these being probably among the smallest stellar orbits.
The law of gravitation having been proved to extend to the stars, a discovery (like that of Neptune in its origin, though unlike it in the labour and originality involved in the calculation) that entrances the imagination became possible, and was realised by Bessel--the discovery of an unknown body by its gravitational disturbance on one that was visible. In 1834 and 1840 he began to suspect a want of uniformity in the proper motion of Sirius and Procyon respectively. In 1844, in a letter to Sir John Herschel,[15] he attributed these irregularities in each case to the attraction of an invisible companion, the period of revolution of Sirius being about half a century. Later he said: "I adhere to the conviction that Procyon and Sirius form real binary systems, consisting of a visible and an invisible star. There is no reason to suppose luminosity an essential quality of cosmical bodies. The visibility of countless stars is no argument against the invisibility of countless others." This grand conception led Peters to compute more accurately the orbit, and to a.s.sign the place of the invisible companion of Sirius. In 1862 Alvan G. Clark was testing a new 18-inch object-gla.s.s (now at Chicago) upon Sirius, and, knowing nothing of these predictions, actually found the companion in the very place a.s.signed to it. In 1896 the companion of Procyon was discovered by Professor Schaeberle at the Lick Observatory.
Now, by the refined parallax determinations of Gill at the Cape, we know that of Sirius to be 0".38. From this it has been calculated that the ma.s.s of Sirius equals two of our suns, and its intrinsic brightness equals twenty suns; but the companion, having a ma.s.s equal to our sun, has only a five-hundredth part of the sun"s brightness.
_Spectroscopic Binaries_.--On measuring the velocity of a star in the line of sight at frequent intervals, periodic variations have been found, leading to a belief in motion round an invisible companion. Vogel, in 1889, discovered this in the case of Spica ( Virginis), whose period is 4d. 0h. 19m., and the diameter of whose orbit is six million miles. Great numbers of binaries of this type have since then been discovered, all of short period.
Also, in 1889, Pickering found that at regular intervals of fifty-two days the lines in the spectrum of of the Great Bear are duplicated, indicating a relative velocity, equal to one hundred miles a second, of two components revolving round each other, of which that apparently single star must be composed.
It would be interesting, no doubt, to follow in detail the acc.u.mulating knowledge about the distances, proper motions, and orbits of the stars; but this must be done elsewhere. Enough has been said to show how results are acc.u.mulating which must in time unfold to us the various stellar systems and their mutual relationships.
_Variable Stars._--It has often happened in the history of different branches of physical science that observation and experiment were so far ahead of theory that hopeless confusion appeared to reign; and then one chance result has given a clue, and from that time all differences and difficulties in the previous researches have stood forth as natural consequences, explaining one another in a rational sequence. So we find parallax, proper motion, double stars, binary systems, variable stars, and new stars all bound together.
The logical and necessary explanation given of the cause of ordinary spectroscopic binaries, and of irregular proper motions of Sirius and Procyon, leads to the inference that if ever the plane of such a binary orbit were edge-on to us there ought to be an eclipse of the luminous partner whenever the non-luminous one is interposed between us. This should give rise either to intermittence in the star"s light or else to variability. It was by supposing the existence of a dark companion to Algol that its discoverer, Goodricke of York,[16] in 1783, explained variable stars of this type. Algol ( Persei) completes the period of variable brightness in 68.8 hours. It loses three-fifths of its light, and regains it in twelve hours. In 1889 Vogel,[17] with the Potsdam spectrograph, actually found that the luminous star is receding before each eclipse, and approaching us after each eclipse; thus entirely supporting Goodricke"s opinion.
There are many variables of the Algol type, and information is steadily acc.u.mulating. But all variable stars do not suffer the sudden variations of Algol. There are many types, and the explanations of others have not proved so easy.
The Harvard College photographs have disclosed the very great prevalence of variability, and this is certainly one of the lines in which modern discovery must progress.
Roberts, in South Africa, has done splendid work on the periods of variables of the Algol type.
_New Stars_.--Extreme instances of variable stars are the new stars such as those detected by Hipparchus, Tycho Brahe, and Kepler, of which many have been found in the last half-century. One of the latest great "Novae" was discovered in Auriga by a Scotsman, Dr. Anderson, on February 1st, 1892, and, with the modesty of his race, he communicated the fact to His Majesty"s Astronomer for Scotland on an unsigned post-card.[18] Its spectrum was observed and photographed by Huggins and many others. It was full of bright lines of hydrogen, calcium, helium, and others not identified. The astounding fact was that lines were shown in pairs, bright and dark, on a faint continuous spectrum, indicating apparently that a dark body approaching us at the rate of 550 miles a second[19] was traversing a cold nebulous atmosphere, and was heated to incandescence by friction, like a meteor in our atmosphere, leaving a luminous train behind it. It almost disappeared, and on April 26th it was of the sixteenth magnitude; but on August 17th it brightened to the tenth, showing the princ.i.p.al nebular band in its spectrum, and no sign of approach or recession. It was as if it emerged from one part of the nebula, cooled down, and rushed through another part of the nebula, rendering the nebular gas more luminous than itself.[20]
Since 1892 one Nova after another has shown a spectrum as described above, like a meteor rushing towards us and leaving a train behind, for this seems to be the obvious meaning of the spectra.
The same may be said of the brilliant Nova Persei, brighter at its best than Capella, and discovered also by Dr. Anderson on February 22nd, 1901. It increased in brightness as it reached the densest part of the nebula, then it varied for some weeks by a couple of magnitudes, up and down, as if pa.s.sing through separate nebular condensations. In February, 1902, it could still be seen with an opera-gla.s.s. As with the other Novae, when it first dashed into the nebula it was vaporised and gave a continuous spectrum with dark lines of hydrogen and helium. It showed no bright lines paired with the dark ones to indicate a train left behind; but in the end its own luminosity died out, and the nebular spectrum predominated.
The nebular illumination as seen in photographs, taken from August to November, seemed to spread out slowly in a gradually increasing circle at the rate of 90" in forty-eight days. Kapteyn put this down to the velocity of light, the original outburst sending its illumination to the nebulous gas and illuminating a spherical sh.e.l.l whose radius increased at the velocity of light. This supposition seems correct, in which case it can easily be shown from the above figures that the distance of this Nova was 300 light years.
_Star Catalogues._--Since the days of very accurate observations numerous star-catalogues have been produced by individuals or by observatories. Bradley"s monumental work may be said to head the list.
Lacaille"s, in the Southern hemisphere, was complementary. Then Piazzi, Lalande, Groombridge, and Bessel were followed by Argelander with his 324,000 stars, Rumker"s Paramatta catalogue of the southern hemisphere, and the frequent catalogues of national observatories.
Later the Astronomische Gesellschaft started their great catalogue, the combined work of many observatories. Other southern ones were Gould"s at Cordova and Stone"s at the Cape.
After this we have a new departure. Gill at the Cape, having the comet 1882.ii. all to himself in those lat.i.tudes, wished his friends in Europe to see it, and employed a local photographer to strap his camera to the observatory equatoreal, driven by clockwork, and adjusted on the comet by the eye. The result with half-an-hour"s exposure was good, so he tried three hours. The result was such a display of sharp star images that he resolved on the Cape Photographic Durchmusterung, which after fourteen years, with Kapteyn"s aid in reducing, was completed. Meanwhile the brothers Henry, of Paris, were engaged in going over Chacornac"s zodiacal stars, and were about to catalogue the Milky Way portion, a serious labour, when they saw Gill"s Comet photograph and conceived the idea of doing the rest of their work by photography. Gill had previously written to Admiral Mouchez, of the Paris Observatory, and explained to him his project for charting the heavens photographically, by combining the work of many observatories. This led Admiral Mouchez to support the brothers Henry in their scheme.[21] Gill, having got his own photographic work underway, suggested an international astrographic chart, the materials for different zones to be supplied by observatories of all nations, each equipped with similar photographic telescopes. At a conference in Paris, 1887, this was decided on, the stars on the charts going down to the fourteenth magnitude, and the catalogues to the eleventh.
[Ill.u.s.tration: GREAT COMET, Nov. 14TH, 1882. (Exposure 2hrs. 20m.) By kind permission of Sir David Gill. From this photograph originated all stellar chart-photography.]
This monumental work is nearing completion. The labour involved was immense, and the highest skill was required for devising instruments and methods to read off the star positions from the plates.
Then we have the Harvard College collection of photographic plates, always being automatically added to; and their annex at Arequipa in Peru.
Such catalogues vary in their degree of accuracy; and fundamental catalogues of standard stars have been compiled. These require extension, because the differential methods of the heliometer and the camera cannot otherwise be made absolute.
The number of stars down to the fourteenth magnitude may be taken at about 30,000,000; and that of all the stars visible in the greatest modern telescopes is probably about 100,000,000.
_Nebulae and Star-cl.u.s.ters._--Our knowledge of nebulae really dates from the time of W. Herschel. In his great sweeps of the heavens with his giant telescopes he opened in this direction a new branch of astronomy. At one time he held that all nebulae might be cl.u.s.ters of innumerable minute stars at a great distance. Then he recognised the different cla.s.ses of nebulae, and became convinced that there is a widely-diffused "shining fluid" in s.p.a.ce, though many so-called nebulae could be resolved by large telescopes into stars. He considered that the Milky Way is a great star cl.u.s.ter, whose form may be conjectured from numerous star-gaugings. He supposed that the compact "planetary nebulae" might show a stage of evolution from the diffuse nebulae, and that his cla.s.sifications actually indicate various stages of development. Such speculations, like those of the ancients about the solar system, are apt to be harmful to true progress of knowledge unless in the hands of the ablest mathematical physicists; and Herschel violated their principles in other directions. But here his speculations have attracted a great deal of attention, and, with modifications, are accepted, at least as a working hypothesis, by a fair number of people.
When Sir John Herschel had extended his father"s researches into the Southern Hemisphere he was also led to the belief that some nebulae were a phosph.o.r.escent material spread through s.p.a.ce like fog or mist.
Then his views were changed by the revelations due to the great discoveries of Lord Rosse with his gigantic refractor,[22] when one nebula after another was resolved into a cl.u.s.ter of minute stars. At that time the opinion gained ground that with increase of telescopic power this would prove to be the case with all nebulae.
In 1864 all doubt was dispelled by Huggins[23] in his first examination of the spectrum of a nebula, and the subsequent extension of this observation to other nebulae; thus providing a certain test which increase in the size of telescopes could never have given. In 1864 Huggins found that all true nebulae give a spectrum of bright lines. Three are due to hydrogen; two (discovered by Copeland) are helium lines; others are unknown. Fifty-five lines have been photographed in the spectrum of the Orion nebula. It seems to be pretty certain that all true nebulae are gaseous, and show almost exactly the same spectrum.
Other nebulae, and especially the white ones like that in Andromeda, which have not yet been resolved into stars, show a continuous spectrum; others are greenish and give no lines.
A great deal has to be done by the chemist before the astronomer can be on sure ground in drawing conclusions from certain portions of his spectroscopic evidence.
The light of the nebulas is remarkably actinic, so that photography has a specially fine field in revealing details imperceptible in the telescope. In 1885 the brothers Henry photographed, round the star Maia in the Pleiades, a spiral nebula 3" long, as bright on the plate as that star itself, but quite invisible in the telescope; and an exposure of four hours revealed other new nebula in the same district. That painstaking and most careful observer, Barnard, with 10 hours" exposure, extended this nebulosity for several degrees, and discovered to the north of the Pleiades a huge diffuse nebulosity, in a region almost dest.i.tute of stars. By establishing a 10-inch instrument at an alt.i.tude of 6,000 feet, Barnard has revealed the wide distribution of nebular matter in the constellation Scorpio over a s.p.a.ce of 4 or 5 square. Barnard a.s.serts that the "nebular hypothesis" would have been killed at its birth by a knowledge of these photographs. Later he has used still more powerful instruments, and extended his discoveries.
The a.s.sociation of stars with planetary nebulae, and the distribution of nebulae in the heavens, especially in relation to the Milky Way, are striking facts, which will certainly bear fruit when the time arrives for discarding vague speculations, and learning to read the true physical structure and history of the starry universe.
_Stellar Spectra._--When the spectroscope was first available for stellar research, the leaders in this branch of astronomy were Huggins and Father Secchi,[24] of Rome. The former began by devoting years of work princ.i.p.ally to the most accurate study of a few stars. The latter devoted the years from 1863 to 1867 to a general survey of the whole heavens, including 4,000 stars. He divided these into four princ.i.p.al cla.s.ses, which have been of the greatest service. Half of his stars belonged to the first cla.s.s, including Sirius, Vega, Regulus, Altair. The characteristic feature of their spectra is the strength and breadth of the hydrogen lines and the extreme faintness of the metallic lines. This cla.s.s of star is white to the eye, and rich in ultra violet light.
The second cla.s.s includes about three-eighths of his stars, including Capella, Pollux, and Arcturus. These stars give a spectrum like that of our sun, and appear yellowish to the eye.
The third cla.s.s includes Herculis, Orionis (Betelgeux), Mira Ceti, and about 500 red and variable stars. The spectrum has fluted bands shaded from blue to red, and sharply defined at the more refrangible edge.
The fourth cla.s.s is a small one, containing no stars over fifth magnitude, of which 152 Schjellerup, in Canes Venatici, is a good example. This spectrum also has bands, but these are shaded on the violet side and sharp on the red side. They are due to carbon in some form. These stars are ruby red in the telescope.
It would appear, then, that all stars are suns with continuous spectra, and the cla.s.ses are differentiated by the character of the absorbent vapours of their atmospheres.
It is very likely that, after the chemists have taught us how to interpret all the varieties of spectrum, it will be possible to ascribe the different spectrum-cla.s.ses to different stages in the life-history of every star. Already there are plenty of people ready to lay down arbitrary a.s.sumptions about the lessons to be drawn from stellar spectra. Some say that they know with certainty that each star begins by being a nebula, and is condensed and heated by condensation until it begins to shine as a star; that it attains a climax of temperature, then cools down, and eventually becomes extinct. They go so far as to declare that they know what cla.s.s of spectrum belongs to each stage of a star"s life, and how to distinguish between one that is increasing and another that is decreasing in temperature.
The more cautious astronomers believe that chemistry is not sufficiently advanced to justify all of these deductions; that, until chemists have settled the lately raised question of the trans.m.u.tation of elements, no theory can be sure. It is also held that until they have explained, without room for doubt, the reasons for the presence of some lines, and the absence of others, of any element in a stellar spectrum; why the arc-spectrum of each element differs from its spark spectrum; what are all the various changes produced in the spectrum of a gas by all possible concomitant variations of pressure and temperature; also the meanings of all the flutings in the spectra of metalloids and compounds; and other equally pertinent matters--until that time arrives the part to be played by the astronomer is one of observation. By all means, they say, make use of "working hypotheses"
to add an interest to years of laborious research, and to serve as a guide to the direction of further labours; but be sure not to fall into the error of calling any mere hypothesis a theory.
_Nebular Hypothesis._--The Nebular Hypothesis, which was first, as it were, tentatively put forward by Laplace as a note in his _Systeme du Monde_, supposes the solar system to have been a flat, disk-shaped nebula at a high temperature in rapid rotation. In cooling it condensed, leaving revolving rings at different distances from the centre. These themselves were supposed to condense into the nucleus for a rotating planet, which might, in contracting, again throw off rings to form satellites. The speculation can be put in a really attractive form, but is in direct opposition to many of the actual facts; and so long as it is not favoured by those who wish to maintain the position of astronomy as the most exact of the sciences--exact in its facts, exact in its logic--this speculation must be recorded by the historian, only as he records the guesses of the ancient Greeks--as an interesting phase in the history of human thought.
Other hypotheses, having the same end in view, are the meteoritic hypothesis of Lockyer and the planetesimal hypothesis that has been largely developed in the United States. These can best be read in the original papers to various journals, references to which may be found in the footnotes of Miss Clerke"s _History of Astronomy during the Nineteenth Century_. The same can be said of Bredichin"s hypothesis of comets" tails, Arrhenius"s book on the applications of the theory of light repulsion, the speculations on radium, the origin of the sun"s heat and the age of the earth, the electron hypothesis of terrestrial magnetism, and a host of similar speculations, all combining to throw an interesting light on the evolution of a modern train of thought that seems to delight in conjecture, while rebelling against that strict mathematical logic which has crowned astronomy as the queen of the sciences.
FOOTNOTES:
[1] _R. S. Phil Trans_., 1810 and 1817-24.
[2] One of the most valuable contributions to our knowledge of stellar parallaxes is the result of Gill"s work (_Cape Results_, vol. iii., part ii., 1900.)