William Herschel and his work/Chapter 12
DOUBLE STARS AND NEBULÆ
With the intuition of genius, Herschel, at an early period in his career, leaped to the conclusion that, as a planet revolves round the sun, so, in the regions of space, stars may revolve round stars, or sun round sun. It was a magnificent idea, apparently beyond proof, and would be reckoned among the useless things of science. "We have already shown," he wrote in 1803, "the possibility that two stars, whatever be their relative magnitudes, may revolve, either in circles or ellipses, round their common centre of gravity; and that, among the multitude of the stars of the heavens, there should be many sufficiently near each other to occasion this mutual revolution, must also appear highly probable." A sun of enormous size and brightness revolving round another sun as big or as bright, but it may be of a different colour, might be and really was regarded as the dream of a poet, imagining things that mathematics, with inexorable logic, gave no countenance to. But imagination sometimes realises truth long before the facts of science make it known. It was so here. "I shall therefore now proceed to give an account of a series of observations on double stars, comprehending a period of about twenty-five years, which, if I am not mistaken, will go to prove that many of them are not merely double in appearance, but must be allowed to be real binary combinations of two stars, intimately held together by the bond of mutual attraction."
Herschel's first catalogue of double stars was presented to the Royal Society in a memoir of fifty pages on January 10, 1782. It was a work of enormous labour to be undertaken and carried out by a hard-working musician during the nights, that followed days of absorbing business. Of the number 269, contained in this catalogue, 227 had not been noted by any astronomer before him. It was not only a new field of research he may be considered to have opened up. He had also two distinct ends in view, which may be said to have been equally novel. One of them was, by means of these double or triple systems, to discover the distances of the stars from our sun, and the other to ascertain whether "small stars revolved round large ones." He failed in the former, he was successful in the latter. The arithmetic of the one was too hard for him; the poetry of the other was reduced to the commonplace of fact, after a waiting period of twenty-five years.
Everyone knows that if a tree and a house be in the same line of sight from a distant spectator, the eye of the spectator may imagine the tree to be at the same distance as the house, but cannot measure the space between them. We cannot see distance; it is an acquirement gained by experience from the sense of touch, and gained so insensibly that we think we see distance in front of us, height or depth, it may be, while, in fact, we only see length and breadth. An observer, seeing two stars so close that, to the naked eye, they seem only one, may consider them both at the same distance. A little reflection, however, soon convinces him that the one star, though shining at a vast distance from the other, may be so placed in a line drawn from our eye to the latter as to be nearly or altogether eclipsed by it. Sometimes these stars are so close that the two pass for one, till an improvement in the telescope separates the companions, and shows them to be distinct. Herschel had this experience, and one of the most singular instances of it is not yet thirty years old. The dog-star Sirius is among the best known stars in our southern skies. Its brightness is forty- to sixty-fold that of the sun, its distance is such that a flash of light from it takes perhaps ten years to reach our eyes, and its weight exceeds that of two of our suns. This vast and brilliant sun was found to indulge in vagaries which were, and some of which still are, the puzzle of astronomers. They could not see, and therefore did not know. But although they could not see, they could imagine what the unseen cause of these vagaries was: for "the eyes of the mind can supply the want of the most powerful telescopes, and lead to astronomical discoveries of the highest importance." Another star in the neighbourhood of Sirius, the mathematicians said, is moving round him. They calculated its orbit, they told observers where to apprehend the disturber, but in vain. At last the eighteen-inch object-glass, made for the Chicago Observatory in the United States, was turned on Sirius by way of trial. Great was the surprise of the manufacturers when they saw that the mighty sun had a fainter but a very bulky companion in his company, and was seen in the direction predicted by mathematicians. It is twice as heavy as our sun, but does not give a fiftieth part of the light. Stars then may be double, or treble, or even quadruple by nature, or by the accident of position.
Comparing his own observations and such others as he could procure, Herschel calculated that the one star moved round the other, or that both moved round their common centre of gravity in the following double stars:—
Castor in about 342 years 2 months.
γ Leonis in about 1200 years,
ε Bootis in not less than 1681 years,
δ Serpentis in about 375 years.
γ Virginis in about 708 years.
Another double star that he carefully examined was Zeta Herculis. It presented him with a sight "which is new in astronomy; it is, the occultation of one star by another." For twenty-one years he continued to keep a watch on the star. After twenty years had passed he could no longer perceive the smaller of the two companions. The following year he found "the apparent disk a little distorted; but there could not be more than about 3 of the apparent diameter of the small star wanting to a complete occultation." But the observations made were not sufficient to determine the nature of the motion that produced these effects. The long period of 1681 years set down against ε Bootis, Herschel himself points out as subject to uncertainties, which it will take long to clear up. A slight mistake in exceedingly small measurements may cause serious errors in the calculated times of revolution.
It should not be forgotten that the King's equerry whom Miss Burney, in her gossip from Windsor Castle, calls Colonel Welbred, foretold that time would do justice to Herschel, and turn the laugh at him against the laughers. And time has done him justice with a most ungrudging hand. Eight years after his death, it was asked by a leader of modern enlightenment, "What length of time must the cosmologist suppose necessary to reduce a gaseous nebula into a permanent planetary system? Experience shows pretty clearly the inutility of such speculations." . . . Of the moon's "origin and internal structure we neither know, nor ever can know, anything whatever. And if such is the result of our researches respecting a body placed almost in our immediate vicinity, there is little reason to hope that we shall be more successful with regard to those whose distances are so great that the most powerful telescopes are required to render them even visible." This was written in 1830; it was ill-natured disparagement of a noble attempt to solve the mysteries of the universe, and to give practical proof of man's kinship with God; it was wholly unscientific. In 1842 another greatly-extolled writer declared that in that region of inquiry there did not exist any discovered, or even, without doubt discoverable phenomenon. The equerries of Windsor might be laughed at and forgiven; the scepticism that prompted men of science to bid their brethren fold their hands and do nothing, was an unpardonable sin against truth. It was of the same nature as the scientific proof that steamboats could not cross the Atlantic, and was belied, as the other was, by facts.
To Herschel then belongs the credit not merely of having suspected the revolution of sun round sun in the far distant realms of space, but also of actually detecting the fact that this was going on among the stars. He has the credit also of having, with imperfect appliances, measured the angles which enabled him to calculate the times of revolution of these systems of suns. It was a beginning, a wonderful beginning of a new departure in man's warfare with ignorance, and with the bonds that tie him down to the earth. He did not know, probably he was so wrapt up in his own conceptions of the usefulness of the telescope, that he could not imagine a more potent revealer of the secrets of the universe than a gigantic mirror at the bottom of a gigantic tube, or an immense eye at the object end of a telescope. A glass prism has done what the telescope could not do, revealed double stars where they were not known to exist, shown their rates of motion to or from us, and where an unseen ball is a companion to a living and a lighted sun, told us what they are made of, and enabled us to weigh them as if they were in the scales of a balance. To be able to do this, or apprehend the way it has been done, or even to know the fact, lifts human nature to a loftier height than it ever attained in the past, and the pioneer in this elevation of mankind was originally a bandsman in the Hanoverian Guards, a musician of Bath. Nor should it be forgotten that the improvement of the telescope, with which these revelations of the secret things of the starry heavens are closely connected, was largely his work. He laboured indefatigably himself; he invited, he also aroused into honourable emulation, the rivalry of others to equal or surpass his achievements.
What Herschel could only suspect or assert, the glass prism has proved. These mighty suns, "in number numberless," are made of the same materials as our earth and our sun—iron, magnesium, hydrogen, sodium, etc. The vast universe is governed by the same laws, and made of the same matter. It is, so to speak, the work of one and the same building hand. To have risen to this simple truth by exploring the suns and systems of the universe is a reward worth all the time and trouble spent in working it out. Mankind, in this respect alone, stands on a loftier platform now than half a century ago. Oneness of plan, manifested in this widespread oneness of working, implies oneness of the worker. A lofty moral truth has resulted from the labours and speculations of which leaders of scientific truth in Europe formerly saw only the inutility. The Maker, Governor, and Upholder of all these worlds and universes is one and the same. Who He is, what is His central seat of power no telescope, no glass prism can reveal. Amid the wonders of infinite space and time, our standards of measurement and knowledge may be said to be our five senses, and if one of these, sight, were taken from us, our sphere of knowledge would be immeasurably reduced in extent. On the other hand, an addition to the senses we have, a quickening of the inner light, might reveal this Builder of worlds. His palace. His living armies, with a distinctness, a fulness hitherto unknown. Herschel evidently thought this when he stood in wondering awe before the hole in the heavens.
That Herschel fell into mistakes regarding double stars cannot and need not be denied. It was unavoidable that the first traveller in an unexplored region, billions of miles distant from our earth, should err in tracing paths, measuring time, and estimating distances. He failed in his calculations with γ Virginis, which he represented as two companions that revolved round a common centre in 708 years. His son by a careful discussion of the observations made since 1718 showed that the time of revolution was not 708 years but 513. It was also predicted that the smaller of the two companions would reach the point where it is nearest the larger in the beginning of 1834. Even these revised calculations proved to be incorrect, for it did not reach that point till two years later. Observations of the star were then renewed for several years; new calculations were made, and the time of revolution of the lesser companion round the greater was found to be 182 years. But it came out that the orbit of 1834, with the time 513 years, was nearly the same, in part of its course, as the true orbit, and was "a curious example, and by no means the first in the history of the progress of discovery, where of two possible courses, each at the moment equally plausible, the wrong has been chosen."
But Herschel's study of the fixed stars and of the unity of plan in nature went farther than we have yet traced. A paper read by him in 1814 contains the following facts, that might almost have been prophecies of wonders in store for men—"Stars although surrounded by a luminous atmosphere, may be looked upon as so many opaque, habitable, planetary globes; differing, from what we know of our own planets, only in their size, and by their intrinsically luminous appearance. They also, like the planets, shine with differently coloured light. That of Arcturus and Aldebaran, for instance, is as different from the light of Sirius and Capella, as that of Mars and Saturn is from the light of Venus and Jupiter. A still greater variety of coloured star-light has already been shewn to exist in many double stars, such as γ Andromedse, β Cygni, and many more. In my sweeps are also recorded the places of 9 deep garnet, 5 bright garnet, and 10 red coloured stars, of Various small magnitudes from the 7th to the 12th.
"By some experiments on the light of a few of the stars of the 1st magnitude, made in 1798, by a prism applied to the eye-glasses of my reflectors, adjustable to any angle, and to any direction, I had the following analyses:
"The light of Sirius consists of red, orange, yellow, green, blue, purple, and violet.
"α Orionis contains the same colours, but the red is more intense, and the orange and yellow are less copious in proportion than they are in Sirius.
"Procyon contains all the colours, but proportionally more blue and purple than Sirius.
"Arcturus contains more red and orange and less yellow in proportion than Sirius.
"Aldebaran contains much orange, and very little yellow.
"α Lyrae contains much yellow, green, blue, and purple."
The foundation of what may be called a new science was thus laid by Herschel more than half a century before anything was built on it.
In that paper also he embodied curious speculations on the growth of stars: "If the nebulosity should subside into the star, as seems to be indicated by the assumed form of the fan-shaped nebulæ, the star would receive an increase of matter proportional to the magnitude and density of the nebulosity in contact with it."
Another of the subjects specially studied by Herschel from an early period in his career was the white clouds or nebulæ seen, even with the naked eye, in various places among the stars. The telescopes of astronomers had not done much to add to their number or reveal their peculiar forms till he took the matter in hand. In 1786 he laid before the Royal Society a "catalogue of a thousand nebulæ and clusters of stars." Three years after, he presented the Society with a "catalogue of a second thousand new nebulæ and clusters of stars"; and in 1802 he added "a catalogue of 500 new nebulæ and clusters of stars." A field of discovery so rich he had been left to reap alone, except in the assistance, the invaluable assistance, which he received from his devoted sister Caroline. He looked upon star-clusters and nebulæ as building stones used by the Creator in constructing the universe; to catalogue, to watch, and to measure these building stones was a long step taken in ascertaining the plan on which the Almighty Architect proceeded. Herschel was laughed at, most unfairly laughed at, as a "lively and amusing" dreamer; science has proved that he was a noble pioneer of modern discoveries, which inspire mankind with awe. The work of observing, measuring, and recording these worlds of wonder, and sometimes of surpassing beauty even when seen in the magic mirror of a reflector, was enormous: but this indefatigable worker, with his like-minded sister-helper, seemed never to weary in his marvellous efforts to lift the curtain that hid Creation's glories from man. What these glories seemed (to him) to mean was unfolded in 1811 in a memoir, which anticipated by many years the doctrine of evolution taught by Darwin, and which showed the progress, slow it might be, "for, in this case, millions of years are perhaps but moments," but sure, of a vast body of gas condensing into a sun or suns with a train of planets around.
When Herschel entered upon this inquiry he believed that these nebulæ, or whitish clouds or milky ways are clusters of stars, too far off to be resolved into separate points of light, but blended so together as to assume the appearance of a little cloud in the depths of space. "Longer experience and a better acquaintance" with them induced him to change his mind. Vast masses of gas, in which a few stars were sometimes seen, or through which they shone from a greater distance, were believed by him to exist in space, besides those which an increase of telescopic power could resolve, as the phrase was, into stars. It was the idea of a far-seeing mind, feeling its way to truth, and, in our own day, it has been proved true. The prism has shown that these inconceivably vast masses of gas exist. Justice to Herschel requires that his rights to the first announcement of this new and startling view of the gradual formation of worlds should not be overlooked, as is sometimes done. "The profound awe," says the discoverer of the gaseous nature of some nebulæ, "which I felt on looking for the first time at that which no eye of man had seen, and which even the scientific imagination could not foreshow," is the well expressed wonder of true science, when it penetrates into the workshops of the Almighty, but Herschel's imagination had done more in 1811 than "foreshow" the discovery made fully by Sir William Huggins in 1864. The imagination of William Herschel penetrated into this secret house of wonders, and gave expression to what was believed to be going on in eternal ages and through infinite space.
There are two magnificent nebulæ to which astronomers have specially turned their telescopes, the one in Orion and the other in Andromeda. Writing in 1811, after thirty-seven years' study of these wonderfully mysterious clouds, Herschel thus speaks of "the great nebula in the constellation of Orion discovered by Huyghens. This highly interesting object engaged my attention already in the beginning of the year 1774, when viewing it with a Newtonian reflector I made a drawing of it, to which I shall have occasion hereafter to refer: and having from time to time reviewed it with my large instruments, it may easily be supposed that it was the very first object to which, in February 1787, I directed my 40-feet telescope. The superior light of this instrument shewed it of such a magnitude and brilliancy that, judging from these circumstances, we can hardly have a doubt of its being the nearest of all the nebulæ in the heavens, and as such will afford us many valuable informations. I shall however now only notice that I have placed it in the present order because it connects in one object the brightest and faintest of all nebulosities, and thereby enables us to draw several conclusions from its various appearance." By nebulosity or nebulous matter he meant "that substance or rather those substances which give out light, whatsoever may be their nature, or of whatever different powers they may be possessed." From a laborious examination of these vast regions of visible nebulous matter, Herschel found reason to conclude that the power of gravitation was condensing the matter towards one or more centres, which shone with greater brilliance than the rest of the mass. A motion of rotation round an axis would also probably result from innumerable particles pressing towards a centre, and the matter which did not condense into a nucleus—perhaps a star or sun— would "remain expanded about the nucleus in the shape of a very extended atmosphere; or it may be of an elastic nature, and be kept from uniting with the nucleus, as their elasticity causes the atmospheres of the planets to be expanded about them. In this case we have another property of the nebulous substance to add to the former qualities of its matter."
No one can read even an outline of these interesting speculations by an adventurer into the workshops of creation, without feeling awed by the boldness and sublimity of his views, as well as desirous of knowing what else he saw in his magic mirror, or thought he saw, of the machinery in motion. What he has told us of a mighty volume of nebulous matter is that "a nucleus, to which these nebulæ seem to approach, is an indication of consolidation," and that the faintness of the light in the parts outside the nucleus arises from "a gradual diminution of the length and density of the nebulous matter, occasioned by its gravitation towards the nucleus into which it probably subsides." He believes that "a pretty bright round nebula about a quarter or one minute in diameter, and looking no bigger than a pea, may have shrunk into itself till it is now nineteen hundred times more dense than at first,—a proportion of density more than double that of water to air." In another case he calculates that "the condensation may have reduced the nebulous matter to less than the one hundred and twenty-two thousandth part of its former bulk." To understand what these figures mean, suppose a sphere whose radius is nearly three thousand millions of miles, or as far as from the sun to our outermost known planet, Neptune, to be filled with gas, luminous or not. It would not occupy more than a fortieth part of the space in the heavens occupied by the great nebula in Orion, and it is doubtful if our best telescopes reveal the whole of that nebula's extent in any direction. It is within such vast spaces that Herschel imagined this world-making process to be going on. Man's imagination quails in his attempt to grasp the space required for such a workshop, the tools employed, or the time taken to condense "nebulous matter" into dazzling suns or dark companions.
We are so much accustomed to feast our eyes on drawings of a few magnificent and singularly shaped nebulæ, that thought is apt to overlook the vast numbers of them scattered over the heavens in all stages of size or progress. Herschel did not fall into this mistake. His object was higher than to satisfy curiosity or to excite wonder. He had the feeling that there was a process going on, of which he believed he could trace not a few of the stages. The smallest and the least wonderful of the nebulæ might thus prove to be as important in tracing out this progress, as the most awe-inspiring. Nor did he look upon all of them as resolvable into stars or masses of shining matter, more or less rare. He believed that some of them were not luminous, but dark; but he made no attempt to explain, as may be at least attempted to-day, how a vast mass of invisible gas may become lighted up, and send its brightness off on a journey of ten or twenty or fifty years, to publish to us the changes that, in process of ages, had taken place in its nature. It was the discovery of world-making he was aiming at in these long and laborious, but not wearisome researches. Others have followed in his footsteps with a better equipment of instruments, if not with a richer endowment of insight or genius. Others still have looked upon his lifelong quest as an attempt to reach the foot of the rainbow ladder, or to master the secret of the philosopher's stone. His papers remain a wonderful monument of ingenious research and marvellous discovery, of lofty imaginings and reasoned conclusions.
These nebulæ and clusters of stars Herschel called milky ways, different from the great Milky Way, in which our solar system is imbedded. He held at first that they are in no respect connected with our milky way, but are star-islands or world-systems, perhaps only in process of formation, at immense distances from our sun, outlying provinces of creation, as it were, in the vast ocean of ether, or constructions only begun in the realms of space. He is supposed to have fallen from this opinion in his later years, and to have imagined that all these milky ways and star-clusters were connected with ours. His latest papers give no indication of this change of view. He appears indeed only to have changed his view in so far as to have regarded our milky way as the greatest of all the milky ways, visible in our telescopes: but on this point he was scarcely justified in speaking, as the distance of the nearest nebula not only was and continues to be unknown, but the means of determining the distances of these white clouds have not yet been discovered. It is thought that the great nebula in Orion, if not the nearest to us, is among the nearest. Herschel maintained this. He had some grounds also for believing that changes had taken place in the positions of the nebulous matter during the thirty-seven years he had been watching, and still greater changes since Huyghens, a century and a half earlier, gave a picture of it in his Systema Saturnium. "The various appearances of this nebula," Herschel writes, "are so instructive that I shall apply them to the subject of the partial opacity of the nebulous matter. . . . For when I formerly saw three fictitious nebulous stars, it will not be contended that there were three small shining nebulosities, just in the three lines, in which I saw them, of which two are now gone, and only one remaining. As well might we ascribe the light surrounding a star, which is seen through a mist, to a quality of shining belonging to that particular part of the mist, which by chance happened to be situated where the star is seen. If then the former nebulosity of the two stars which have ceased to be nebulous can only be ascribed to an effect of the transit or penetration through nebulous matter which deflected and scattered it, we have now a direct proof that this matter can exist in a state of opacity, and may possibly be diffused in many parts of the heavens without our being able to perceive it."
It would be unjust to Herschel to pass over the condemnation of his views, pronounced by Sir David Brewster in his Life of Sir Isaac Newton, Without mentioning the name of William Herschel, or of La Place, who advocated the same views. Sir David writes as one who felt sure that Newton, for mathematical reasons alone, would have taken a side against this Nebular Hypothesis. In the last of the famous four letters written by Sir Isaac to Dr. Bentley, the great classical scholar and the author of Phalaris, he enters into a mathematical criticism of the opinion of Plato "that the motion of the planets is such as if they had all been created by God in some region very remote from our system, and let fall from thence towards the sun, their falling motion being turned aside into a transverse one whenever they arrived at their several orbits." This, of course, is wholly unlike Herschel's theory, or that of Laplace. But of these letters Sir David says: "In the present day they possess a peculiar interest. They show that the Nebular Hypothesis, the dull and dangerous heresy of the age, is incompatible with the established laws of the material universe, and that an omnipotent arm was required to give the planets their position and motions in space, and a presiding intelligence to assign to them the different functions they had to perform."
These views of Sir David Brewster, eminent man of science though he was and sincere believer in an almighty arm ruling all the motions of material bodies, do not seem justified by facts. Even his great name is not weighty enough to counterbalance that of Laplace, when the former affirms and the latter denies that the Nebular Hypothesis "is incompatible with the established laws of the material universe." Newton's speculations on Plato's dream of the origin of planets had nothing to do with the hypothesis in question. It may be "a dull and dangerous heresy," as Sir David believed, "but it denies neither an almighty arm nor a presiding mind." Recent discoveries have given more probability to the theory—if we are entitled to use that name: and Herschel's inductions from observed and classified facts have gone far to prove that Laplace's imaginings rest on a more solid foundation than theories, at their birth, can usually boast of.
In pursuit of his favourite study—the plan of the Creator in constructing the Temple of the Heavens—Herschel, with fuller knowledge, and after many years of labour, departed from Cassini's simple classification of nebulæ, and adopted another in closer agreement with facts It was as follows:—
As he entered these nebulæ on a star map, it was evident to the eye that the parts of the heavens at a distance from the Milky Way are most abundant in white clouds. Of a connection between them and the Milky Way he does not appear to have been certain. We must leave it as he left it—in uncertainty and doubt. Future ages may determine whether the whole material universe, designed by one mind, governed by the same laws, built of the same materials, and upheld for purposes in which the mighty littleness of man seems to play a not unimportant part, moral as well as intellectual, has been spread out before our eyes. We can only look on in wondering adoration at the glory and vastness of a temple, built by Almighty Power and Wisdom, the forth-puttings of whose hand we can see and trace, but whose palace and presence are hidden in brightness impenetrable to our sight.
Astronomy has made vast strides in knowledge of the stars since Herschel's death. Other magicians, imbued with his spirit, and wielding a more wonderful rod of power than his 40-feet reflector, have arisen to walk in his footsteps, and to tread the paths of discovery, which more or less dimly he saw and walked in—double stars; treble systems; eclipses of suns; youthful stars; dark or dying worlds; star charts; photographic plates, and vast volumes of gas, lighted or dark. More even than in his days have the barren heavens proved to be a land of wonders to curious man.
- Arago, Biographies, etc., p. 224.
- "One thing very remarkable I must tell you, γ Virginis is now a single star in both the twenty-foot, and the seven-foot equatorial!!!" (Sir John Herschel, March 8, 1836). He means that one of the two suns had eclipsed the other.
- Edin. Rev. li. 101.
- Comte, Nineteenth Century (1897), 908.
- Edin. Rev., 1848, 132-33.
- "The reason for not having a more circumstantial account of such a number of objects, is that they crowded upon me at the time of sweeping in such quick succession that of sixty-one I could but just secure the place in the heavens, and of the remaining three hundred and sixty-three, I had only time to add the relative size" (Phil. Trans. for 1811, p. 290).
- Phil. Trans. for 1811, p. 270.
- "Sir William Herschel supposed that they [nebulæ] were all really star-clusters, but so enormously remote that even the most powerful telescopes could not render visible the stars composing them" (Wallace, The Wonderful Century, p. 44). This is a singular statement to come from the gifted author or co-author of the Darwinian theory. The reduction of the immensely vast to the comparatively small was Herschel's view of development or evolution in the realms of space; the growth of organic life from the simple cell to the living forms of earth—the inverse process—is the idea or hypothesis of natural science to-day. See Phil. Trans. 1791, pp. 73-83.
- Phil. Trans. for 1811, pp. 278, 279, 277, 313. "The nature of diffused nebulosity is such that we often see it joined to real nebulæ." He means apparently gas sometimes very rare joined to matter condensed or condensing into stars.
- Phil. Trans., 1811, pp. 308, 310, 311, 315, 316, 318.
- Life of Newton, ii. 130.