THE EXPLORER OF THE HEAVENS.
“A knowledge of the construction of the heavens,” Herschel wrote
in 1811, “has always been the ultimate object of my observations.”
The “Construction of the Heavens”! A phrase of profound and novel
import, for the invention of which he was ridiculed by Brougham in the
_Edinburgh Review_; yet expressing, as it had never been expressed
before, the essential idea of sidereal astronomy. Speculation there had
been as to the manner in which the stars were grouped together; but the
touchstone of reality had yet to be applied to them. This unattempted,
and all but impossible enterprise Herschel deliberately undertook. It
presented itself spontaneously to his mind as worth the expenditure
of a life’s labour; and he spared nothing in the disbursement. The
hope of its accomplishment inspired his early exertions, carried him
through innumerable difficulties, lent him audacity, fortified him in
perseverance. For this,
“He left behind the painted buoy
That tosses at the harbour’s mouth,”
and burst his way into an unnavigated ocean.
Herschel has had very few equals in his strength of controlled
imagination. He held the balance, even to a nicety, between the
real and the ideal. Meditation served in him to prescribe and guide
experience; experience to ripen the fruit of meditation.
“We ought,” he wrote in 1785, “to avoid two opposite extremes. If we
indulge a fanciful imagination, and build worlds of our own, we must
not wonder at our going wide from the path of truth and nature. On the
other hand, if we add observation to observation without attempting to
draw, not only certain conclusions, but also conjectural views from
them, we offend against the very end for which only observations ought
to be made.”
This was consistently his method. If thought outran sight, he laboured
earnestly that it should be overtaken by it: while sight, in turn,
often took the initiative, and suggested thought. He was much more than
a simple explorer. “Even at the telescope,” Professor Holden says, “his
object was not discovery merely, but to know the inner constitution of
the heavens.” He divined, at the same time that he observed.
The antique conception of the heavens as a hollow sphere upon which
the celestial bodies are seen projected, survived then, and survives
now, as a convenient fiction for practical purposes. But in the
eighteenth century the fiction assumed to the great majority a sort of
quasi-reality. Herschel made an exception in being vividly impressed
with the _depth_ of space. How to sound that depth was the first
problem that he attacked. As a preliminary to further operations, he
sought to fix a unit of sidereal measurement. The distances from the
earth to the stars were then altogether unknown. All that had been
ascertained was that they must be very great. Instrumental refinements
had not, in fact, been carried far enough to make the inquiry
profitable. Herschel did not underrate its difficulty. He recognised
that, in pursuing it, _one hundredth of a second of arc_ “became a
quantity to be considered.” Justly arguing, however, that previous
experiments on stellar parallax had been unsatisfactory and indecisive,
he determined to try again.
He chose the “double star method,” invented by Galileo, but never, so
far, effectually put to trial. The principle of it is perfectly simple,
depending upon the perspective shifting to a spectator in motion, of
objects at different distances from him. In order to apprehend it,
one need only walk up and down before a lamp placed in the middle of
a room, watching its apparent change of position relative to another
lamp at the end of the same room. Just in the same way, a star observed
from opposite sides of the earth’s orbit is sometimes found to alter
its situation very slightly by comparison with another star close to it
in the sky, but indefinitely remote from it in space. Half the small
oscillation thus executed is called that star’s “annual parallax.” It
represents the minute angle under which the radius of the terrestrial
orbit would appear at the star’s actual distance. So vast, however, is
the scale of the universe, that this tell-tale swing to and fro is,
for the most part, imperceptible even with modern appliances, and was
entirely inaccessible to Herschel’s observations. Yet they did not
remain barren of results.
“As soon as I was fully convinced,” he wrote in 1781, “that in the
investigation of parallax the method of double stars would have many
advantages above any other, it became necessary to look out for proper
stars. This introduced a new series of observations. I resolved to
examine every star in the heavens with the utmost attention that I
might fix my observations upon those that would best answer my end.
The subject promises so rich a harvest that I cannot help inviting
every lover of astronomy to join with me in observations that must
inevitably lead to new discoveries. I took some pains to find out
what double stars had been recorded by astronomers; but my situation
permitted me not to consult extensive libraries, nor, indeed, was it
very material; for as I intended to view the heavens myself, Nature,
that great volume, appeared to me to contain the best catalogue.”
On January 10th, 1782, he presented to the Royal Society a catalogue
of 269 double stars, of which 227 were of his own finding; and a
second list of 434 followed in December, 1784. All were arranged in
six classes, according to the distance apart of their components,
ranging from one up to 120 seconds. The close couples he regarded as
especially adapted for parallax-determinations; the wider ones might
serve for criteria of stellar proper movements, or even of the sun’s
transport through space. For the purpose of measuring the directions
in which their members lay towards each other--technically called
“position-angles”--and the intervals separating them, he invented
two kinds of micrometers, and notes were added as to their relative
brightness and colours. He was the first to observe the lovely
contrasted or harmonised tints displayed by some of these objects.
Herschel’s double stars actually fulfilled none of the functions
assigned to them. He was thus left without any definite unit of
measurement for sidereal space; and he never succeeded in supplying the
want. In 1814 he was “still engaged,” though vainly, “in ascertaining
a scale whereby the extent of the universe, so far as it is possible
for us to penetrate into space, may be fathomed.” He knew only that
the distances of the stars nearest the earth could not be less, and
might be a great deal more, than light-waves, propagated at the rate
of 186,300 miles a second, would traverse in three or four years. Only
the _manner_ of stellar arrangement, then, remained open to his zealous
investigations.
The initial question presenting itself to an intelligent spectator of
the nocturnal sky is: What relation does the dim galactic star-stream
bear to the constellations amidst which it flows? And this question
our interior position makes very difficult to be answered. We see
the starry universe, it has been well said, “not in _plan_ but in
_section_.” The problem is, from that section to determine the plan--to
view the whole mentally as it would show visually from the outside. The
general appearance to ourselves of the Milky Way leaves it uncertain
whether it represents the projection upon the heavens of an immense
stratum of equally scattered stars, or a ring-like accumulation,
towards the middle of which our sun is situated. Herschel gave his
preference, to begin with, to the former hypothesis, and then, with
astonishing boldness and ingenuity, attempted to put it experimentally
to the proof.
His method of “star-gauging” was described in 1784. It consisted in
counting the stars visible in successive fields of his twenty-foot
telescope, and computing the corresponding depths of space. Admitting
an average regularity of distribution, this was easily done. If the
stars did not really lie closer together in one region than in another,
then the more of them there were to be seen along a given line of
vision, the further the system could be inferred to extend in that
particular direction. The ratio of its extension would also be given.
It would vary with the cube-roots of the number of stars in each count.
Guided by this principle, Herschel ventured to lay down the boundaries
of the stellar aggregation to which our sun belongs. So far as he
“had yet gone round it,” in 1785, he perceived it to be “everywhere
terminated, and in most places very narrowly too.” The differences,
however, between his enumerations in various portions of the sky
were enormous. In the Milky Way zone the stars presented themselves
in shoals. He met fields--of just one quarter the area of the
moon--containing nearly 600; so that, in fifteen minutes, 116,000
were estimated to have marched past his stationary telescope. Here,
the calculated “length of his sounding-line” was nearly 500 times the
distance of Sirius, his standard star. Towards the galactic poles, on
the contrary, stars were comparatively scarce; and the transparent
blackness of the sky showed that in those quarters the supply of
stars was completely exhausted. At right angles to the Milky Way,
then, the stellar system might be termed shallow, while in its plane,
it stretched out on all sides to an inconceivable, though not to an
illimitable extent. Its shape appeared, accordingly, to be that of a
flat disc, of very irregular contour, and with a deep cleft matching
the bifid section of the Milky Way between Cygnus and Scorpio.
Herschel regarded this conclusion only “as an example to illustrate
the method.” Yet it was derived from the reckoning-up of 90,000 stars
in 2,400 telescopic fields! Its validity rested on the assumption
that stellar crowding indicated, not more stars in a given space, but
more space stocked in the same proportion with stars. But his hope
of thus getting a true mean result collapsed under the weight of his
own observations. “It would not be difficult,” he stated in 1785, “to
point out two or three hundred gathering clusters in our system.” The
action of a “clustering power” drawing its component stars “into many
separate allotments” grew continually clearer to him, and he admitted
unreservedly in 1802 that the Milky Way “consists of stars very
differently scattered from those immediately about us.”
In 1811, he expressly abandoned his original hypothesis. “I must freely
confess,” he wrote, “that by continuing my sweeps of the heavens
my opinion of the arrangement of the stars has undergone a gradual
change. An equal scattering of the stars may be admitted in certain
calculations; but when we examine the Milky Way, or closely compressed
clusters, it must be given up.”
And in 1817: “Gauges, which on a supposition of an equality of
scattering, were looked upon as gauges of distance, relate, in fact,
more immediately to the scattering of the stars, of which they give
valuable information.”
The “disc-theory” was then virtually withdrawn not many years after it
had been propounded. “The subtlety of nature,” according to Bacon’s
aphorism, “transcends the subtlety both of the intellect and of the
senses.” Herschel very soon perceived the inadequacy of his colossal
experiment; and he tranquilly acquiesced, not being among those who
seek to entrench theory against evidence. He found that he had
undervalued the complexity of the problem. Yet it remained before his
mind to the end. The supreme object of his scientific life was to
ascertain the laws of stellar distribution in cubical space, and he
devoted to the subject the two concluding memoirs of the sixty-nine
contributed by him to the “Philosophical Transactions.” He was in his
eightieth year when he opened, with youthful freshness, a new phase of
arduous investigation.
“The construction of the heavens,” he wrote in June, 1817, “can only
be known when we have the situation of each body defined by its three
dimensions. Of these three, the ordinary catalogues give but two,
leaving the distance or profundity undetermined.” This element of
“profundity” he went on to determine by the absolutely novel method of
what may be called “photometric enumeration.”
He began by asserting what is self-evident--that faint stars are, “one
with another,” more remote than bright ones; and he argued thence,
reasonably enough, that the relative mean distances of the stars, taken
order by order, might be inferred from their relative mean magnitudes.
Next he pointed out that more space would be available for their
accommodation in proportion to the cubes of their mean distances.
Here lies the value of the method. It sets up, as Herschel said, “a
standard of reference” with regard to stellar distribution. It makes it
possible to compare actual stellar density, at a given mean distance,
with a “certain properly modified equality of scattering.” By patiently
calling over the roll of successive magnitudes, information may be
obtained regarding over- and under-populated districts of space.
Herschel’s reasonings on the subject are perfectly valid, but for
practical purposes far in advance of the time. Their application
demanded a knowledge of stellar light-gradations, which, even now, has
been only partially attained. His surprising anticipation of this mode
of inquiry came, therefore, to nothing.
His device of “limiting apertures” was a simultaneous invention. It
was designed as a measure of relative star-distances. Pointing two
similar telescopes upon two unequal stars, he equalised them to the
eye by stopping down the aperture of the instrument directed towards
the brighter object. Assuming each to emit the same quantity of light,
their respective distances would then be inversely as the diameters of
the reflecting surfaces by which they were brought to the same level of
apparent lustre. But the enormous real diversities of stellar size and
brightness render this plan of action wholly illusory. Even for average
estimates, proper motion is apparently a safer criterion of distance
than magnitude.
Herschel employed the method of apertures with better success to
ascertain the comparative extent of natural and telescopic vision. The
boundary of the former was placed at “the twelfth order of distance.”
Sirius, that is to say, removed to twelve times its actual remoteness,
would be a barely discernible object to the naked eye. The same star
carried seventy-five times further away still, could be seen as a faint
light-speck with his twenty-foot telescope; and, transported 192 times
beyond the visual limit, would make a similar appearance in the field
of the forty-foot. These figures, multiplied by twelve, represented,
in his expressive phrase, the “space-penetrating power” of his
instruments. Their range extended respectively to 900 and 2,300 times
the distance of his “standard star.” He estimated, moreover, that,
through the agency of the larger, light might become sensible to the
eye after a journey lasting nearly seven thousand years! So that, as he
said, his telescopes penetrated both time and space.
His last observation of the Milky Way showed it to be in parts
“fathomless,” even with the forty-foot. No sky-background could be
seen, but only the dim glow of “star-dust.” This effect he attributed
to the immeasurable extension, in those directions, of the stellar
system. The serried orbs composing it, as they lay further and further
from the eye, became at last separately indistinguishable. Herschel, as
has been said, formulated no second theory of galactic structure after
that of 1784–5 had been given up. What he thought on the subject, with
ripened experience for his guide, can only be gathered piecemeal from
his various writings. The general appearance of
That “broad and ample road, whose dust is gold,
And pavement stars,”
he described as “that of a zone surrounding our situation in the
solar system, in the shape of a succession of differently condensed
patches of brightness, intermixed with others of a fainter tinge.”
And he evidently considered this _seeming_ to be in fair accord with
reality. The “patches of brightness” stood for genuine clusters,
incipient, visibly forming, or formed. They are made up of stars not
less lustrous, but much more closely collected than Sirius, Arcturus,
or Capella. The smallness of galactic stars would thus be an effect
of distance, while their crowding is a physical fact. The whole of
these clusters are (on Herschel’s view) aggregated into an irregular,
branching ring, distinct from, although bound together into one system
with the brilliants of the constellations. “Our sun,” he emphatically
affirmed in 1817, “with all the stars we can see with the eye, are
deeply immersed in the Milky Way, and form a component part of it.”
He took leave of the subject which had engrossed so many of his
thoughts in a paper read before the Royal Society, June 11th, 1818.
In it he showed how the “equalising” principle could be applied to
determine the relative distances of “globular and other clusters,”
provided only that their component stars are of the rank of Sirius.
It is improbable, however, that this condition is fulfilled. In open
groups, such as the Pleiades, enormous suns are most likely connected
with minute self-luminous bodies; but the stars compressed into
“globular clusters” appear to be more uniform, and may, perhaps, be
intermediate in magnitude. Yet here again, the only thing certain is
the prevalence of endless variety. Celestial systems are not turned out
by the dozen, like articles from a factory. Each differs from the rest
in scale, in structure, in mechanism. Attempts to reduce all to any
common standard must then prove futile. Disparities of distance are of
course concerned in producing their varieties of aspect, coarse-looking
“balls of stars” being, necessarily, on the whole, less remote than
those of smoother texture. Finer graining, however, may also be due to
a composition out of smaller and closer masses. The two causes concur,
and the share of each in producing a certain effect cannot, in any
individual case, be apportioned.
Herschel was indeed far too philosophical to adopt rigid lines of
argument. His reasoning did not extend “so far as to exclude a
real difference, not only in the size, but also in the number and
arrangement of the stars in different clusters.” Nevertheless,
the discussion founded upon it is no longer convincing. To modern
astronomers it appears to travel quite wide of the mark. Its interest
consists in the proof given by it that the problem of sidereal
distances, the original incentive to Herschel’s reviews of the
heavens, attracted his attention to the very end of his thinking life.
Throughout his long career, the profundities of the universe haunted
him. He sought, _per fas, per nefas_, trustworthy measures of the
“third dimension” of celestial space. The object of his search was out
of reach, and has not even now been fully attained; but the path it led
him by was strewn with discoveries.
The nets spread in his “sweeps” brought in, besides double stars,
plentiful takes of the filmy objects called “nebulæ.” He recognised
with amazement their profusion in certain tracts of the sky; increased
telescopic light-grasp never failed to render a further supply visible;
the heavens teemed with them. He presented a catalogue of 1,000 to the
Royal Society in 1786, a second equally comprehensive in 1789, and a
supplementary list of 500 in 1802. Their natural history fascinated
him. What they were, what they had been, and what they should come to,
formed the subject of many of those ardent meditations which supplied
motive power for his researches. He not only laid the foundation of
nebular science, but carried the edifice to a considerable height,
distinguishing the varieties of its objects, and classifying them
according to their gradations of brightness. Some presented a most
fantastic appearance.
“I have seen,” he wrote in 1784, “double and treble nebulæ variously
arranged; large ones with small, seeming attendants; narrow, but much
extended lucid nebulæ or bright dashes; some of the shape of a fan,
resembling an electric brush, issuing from a lucid point; others of
the cometic shape, with a seeming nucleus in the centre, or like
cloudy stars surrounded with a nebulous atmosphere; a different sort,
again, contained a nebulosity of the milky kind, like that wonderful,
inexplicable phenomenon about Theta Orionis; while others shine with a
fainter mottled kind of light which denotes their being resolvable into
stars.”
He, “through the mystic dome,” discerned
“Regions of lucid matter taking form,
Brushes of fire, hazy gleams,
Clusters and beds of worlds, and bee-like swarms
Of suns and starry streams.”
Annular and planetary nebulæ were _as such_, first described by him.
“Among the curiosities of the heavens,” he announced in 1785, “should
be placed a nebula that has a regular concentric dark spot in the
middle, and is probably a ring of stars.” This was the famous annular
nebula in Lyra, then a unique specimen, now the type of a class.
The planetary kind, so-called from their planet-like discs, were
always more or less of an enigma to him. The vividness and uniformity
of their light appeared to cut them off from true nebulæ; on mature
consideration, he felt driven to suppose them “compressed star-groups.”
“If it were not, perhaps, too hazardous,” he went on, “to pursue
a former surmise of a renewal in what I figuratively called the
laboratories of the universe, the stars forming these extraordinary
nebulæ, by some decay or waste of nature, being no longer fit for their
former purposes, and having their projectile forces, if any such they
had, retarded in each other’s atmospheres, rush at last together, and
either in succession, or by one general tremendous shock, unite into a
new body. Perhaps the extraordinary and sudden blaze of a new star in
Cassiopeia’s Chair, in 1572, might possibly be of such a nature.”
At that early stage of his inquiries, Herschel regarded all nebulæ
indiscriminately as composed of genuine stars. It was almost inevitable
that he should do so. For each gain in telescopic power had the effect
of transferring no insignificant proportion of them from the nebular
to the stellar order. There was no apparent reason for drawing a line
anywhere. The inference seemed irresistible, that resolvability was
simply a question of optical improvement. As Messier’s _nébuleuses sans
étoiles_ had yielded to Herschel’s telescopes, so--it might fairly
be anticipated--the “milky” streaks and patches seen by Herschel
would curdle into stars under the compulsion of the still mightier
instruments of the future. He was led on--to use his own expressions
in 1791--“by almost imperceptible degrees from evident clusters,
such as the Pleiades, to spots without a trace of stellar formation,
the gradations being so well connected as to leave no doubt that all
these phenomena were equally stellar.” They were what Lambert and Kant
had supposed them to be--island-universes, vast congeries of suns,
independently organised, and of galactic rank. They were, each and all,
glorious systems, barely escaping total submergence in the illimitable
ocean of space. Under the influence of these grandiose ideas, Herschel
told Miss Burney, in 1786, that with his “large twenty-foot” he had
“discovered 1,500 universes!” Fifteen hundred “whole sidereal systems,
some of which might well outvie our Milky Way in grandeur.”
His contemplations of the heavens showed him everywhere traces of
progress--of progress rising towards perfection, then sinking into
decay, though with a sure prospect of renovation. He was thus led to
arrange the nebulæ in a presumed order of development. The signs of
interior condensation traceable in nearly all, he attributed to the
persistent action of central forces. Condensation, then, gave evidence
of age. Aggregated stars drew closer and closer together with time. So
that scattered or branching formations were to be regarded as at an
early stage of systemic existence; globular clusters, as representing
universes still in the prime of life; while objects of the planetary
kind were set down as “very aged, and drawing on towards a period of
change, or dissolution.”
Our own nebula he characterised as “a very extensive, branching
congeries of many millions of stars,” bearing upon it “fewer marks of
profound antiquity than the rest.” Yet, in certain regions, he found
“reason to believe that the stars are now drawing towards various
secondary centres, and will in time separate into different clusters.”
As an example of the ravages of time upon the galactic structure,
he adverted to a black opening, four degrees wide, in the Zodiacal
Scorpion, bordered on the west by an exceedingly compact cluster
(Messier’s No. 80), possibly formed, he thought, of stars drawn from
the adjacent vacancy. The chasm was to him one of the most impressive
of celestial phenomena. His sister preserved an indelible recollection
of hearing him, in the course of his observations, after a long, awful
silence, exclaim, “Hier ist wahrhaftig ein Loch im Himmel!” (Here,
truly, is a hole in the sky); and he recurred to its examination
night after night and year after year, without ever clearing up, to
his complete satisfaction, the mystery of its origin. The cluster
significantly located at its edge was lit up in 1860 by the outburst of
a temporary star.
This was not the sole instance noted by Herschel of the conjunction
of a chasm with a cluster; and chasms and clusters alike told the
same story of dilapidation. He foresaw, accordingly, as inevitable,
the eventual “breaking-up” of the Milky Way into many small, but
independent nebulæ. “The state into which the incessant action of the
clustering power has brought it at present,” he wrote in 1814, “is a
kind of chronometer that may be used to measure the time of its past
and future existence; and although we do not know the rate of going of
this mysterious chronometer, it is, nevertheless, certain that since
the breaking up of the Milky Way affords a proof that it cannot last
for ever, it equally bears witness that its past duration cannot be
admitted to be infinite.”
Thus the idea of estimating the relative “ages” of celestial
objects--of arranging them according to their progress in development,
originated with Herschel in 1789. “This method of viewing the heavens,”
he added, “seems to throw them into a new kind of light. They are
now seen to resemble a luxuriant garden which contains the greatest
variety of productions in different flourishing beds; and one
advantage we may at least reap from it is that we can, as it were,
extend the range of our experience to an immense duration. For, is
it not almost the same thing whether we live successively to witness
the germination, blooming, foliage, fecundity, fading, withering, and
corruption of a plant, or whether a vast number of specimens, selected
from every stage through which the plant passes in the course of its
existence, be brought at once to our view?”
But while he followed the line of continuity thus vividly traced,
another crossing, and more or less interfering with it, opened out
before him. The discovery of a star in Taurus, “surrounded with a
faintly luminous atmosphere,” led him, in 1791, to revise his previous
opinions regarding the nature of nebulæ. He was not at all ashamed of
this fresh start. No fear of “committing himself” deterred him from
imparting the thoughts that accompanied his multudinous observations.
He felt committed to nothing but truth. He was advancing into an
untrodden country. At every step he came upon unexpected points of
view. The bugbear of inconsistency could not prevent him from taking
advantage of each in turn to gain a wider prospect.
Until 1791 Herschel never doubted that gradations of distance fully
accounted for gradations of nebular resolvability. He had been led on,
he explained, by almost imperceptible degrees from evident clusters
to spots without a trace of stellar formation, no break anywhere
suggesting the possibility of a radical difference of constitution.
“When I pursued these researches,” he went on, “I was in the situation
of a natural philosopher who follows the various species of animals
and insects from the height of their perfection down to the lowest ebb
of life; when, arriving at the vegetable kingdom, he can scarcely point
out to us the precise boundary where the animal ceases and the plant
begins; and may even go so far as to suspect them not to be essentially
different. But, recollecting himself, he compares, for instance,
one of the human species to a tree, and all doubt upon the subject
vanishes. In the same manner we pass by gentle steps from a coarse
cluster to an object such as the nebula in Orion, where we are still
inclined to remain in the once adopted idea of stars exceedingly remote
and inconceivably crowded, as being the occasion of that remarkable
appearance. It seems, therefore, to require a more dissimilar object
to set us right again. A glance like that of the naturalist, who casts
his eye from the perfect animal to the perfect vegetable, is wanting
to remove the veil from the mind of the astronomer. The object I have
mentioned above is the phenomenon that was wanting. View, for instance,
the nineteenth cluster of my sixth class, and afterwards cast your eye
on this cloudy star, and the result will be no less decisive than that
of the naturalist. Our judgment, I venture to say, will be that _the
nebulosity about the star is not of a starry nature_.”
In this manner he inferred the existence of real nebulous matter--of
a “shining fluid” of unknown and unimaginable properties. Was it
perhaps, he asked himself, a display of electrical illumination, like
the aurora borealis, or did it rather resemble the “magnificent cone of
the zodiacal light?” A boundless field of speculation was thrown open.
“These nebulous stars,” he added, “may serve as a clue to unravel
other mysterious phenomena.”
As their close allies, he now recognised planetary nebulæ, the
“milkiness, or soft tint of their light,” agreeing much better with the
supposition of a fluid, than of a stellar condition. And he rightly
placed in the same category the Orion nebula, and certain “diffused
nebulosities” which he had observed just to tarnish the sky over wide
areas. These last might, he considered, be quite near the earth, and
the object in Orion not more distant than perhaps an average second
magnitude star.
The relations of the sidereal to the nebular “principle” exercised
Herschel’s thoughts during many years. He had no sooner reasoned out
the existence in interstellar space of a rarefied, self-luminous
substance, than he began to interrogate himself as to its probable
function. Nature was to him the expression of Supreme Reason. He could
only conceive of her doings as directed towards an intelligible end.
Hence his confidence that rational investigation must lead to truth.
Already in 1791 he hinted at the conclusion which he foresaw. The
envelope of a “cloudy star” was, he declared, “more fit to produce
a star by its condensation than to depend upon the star for its
existence.” And the surmise was confirmed by his detection, in a
planetary nebula, of a sharp nucleus, or “generating star,” possibly to
be completed in time by the further accumulation of luminous matter.
His conjectures developed in 1811 into a formal theory. The cosmical
fluid was met with in all stages of condensation. Nebulous tracts of
almost evanescent lustre were connected in an unbroken series with
slightly “burred” objects, wanting only a few last touches to make them
finished stars. The extremes, as he said, had been, by his “critical
examination of the nebulous system,” “connected by such nearly allied
intermediate steps, as will make it highly probable that every
succeeding state of the nebulous matter is the result of the action of
gravitation upon it while in a foregoing one.”
In 1814 he traced the progress towards maturity of binary systems.
Originating in double nebulæ incompletely dissevered--Siamese-twin
objects, of which he had collected 139 examples--they next appeared
as nebulously-connected stars, finally as a pair materially isolated,
and linked together by the sole tie of gravitation. Scattered clusters
represented, in his scheme of celestial progress, a state antecedent
to that of globular clusters. “The still remaining irregularity of
their arrangement,” he said, “additionally proves that the action of
the clustering power has not been exerted long enough to produce a more
artificial construction.” He made, too, the important admission that
clusters apparently “in, or very near the Milky Way,” were truly part
and parcel of that complex agglomeration.
But what of his “fifteen hundred universes,” which had now logically
ceased to exist? The stellar and nebular “principles” had virtually
coalesced; both were included in the galactic system. The question
of “island universes” was accordingly left in abeyance; although
Herschel certainly believed in 1818 that among the multitude of
“ambiguous objects”--we should call them irresolvable nebulæ--many
exterior firmaments were included. Yet what he had ascertained about
the distribution of nebulæ should alone have sufficed to shatter this
remnant of a conviction.
The fact became clear to him during the progress of his “sweeps” that
nebulæ, to some extent, _replace stars_. He found them to occur in
“parcels,” more or less embedded with stars, “beds” and “parcels”
together being surrounded by blank spaces. This arrangement grew so
familiar to him that he used to notify his assistant, when stars
thinned out in the zone he was traversing, “to prepare for nebulæ.”
A wider relationship, brought within view by the large scale of
his labours, was defined by his fortunate habit of charting, for
convenience of identification, each newly-discovered batch of nebulæ.
“A very remarkable circumstance,” he wrote in 1784, “attending the
nebulæ and clusters of stars, is that they are arranged into strata,
which seem to run on to a great length; and some of them I have already
been able to pursue, so as to guess pretty well at their form and
direction. It is probable enough that they may surround the whole
apparent sphere of the heavens, not unlike the Milky Way.”
In the following year he spoke no longer of a zone, but of two vast
groupings of nebulæ about the opposite poles of the Milky Way. That
is to say, where stars are scarcest nebulæ are most abundant. The
correspondence did not escape him; but he did not recognise its
architectonic meaning. He had traced out the main plan of the stellar
world; he had discovered, not merely thousands of nebulæ, but the
nebular system; he had shown that stars and nebulæ were intimately
associated; he had even made it clear that nebular distribution was
governed by the lines of galactic structure. It only remained to draw
the obvious inference that these related parts made up one whole--that
no more than a single universe is laid open to human contemplation.
This was done by Whewell thirty years after his death.