FOUNDATION OF SIDEREAL ASTRONOMY

Until nearly a hundred years ago the stars were regarded by practical
astronomers mainly as a number of convenient fixed points by which the
motions of the various members of the solar system could be determined
and compared. Their recognised function, in fact, was that of milestones
on the great celestial highway traversed by the planets, as well as on
the byways of space occasionally pursued by comets. Not that curiosity
as to their nature, and even conjecture as to their origin, were at any
period absent. Both were from time to time powerfully stimulated by the
appearance of startling novelties in a region described by philosophers
as "incorruptible," or exempt from change. The catalogue of Hipparchus
probably, and certainly that of Tycho Brahe, some seventeen centuries
later, owed each its origin to the temporary blaze of a new star. The
general aspect of the skies was thus (however imperfectly) recorded from
age to age, and with improved appliances the enumeration was rendered
more and more accurate and complete; but the secrets of the stellar
sphere remained inviolate.

In a qualified though very real sense, Sir William Herschel may be
called the Founder of Sidereal Astronomy. Before his time some curious
facts had been noted, and some ingenious speculations hazarded,
regarding the condition of the stars, but not even the rudiments of
systematic knowledge had been acquired. The facts ascertained can be
summed up in a very few sentences.

Giordano Bruno was the first to set the suns of space in motion; but in
imagination only. His daring surmise was, however, confirmed in 1718,
when Halley announced[3] that Sirius, Aldebaran, Betelgeux, and Arcturus
had unmistakably shifted their quarters in the sky since Ptolemy
assigned their places in his catalogue. A similar conclusion was reached
by J. Cassini in 1738, from a comparison of his own observations with
those made at Cayenne by Richer in 1672; and Tobias Mayer drew up in
1756 a list showing the direction and amount of about fifty-seven proper
motions,[4] founded on star-places determined by Olaus Römer fifty years
previously. Thus the stars were no longer regarded as "fixed," but the
question remained whether the movements perceived were real or only
apparent; and this it was not yet found possible to answer. Already, in
the previous century, the ingenious Robert Hooke had suggested an
"alteration of the very system of the sun,"[5] to account for certain
suspected changes in stellar positions; Bradley in 1748, and Lambert in
1761, pointed out that such apparent displacements (by that time well
ascertained) were in all probability a combined effect of motions both
of sun and stars; and Mayer actually attempted the analysis, but without
result.

On the 13th of August, 1596, David Fabricius, an unprofessional
astronomer in East Friesland, saw in the neck of the Whale a star of the
third magnitude, which by October had disappeared. It was, nevertheless,
visible in 1603, when Bayer marked it in his catalogue with the Greek
letter Omicron, and was watched, in 1638-39, through its phases of
brightening and apparent extinction by a Dutch professor named
Holwarda.[6] From Hevelius this first-known periodical star received the
name of "Mira," or the Wonderful, and Boulliaud in 1667 fixed the length
of its cycle of change at 334 days. It was not a solitary instance. A
star in the Swan was perceived by Janson in 1600 to show fluctuations of
light, and Montanari found in 1669 that Algol in Perseus shared the same
peculiarity to a marked degree. Altogether the class embraced in 1782
half-a-dozen members. When it is added that a few star-couples had been
noted in singularly, but it was supposed accidentally, close
juxtaposition, and that the failure of repeated attempts to measure
stellar parallaxes pointed to distances _at least_ 400,000 times that of
the earth from the sun,[7] the picture of sidereal science, when the
last quarter of the eighteenth century began, is practically complete.
It included three items of information: that the stars have motions,
real or apparent; that they are immeasurably remote; and that a few
shine with a periodically variable light. Nor were these scantily
collected facts ordered into any promise of further development. They
lay at once isolated and confused before the inquirer. They needed to be
both multiplied and marshalled, and it seemed as if centuries of patient
toil must elapse before any reliable conclusions could be derived from
them. The sidereal world was thus the recognised domain of far-reaching
speculations, which remained wholly uncramped by systematic research
until Herschel entered upon his career as an observer of the heavens.

The greatest of modern astronomers was born at Hanover, November 15,
1738. He was the fourth child of Isaac Herschel, a hautboy-player in the
band of the Hanoverian Guard, and was early trained to follow his
father's profession. On the termination, however, of the disastrous
campaign of 1757, his parents removed him from the regiment, there is
reason to believe, in a somewhat unceremonious manner. Technically,
indeed, he incurred the penalties of desertion, remitted--according to
the Duke of Sussex's statement to Sir George Airy--by a formal pardon
handed to him personally by George III. on his presentation in 1782.[8]
At the age of nineteen, then, his military service having lasted four
years, he came to England to seek his fortune. Of the life of struggle
and privation which ensued little is known beyond the circumstances that
in 1760 he was engaged in training the regimental band of the Durham
Militia, and that in 1765 he was appointed organist at Halifax. In the
following year he removed to Bath as oboist in Linley's orchestra, and
in October 1767 was promoted to the post of organist in the Octagon
Chapel. The tide of prosperity now began to flow for him. The most
brilliant and modish society in England was at that time to be met at
Bath, and the young Hanoverian quickly found himself a favourite and the
fashion in it. Engagements multiplied upon him. He became director of
the public concerts; he conducted oratorios, engaged singers, organised
rehearsals, composed anthems, chants, choral services, besides
undertaking private tuitions, at times amounting to thirty-five or even
thirty-eight lessons a week. He in fact personified the musical activity
of a place then eminently and energetically musical.

But these multifarious avocations did not take up the whole of his
thoughts. His education, notwithstanding the poverty of his family, had
not been neglected, and he had always greedily assimilated every kind of
knowledge that came in his way. Now that he was a busy and a prosperous
man, it might have been expected that he would run on in the deep
professional groove laid down for him. On the contrary, his passion for
learning seemed to increase with the diminution of the time available
for its gratification. He studied Italian, Greek, mathematics;
Maclaurin's Fluxions served to "unbend his mind"; Smith's Harmonics and
Optics and Ferguson's Astronomy were the nightly companions of his
pillow. What he read stimulated without satisfying his intellect. He
desired not only to know, but to discover. In 1772 he hired a small
telescope, and through it caught a preliminary glimpse of the rich and
varied fields in which for so many years he was to expatiate.
Henceforward the purpose of his life was fixed: it was to obtain "a
knowledge of the construction of the heavens";[9] and this sublime
ambition he cherished to the end.

A more powerful instrument was the first desideratum; and here his
mechanical genius came to his aid. Having purchased the apparatus of a
Quaker optician, he set about the manufacture of specula with a zeal
which seemed to anticipate the wonders they were to disclose to him. It
was not until fifteen years later that his grinding and polishing
machines were invented, so the work had at that time to be entirely done
by hand. During this tedious and laborious process (which could not be
interrupted without injury, and lasted on one occasion sixteen hours),
his strength was supported by morsels of food put into his mouth by his
sister,[10] and his mind amused by her reading aloud to him the Arabian
Nights, Don Quixote, or other light works. At length, after repeated
failures, he found himself provided with a reflecting telescope--a
5-1/2-foot Gregorian--of his own construction. A copy of his first
observation with it, on the great Nebula in Orion--an object of
continual amazement and assiduous inquiry to him--is preserved by the
Royal Society. It bears the date March 4, 1774.[11]

In the following year he executed his first "review of the heavens,"
memorable chiefly as an evidence of the grand and novel conceptions
which already inspired him, and of the enthusiasm with which he
delivered himself up to their guidance. Overwhelmed with professional
engagements, he still contrived to snatch some moments for the stars;
and between the acts at the theatre was often seen running from the
harpsichord to his telescope, no doubt with that "uncommon precipitancy
which accompanied all his actions."[12] He now rapidly increased the
power and perfection of his telescopes. Mirrors of seven, ten, even
twenty feet focal length, were successively completed, and unprecedented
magnifying powers employed. His energy was unceasing, his perseverance
indomitable. In the course of twenty-one years no less than 430
parabolic specula left his hands. He had entered upon his forty-second
year when he sent his first paper to the _Philosophical Transactions_;
yet during the ensuing thirty-nine years his contributions--many of them
elaborate treatises--numbered sixty-nine, forming a series of
extraordinary importance to the history of astronomy. As a mere explorer
of the heavens his labours were prodigious. He discovered 2,500 nebulæ,
806 double stars, passed the whole firmament in review four several
times, counted the stars in 3,400 "gauge-fields," and executed a
photometric classification of the principal stars, founded on an
elaborate (and the first systematically conducted) investigation of
their relative brightness. He was as careful and patient as he was
rapid; spared no time and omitted no precaution to secure accuracy in
his observations; yet in one night he would examine, singly and
attentively, up to 400 separate objects.

The discovery of Uranus was a mere incident of the scheme he had marked
out for himself--a fruit, gathered as it were by the way. It formed,
nevertheless, the turning-point in his career. From a star-gazing
musician he was at once transformed into an eminent astronomer; he was
relieved from the drudgery of a toilsome profession, and installed as
Royal Astronomer, with a modest salary of £200 a year; funds were
provided for the construction of the forty-foot reflector, from the
great space-penetrating power of which he expected unheard-of
revelations; in fine, his future work was not only rendered possible,
but it was stamped as authoritative.[13] On Whit-Sunday 1782, William
and Caroline Herschel played and sang in public for the last time in St.
Margaret's Chapel, Bath; in August of the same year the household was
moved to Datchet, near Windsor, and on April 3, 1786, to Slough. Here
happiness and honours crowded on the fortunate discoverer. In 1788 he
married Mary, only child of James Baldwin, a merchant of the city of
London, and widow of Mr. John Pitt--a lady whose domestic virtues were
enhanced by the possession of a large jointure. The fruit of their union
was one son, of whose work--the worthy sequel of his father's--we shall
have to speak further on. Herschel was created a Knight of the
Hanoverian Guelphic Order in 1816, and in 1821 he became the first
President of the Royal Astronomical Society, his son being its first
Foreign Secretary. But his health had now for some years been failing,
and on August 25, 1822, he died at Slough, in the eighty-fourth year of
his age, and was buried in Upton churchyard.

His epitaph claims for him the lofty praise of having "burst the
barriers of heaven." Let us see in what sense this is true.

The first to form any definite idea as to the constitution of the
stellar system was Thomas Wright, the son of a carpenter living at
Byer's Green, near Durham. With him originated what has been called the
"Grindstone Theory" of the universe, which regarded the Milky Way as the
projection on the sphere of a stratum or disc of stars (our sun
occupying a position near the centre), similar in magnitude and
distribution to the lucid orbs of the constellations.[14] He was
followed by Kant,[15] who transcended the views of his predecessor by
assigning to nebulæ the position they long continued to occupy, rather
on imaginative than scientific grounds, of "island universes," external
to, and co-equal with, the Galaxy. Johann Heinrich Lambert,[16] a
tailor's apprentice from Mühlhausen, followed, but independently. The
conceptions of this remarkable man were grandiose, his intuitions bold,
his views on some points a singular anticipation of subsequent
discoveries. The sidereal world presented itself to him as a hierarchy
of systems, starting from the planetary scheme, rising to throngs of
suns within the circuit of the Milky Way--the "ecliptic of the stars,"
as he phrased it--expanding to include groups of many Milky Ways; these
again combining to form the unit of a higher order of assemblage, and so
onwards and upwards until the mind reels and sinks before the immensity
of the contemplated creations.

"Thus everything revolves--the earth round the sun; the sun round the
centre of his system; this system round a centre common to it with other
systems; this group, this assemblage of systems, round a centre which is
common to it with other groups of the same kind; and where shall we have
done?"[17]

The stupendous problem thus speculatively attempted, Herschel undertook
to grapple with experimentally. The upshot of this memorable inquiry was
the inclusion, for the first time, within the sphere of human knowledge,
of a connected body of facts, and inferences from facts, regarding the
sidereal universe; in other words, the foundation of what may properly
be called a science of the stars.

Tobias Mayer had illustrated the perspective effects which must ensue in
the stellar sphere from a translation of the solar system, by comparing
them to the separating in front and closing up behind of trees in a
forest to the eye of an advancing spectator;[18] but the appearances
which he thus correctly described he was unable to detect. By a more
searching analysis of a smaller collection of proper motions, Herschel
succeeded in rendering apparent the very consequences foreseen by Mayer.
He showed, for example, that Arcturus and Vega did, in fact, appear to
recede from, and Sirius and Aldebaran to approach, each other by very
minute amounts; and, with a striking effort of divinatory genius, placed
the "apex," or point of direction of the sun's motion, close to the star
Lambda in the constellation Hercules,[19] within a few degrees of
the spot indicated by later and indefinitely more refined methods of
research. He resumed the subject in 1805,[20] but though employing a
more rigorous method, was scarcely so happy in his result. In 1806,[21]
he made a preliminary attempt to ascertain the speed of the sun's
journey, fixing it, by doubtless much too low an estimate, at about
three miles a second. Yet the validity of his general conclusion as to
the line of solar travel, though long doubted, has been triumphantly
confirmed. The question as to the "secular parallax" of the fixed stars
was in effect answered.

With their _annual_ parallax, however, the case was very different. The
search for it had already led Bradley to the important discoveries of
the aberration of light and the nutation of the earth's axis; it was now
about to lead Herschel to a discovery of a different, but even more
elevated character. Yet in neither case was the object primarily sought
attained.

From the very first promulgation of the Copernician theory the seeming
immobility of the stars had been urged as an argument against its truth;
for if the earth really travelled in a vast orbit round the sun, objects
in surrounding space should appear to change their positions, unless
their distances were on a scale which, to the narrow ideas of the
universe then prevailing, seemed altogether extravagant.[22] The
existence of such apparent or "parallactic" displacements was
accordingly regarded as the touchstone of the new views, and their
detection became an object of earnest desire to those interested in
maintaining them. Copernicus himself made the attempt; but with his
"Triquetrum," a jointed wooden rule with the divisions marked in ink,
constructed by himself,[23] he was hardly able to measure angles of ten
minutes, far less fractions of a second. Galileo, a more impassioned
defender of the system, strained his ears, as it were, from Arcetri, in
his blind and sorrowful old age, for news of a discovery which two more
centuries had still to wait for. Hooke believed he had found a parallax
for the bright star in the Head of the Dragon; but was deceived. Bradley
convinced himself that such effects were too minute for his instruments
to measure. Herschel made a fresh attempt by a practically untried
method.

It is a matter of daily experience that two objects situated at
different distances seem to a beholder in motion to move relatively to
each other. This principle Galileo, in the third of his Dialogues on the
Systems of the World,[24] proposed to employ for the determination of
stellar parallax; for two stars, lying apparently close together, but in
reality separated by a great gulf of space, must shift their mutual
positions when observed from opposite points of the earth's orbit; or
rather, the remoter forms a virtually fixed point, to which the
movements of the other can be conveniently referred. By this means
complications were abolished more numerous and perplexing than Galileo
himself was aware of, and the problem was reduced to one of simple
micrometrical measurement. The "double-star method" was also suggested
by James Gregory in 1675, and again by Wallis in 1693;[25] Huygens
first, and afterwards Dr. Long of Cambridge (about 1750), made futile
experiments with it; and it eventually led, in the hands of Bessel, to
the successful determination of the parallax of 61 Cygni.

Its advantages were not lost upon Herschel. His attempt to assign
definite distances to the nearest stars was no isolated effort, but part
of the settled plan upon which his observations were conducted. He
proposed to sound the heavens, and the first requisite was a knowledge
of the length of his sounding-line. Thus it came about that his special
attention was early directed to double stars.

"I resolved," he writes,[26] "to examine every star in the heavens with
the utmost attention and a very high power, that I might collect such
materials for this research as would enable me to fix my observations
upon those that would best answer my end. The subject has already proved
so extensive, and still promises so rich a harvest to those who are
inclined to be diligent in the pursuit, that I cannot help inviting
every lover of astronomy to join with me in observations that must
inevitably lead to new discoveries."

The first result of these inquiries was a classed catalogue of 269
double stars presented to the Royal Society in 1782, followed, after
three years, by an additional list of 434. In both these collections the
distances separating the individuals of each pair were carefully
measured, and (with a few exceptions) the angles made with the
hour-circle by the lines joining their centres (technically called
"angles of position") were determined with the aid of a "revolving-wire
micrometer," specially devised for the purpose. Moreover, an important
novelty was introduced by the observation of the various colours visible
in the star-couples, the singular and vivid contrasts of which were now
for the first time described.

Double stars were at that time supposed to be a purely optical
phenomenon. Their components, it was thought, while in reality
indefinitely remote from each other, were brought into fortuitous
contiguity by the chance of lying nearly in the same line of sight from
the earth. Yet Bradley had noticed a change of 30°, between 1718 and
1759, in the position-angle of the two stars forming Castor, and was
thus within a hair's breadth of the discovery of their physical
connection.[27] While the Rev. John Michell, arguing by the doctrine of
probabilities, wrote as follows in 1767:--"It is highly probable in
particular, and next to a certainty in general, that such double stars
as appear to consist of two or more stars placed very near together, do
really consist of stars placed near together, and under the influence of
some general law."[28] And in 1784:[29] "It is not improbable that a few
years may inform us that some of the great number of double, triple
stars, etc., which have been observed by Mr. Herschel, are systems of
bodies revolving about each other."

This remarkable speculative anticipation had a practical counterpart in
Germany. Father Christian Mayer, a Jesuit astronomer at Mannheim, set
himself, in January 1776, to collect examples of stellar pairs, and
shortly after published the supposed discovery of "satellites" to many
of the principal stars.[30] But his observations were neither exact nor
prolonged enough to lead to useful results in such an inquiry. His
disclosures were derided; his planet-stars treated as results of
hallucination. _On n'a point cru à des choses aussi extraordinaires_,
wrote Lalande[31] within one year of a better-grounded announcement to
the same effect.

Herschel at first shared the general opinion as to the merely optical
connection of double stars. Of this the purpose for which he made his
collection is in itself sufficient evidence, since what may be called
the _differential_ method of parallaxes depends, as we have seen, for
its efficacy upon disparity of distance. It was "much too soon," he
declared in 1782,[32] "to form any theories of small stars revolving
round large ones;" while in the year following,[33] he remarked that the
identical proper motions of the two stars forming, to the naked eye, the
single bright orb of Castor could only be explained as both equally due
to the "systematic parallax" caused by the sun's movement in space.
Plainly showing that the notion of a physical tie, compelling the two
bodies to travel together, had not as yet entered into his speculations.
But he was eminently open to conviction, and had, moreover, by
observations unparalleled in amount as well as in kind, prepared ample
materials for convincing himself and others. In 1802 he was able to
announce the fact of his discovery, and in the two ensuing years, to lay
in detail before the Royal Society proofs, gathered from the labours of
a quarter of a century, of orbital revolution in the case of as many as
fifty double stars, henceforth, he declared, to be held as real binary
combinations, "intimately held together by the bond of mutual
attraction."[34] The fortunate preservation in Dr. Maskelyne's note-book
of a remark made by Bradley about 1759, to the effect that the line
joining the components of Castor was an exact prolongation of that
joining Castor with Pollux, added eighteen years to the time during
which the pair were under scrutiny, and confirmed the evidence of change
afforded by more recent observations. Approximate periods were fixed for
many of the revolving suns--for Castor 342 years; for Gamma Leonis,
1200, Delta Serpentis, 375, Eta Bootis, 1681 years; Eta Lyræ
was noted as a "double-double-star," a change of relative
situation having been detected in each of the two pairs composing the
group; and the occultation was described of one star by another in the
course of their mutual revolutions, as exemplified in 1795 by the
rapidly circulating system of Zeta Herculis.

Thus, by the sagacity and perseverance of a single observer, a firm
basis was at last provided upon which to raise the edifice of sidereal
science. The analogy long presumed to exist between the mighty star of
our system and the bright points of light spangling the firmament was
shown to be no fiction of the imagination, but a physical reality; the
fundamental quality of attractive power was proved to be common to
matter so far as the telescope was capable of exploring, and law,
subordination, and regularity to give testimony of supreme and
intelligent design no less in those limitless regions of space than in
our narrow terrestrial home. The discovery was emphatically (in Arago's
phrase) "one with a future," since it introduced the element of precise
knowledge where more or less probable conjecture had previously held
almost undivided sway; and precise knowledge tends to propagate itself
and advance from point to point.

We have now to speak of Herschel's pioneering work in the skies. To
explore with line and plummet the shining zone of the Milky Way, to
delineate its form, measure its dimensions, and search out the
intricacies of its construction, was the primary task of his life, which
he never lost sight of, and to which all his other investigations were
subordinate. He was absolutely alone in this bold endeavour. Unaided, he
had to devise methods, accumulate materials, and sift out results. Yet
it may safely be asserted that all the knowledge we possess on this
sublime subject was prepared, and the greater part of it anticipated, by
him.

The ingenious method of "star-gauging," and its issue in the delineation
of the sidereal system as an irregular stratum of evenly-scattered suns,
is the best-known part of his work. But it was, in truth, only a first
rude approximation, the principle of which maintained its credit in the
literature of astronomy a full half-century after its abandonment by its
author. This principle was the general equality of star distribution. If
equal portions of space really held equal numbers of stars, it is
obvious that the number of stars visible in any particular direction
would be strictly proportional to the range of the system in that
direction, apparent accumulation being produced by real extent. The
process of "gauging the heavens," accordingly, consisted in counting the
stars in successive telescopic fields, and calculating thence the depths
of space necessary to contain them. The result of 3,400 such operations
was the plan of the Galaxy familiar to every reader of an astronomical
text-book. Widely-varying evidence was, as might have been expected,
derived from an examination of different portions of the sky. Some
fields of view were almost blank, while others (in or near the Milky
Way) blazed with the radiance of many hundred stars compressed into an
area about one-fourth that of the full-moon. In the most crowded parts
116,000 were stated to have been passed in review within a quarter of an
hour. Here the "length of his sounding-line" was estimated by Herschel
at about 497 times the distance of Sirius--in other words, the bounding
orb, or farthest sun of the system in that direction, so far as could be
seen with the 20-foot reflector, was thus inconceivably remote. But
since the distance of Sirius, no less than of every other fixed star,
was as yet an unknown quantity, the dimensions inferred for the Galaxy
were of course purely relative; a knowledge of its form and structure
might (admitting the truth of the fundamental hypothesis) be obtained,
but its real or absolute size remained altogether undetermined.

Even as early as 1785, however, Herschel perceived traces of a tendency
which completely invalidated the supposition of any approach to an
average uniformity of distribution. This was the action of what he
called a "clustering power" in the Milky Way. "Many gathering
clusters"[35] were already discernible to him even while he endeavoured
to obtain a "true _mean_ result" on the assumption that each star in
space was separated from its neighbours as widely as the sun from
Sirius. "It appears," he wrote in 1789, "that the heavens consist of
regions where suns are gathered into separate systems"; and in certain
assemblages he was able to trace "a course or tide of stars setting
towards a centre," denoting, not doubtfully, the presence of attractive
forces.[36] Thirteen years later, he described our sun and his
constellated companions as surrounded by "a magnificent collection of
innumerable stars, called the Milky Way, which must occasion a very
powerful balance of opposite attractions to hold the intermediate stars
at rest. For though our sun, and all the stars we see, may truly be said
to be in the plane of the Milky Way, yet I am now convinced, by a long
inspection and continued examination of it, that the Milky Way itself
consists of stars very differently scattered from those which are
immediately about us." "This immense aggregation," he added, "is by no
means uniform. Its component stars show evident signs of clustering
together into many separate allotments."[37]

The following sentences, written in 1811, contain a definite
retractation of the view frequently attributed to him:--

"I must freely confess," he says, "that by continuing my sweeps of the
heavens my opinion of the arrangement of the stars and their magnitudes,
and of some other particulars, has undergone a gradual change; and
indeed, when the novelty of the subject is considered, we cannot be
surprised that many things formerly taken for granted should on
examination prove to be different from what they were generally but
incautiously supposed to be. For instance, an equal scattering of the
stars may be admitted in certain calculations; but when we examine the
Milky Way, or the closely compressed clusters of stars of which my
catalogues have recorded so many instances, this supposed equality of
scattering must be given up."[38]

Another assumption, the fallacy of which he had not the means of
detecting since become available, was retained by him to the end of his
life. It was that the brightness of a star afforded an approximate
measure of its distance. Upon this principle he founded in 1817 his
method of "limiting apertures,"[39] by which two stars, brought into
view in two precisely similar telescopes, were "equalised" by covering a
certain portion of the object-glass collecting the more brilliant rays.
The distances of the orbs compared were then taken to be in the ratio of
the reduced to the original apertures of the instruments with which they
were examined. If indeed the absolute lustre of each were the same, the
result might be accepted with confidence; but since we have no warrant
for assuming a "standard star" to facilitate our computations, but much
reason to suppose an indefinite range, not only of size but of intrinsic
brilliancy, in the suns of our firmament, conclusions drawn from such a
comparison are entirely worthless.

In another branch of sidereal science besides that of stellar
aggregation, Herschel may justly be styled a pioneer. He was the first
to bestow serious study on the enigmatical objects known as "nebulæ."
The history of the acquaintance of our race with them is comparatively
short. The only one recognised before the invention of the telescope was
that in the girdle of Andromeda, certainly familiar in the middle of the
tenth century to the Persian astronomer Abdurrahman Al-Sûfi; and marked
with dots on Spanish and Dutch constellation-charts of the fourteenth
and fifteenth centuries.[40] Yet so little was it noticed that it might
practically be said--as far as Europe is concerned--to have been
discovered in 1612 by Simon Marius (Mayer of Genzenhausen), who aptly
described its appearance as that of a "candle shining through horn." The
first mention of the great Orion nebula is by a Swiss Jesuit named
Cysatus, who succeeded Father Scheiner in the chair of mathematics at
Ingolstadt. He used it, apparently without any suspicion of its novelty,
as a term of comparison for the comet of December 1618.[41] A novelty,
nevertheless, to astronomers it still remained in 1656, when Huygens
discerned, "as it were, an hiatus in the sky, affording a glimpse of a
more luminous region beyond."[42] Halley in 1716 knew of six nebulæ,
which he believed to be composed of a "lucid medium" diffused through
the ether of space.[43] He appears, however, to have been unacquainted
with some previously noticed by Hevelius. Lacaille brought back with him
from the Cape a list of forty-two--the first-fruits of observation in
Southern skies--arranged in three numerically equal classes;[44] and
Messier (nicknamed by Louis XV. the "ferret of comets"), finding such
objects a source of extreme perplexity in the pursuit of his chosen
game, attempted to eliminate by methodising them, and drew up a
catalogue comprising, in 1781, 103 entries.[45]

These preliminary attempts shrank into insignificance when Herschel
began to "sweep the heavens" with his giant telescopes. In 1786 he
presented to the Royal Society a descriptive catalogue of 1,000 nebulæ
and clusters, followed, three years later, by a second of as many more;
to which he added in 1802 a further gleaning of 500. On the subject of
their nature his views underwent a remarkable change. Finding that his
potent instruments resolved into stars many nebulous patches in which no
signs of such a structure had previously been discernible, he naturally
concluded that "resolvability" was merely a question of distance and
telescopic power. He was (as he said himself) led on 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. The singular variety of their appearance was thus described by
him:--

"I have seen," he says, "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, contain 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."[46]

"These curious objects" he considered to be "no less than whole sidereal
systems,"[47] some of which might "well outvie our Milky Way in
grandeur." He admitted, however, a wide diversity in condition as well
as compass. The system to which our sun belongs he described as "a very
extensive branching congeries of many millions of stars, which probably
owes its origin to many remarkably large as well as pretty closely
scattered small stars, that may have drawn together the rest."[48] But
the continued action of this same "clustering power" would, he supposed,
eventually lead to the breaking-up of the original majestic Galaxy into
two or three hundred separate groups, already visibly gathering. Such
minor nebulæ, due to the "decay" of other "branching nebulæ" similar to
our own, he recognised by the score, lying, as it were, stratified in
certain quarters of the sky. "One of these nebulous beds," he informs
us, "is so rich that in passing through a section of it, in the time of
only thirty-six minutes, I detected no less than thirty-one nebulæ, all
distinctly visible upon a fine blue sky." The stratum of Coma Berenices
he judged to be the nearest to our system of such layers; nor did the
marked aggregation of nebulæ towards both poles of the circle of the
Milky Way escape his notice.

By a continuation of the same process of reasoning, he was enabled (as
he thought) to trace the life-history of nebulæ from a primitive loose
and extended formation, through clusters of gradually increasing
compression, down to the kind named by him "Planetary" because of the
defined and uniform discs which they present. These he regarded as "very
aged, and drawing on towards a period of change or dissolution."[49]

"This method of viewing the heavens," he concluded, "seems to throw them
into a new kind of light. They now are 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, to continue the simile which I have borrowed from
the vegetable kingdom, 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?"[50]

But already this supposed continuity was broken. After mature
deliberation on the phenomena presented by nebulous stars, Herschel was
induced, in 1791, to modify essentially his original opinion.

"When I pursued these researches," he says, "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
before him. In the same manner we pass through gentle steps from a
coarse cluster of stars, such as the Pleiades ... till we find ourselves
brought 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 for this purpose.
View, for instance, the 19th cluster of my 6th class, and afterwards
cast your eye on this cloudy star, and the result will be no less
decisive than that of the naturalist we have alluded to. Our judgment, I
may venture to say, will be, that _the nebulosity about the star is not
of a starry nature_."[51]

The conviction thus arrived at of the existence in space of a widely
diffused "shining fluid" (a conviction long afterwards fully justified
by the spectroscope) led him into a field of endless speculation. What
was its nature? Should it "be compared to the coruscation of the
electric fluid in the aurora borealis? or to the more magnificent cone
of the zodiacal light?" Above all, what was its function in the cosmos?
And on this point he already gave a hint of the direction in which his
mind was moving by the remark that this self-luminous matter seemed
"more fit to produce a star by its condensation, than to depend on the
star for its existence."[52]

This was not a novel idea. Tycho Brahe had tried to explain the blaze of
the star of 1572 as due to a sudden concentration of nebulous material
in the Milky Way, even pointing out the space left dark and void by the
withdrawal of the luminous stuff; and Kepler, theorising on a similar
stellar apparition in 1604, followed nearly in the same track. But under
Herschel's treatment the nebular origin of stars first acquired the
consistency of a formal theory. He meditated upon it long and earnestly,
and in two elaborate treatises, published respectively in 1811 and 1814,
he at length set forth the arguments in its favour. These rested
entirely upon the "principle of continuity." Between the successive
classes of his assortment of developing objects there was, as he said,
"perhaps not so much difference as would be in an annual description of
the human figure, were it given from the birth of a child till he comes
to be a man in his prime."[53] From diffused nebulosity, barely visible
in the most powerful light-gathering instruments, but which he estimated
to cover nearly 152 square degrees of the heavens,[54] to planetary
nebulæ, supposed to be already centrally solid, instances were alleged
of every stage and phase of condensation. The validity of his reasoning,
however, was evidently impaired by his confessed inability to
distinguish between the dim rays of remote clusters and the milky light
of true gaseous nebulæ.

It may be said that such speculations are futile in themselves, and
necessarily barren of results. But they gratify an inherent tendency of
the human mind, and, if pursued in a becoming spirit, should be neither
reproved nor disdained. Herschel's theory still holds the field, the
testimony of recent discoveries with regard to it having proved strongly
confirmatory of its principle, although not of its details. Strangely
enough, it seems to have been propounded in complete independence of
Laplace's nebular hypothesis as to the origin of the solar system.
Indeed, it dated, as we have seen, in its first inception, from 1791,
while the French geometrician's view was not advanced until 1796.

We may now briefly sum up the chief results of Herschel's long years of
"watching the heavens." The apparent motions of the stars had been
disentangled; one portion being clearly shown to be due to a translation
towards a point in the constellation Hercules of the sun and his
attendant planets; while a large balance of displacement was left to be
accounted for by real movements, various in extent and direction, of the
stars themselves. By the action of a central force similar to, if not
identical with, gravity, suns of every degree of size and splendour, and
sometimes brilliantly contrasted in colour, were seen to be held
together in systems, consisting of two, three, four, even six members,
whose revolutions exhibited a wide range of variety both in period and
in orbital form. A new department of physical astronomy was thus
created,[55] and rigid calculation for the first time made possible
within the astral region. The vast problem of the arrangement and
relations of the millions of stars forming the Milky Way was shown to be
capable of experimental treatment, and of at least partial solution,
notwithstanding the variety and complexity seen to prevail, to an extent
previously undreamt of, in the arrangement of that majestic system. The
existence of a luminous fluid, diffused through enormous tracts of
space, and intimately associated with stellar bodies, was virtually
demonstrated, and its place and use in creation attempted to be divined
by a bold but plausible conjecture. Change on a stupendous scale was
inferred or observed to be everywhere in progress. Periodical stars
shone out and again decayed; progressive ebbings or flowings of light
were indicated as probable in many stars under no formal suspicion of
variability; forces were everywhere perceived to be at work, by which
the very structure of the heavens themselves must be slowly but
fundamentally modified. In all directions groups were seen to be formed
or forming; tides and streams of suns to be setting towards powerful
centres of attraction; new systems to be in process of formation, while
effete ones hastened to decay or regeneration when the course appointed
for them by Infinite Wisdom was run. And thus, to quote the words of the
observer who "had looked farther into space than ever human being did
before him,"[56] the state into which the incessant action of the
clustering power has brought the Milky Way at present, 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 parts 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.[57]


FOOTNOTES:

[Footnote 3: _Phil. Trans._, vol. xxx., p. 737.]

[Footnote 4: Out of eighty stars compared, fifty-seven were found to
have changed their places by more than 10". Lesser discrepancies were at
that time regarded as falling within the limits of observational error.
_Tobiæ Mayeri Op. Inedita_, t. i., pp. 80, 81, and Herschel in _Phil.
Trans._, vol. lxxiii., pp. 275-278.]

[Footnote 5: _Posthumous Works_, p. 701.]

[Footnote 6: Arago in _Annuaire du Bureau des Longitudes_, 1842, p.
313.]

[Footnote 7: Bradley to Halley, _Phil. Trans._, vol. xxxv. (1728), p.
660. His observations were directly applicable to only two stars,
Gamma Draconis and Eta Ursæ Majoris, but some lesser ones
were included in the same result.]

[Footnote 8: Holden, _Sir William Herschel, his Life and Works_, p. 17.]

[Footnote 9: _Phil. Trans._, vol. ci., p. 269.]

[Footnote 10: Caroline Lucretia Herschel, born at Hanover, March 16,
1750, died in the same place, January 9, 1848. She came to England in
1772, and was her brother's devoted assistant, first in his musical
undertakings, and afterwards, down to the end of his life, in his
astronomical labours.]

[Footnote 11: Holden, _op. cit._, p. 39.]

[Footnote 12: _Memoir of Caroline Herschel_, p. 37.]

[Footnote 13: See Holden's _Sir William Herschel_, p. 54.]

[Footnote 14: _An Original Theory or New Hypothesis of the Universe_,
London, 1750. See also De Morgan's summary of his views in
_Philosophical Magazine_, April, 1848.]

[Footnote 15: _Allgemeine Naturgeschichte und Theorie des Himmels_,
1755.]

[Footnote 16: _Cosmologische Briefe_, Augsburg, 1761.]

[Footnote 17: _The System of the World_, p. 125, London, 1800 (a
translation of _Cosmologische Briefe_). Lambert regarded nebulæ as
composed of stars crowded together, but _not_ as external universes. In
the case of the Orion nebula, indeed, he throws out such a conjecture,
but afterwards suggests that it may form a centre for that one of the
subordinate systems composing the Milky Way to which our sun belongs.]

[Footnote 18: _Opera Inedita_, t. i., p. 79.]

[Footnote 19: _Phil. Trans._, vol. lxxiii. (1783), p. 273. Pierre
Prévost's similar investigation, communicated to the Berlin Academy of
Sciences four months later, July 3, 1783, was inserted in the _Memoirs_
of that body for 1781, and thus _seems_ to claim a priority not its due.
Georg Simon Klügel at Halle gave about the same time an analytical
demonstration of Herschel's result. Wolf, _Gesch. der Astronomie_, p.
733.]

[Footnote 20: _Phil. Trans._, vol. xcv., p. 233.]

[Footnote 21: _Ibid._, vol. xcvi., p. 205.]

[Footnote 22: "Ingens bolus devorandus est," Kepler admitted to Herwart
in May, 1603.]

[Footnote 23: Described in "Præfatio Editoris" to _De Revolutionibus_,
p. xix. (ed. 1854).]

[Footnote 24: _Opere_, t. i., p. 415.]

[Footnote 25: _Phil. Trans._, vol. xvii., p. 848.]

[Footnote 26: _Ibid._, vol. lxxii., p. 97.]

[Footnote 27: Doberck, _Observatory_, vol. ii., p. 110.]

[Footnote 28: _Phil. Trans._, vol. lvii., p. 249.]

[Footnote 29: _Ibid._, vol. lxxiv., p. 56.]

[Footnote 30: _Beobachtungen von Fixsterntrabanten_, 1778; and _De Novis
in Coelo Sidereo Phænomenis_, 1779.]

[Footnote 31: _Bibliographie_, p. 569.]

[Footnote 32: _Phil. Trans._, vol. lxxii., p. 162.]

[Footnote 33: _Ibid._, vol. lxxiii., p. 272.]

[Footnote 34: _Ibid._, vol. xciii., p. 340.]

[Footnote 35: _Phil. Trans._, vol. lxxv., p. 255.]

[Footnote 36: _Ibid._, vol. lxxix., pp. 214, 222.]

[Footnote 37: _Ibid._, vol. xcii., pp. 479, 495.]

[Footnote 38: _Phil. Trans._, vol. ci., p. 269.]

[Footnote 39: _Ibid._, vol. cvii., p. 311.]

[Footnote 40: Bullialdus, _De Nebulosâ Stellâ in Cingulo Andromedæ_
(1667); see also G. P. Bond, _Mém. Am. Ac._, vol. iii., p. 75, Holden's
Monograph on the Orion Nebula, _Washington Observations_, vol. xxv.,
1878 (pub. 1882), and Lady Huggins's drawing, _Atlas of Spectra_, p.
119.]

[Footnote 41: _Mathemata Astronomica_, p. 75.]

[Footnote 42: _Systema Saturnium_, p. 9.]

[Footnote 43: _Phil. Trans._, vol. xxix., p. 390.]

[Footnote 44: _Mém. Ac. des Sciences_, 1755.]

[Footnote 45: _Conn. des Temps_, 1784 (pub. 1781), p. 227. A previous
list of forty-five had appeared in _Mém. Ac. des Sciences_, 1771.]

[Footnote 46: _Phil. Trans._, vol. lxxiv., p. 442.]

[Footnote 47: _Ibid._, vol. lxxix., p. 213.]

[Footnote 48: _Ibid._, vol. lxxv., p. 254.]

[Footnote 49: _Ibid._, vol. lxxix., p. 225.]

[Footnote 50: _Phil. Trans._, vol. lxxix., p. 226.]

[Footnote 51: _Ibid._, vol. lxxxi., p. 72.]

[Footnote 52: _Ibid._, p. 85.]

[Footnote 53: _Phil. Trans._, vol. ci., p. 271.]

[Footnote 54: _Ibid._, p. 277.]

[Footnote 55: J. Herschel, _Phil. Trans._, vol. cxvi., part iii., p. 1.]

[Footnote 56: His own words to the poet Campbell cited by Holden, _Life
and Works_, p. 109.]

[Footnote 57: _Phil. Trans._, vol. civ., p. 283.]


A Popular History of Astronomy During the
Nineteenth Century, by Agnes M. (Agnes Mary) Clerke