Sir John Frederick William Herschel, only child--and, as an
astronomer, almost the only rival--of Sir William Herschel, was
born at Slough, in Ireland, on March 7, 1792. At first privately
educated, in 1813 he graduated from St. John's College, Cambridge,
as senior wrangler and first Smith's prizeman. He chose the law as
his profession; but in 1816 reported that, under his father's
direction, he was going "to take up stargazing." He then began a
re-examination of his father's double stars. In 1825 he wrote that
he was going to take nebulæ under his especial charge. He embarked
in 1833 with his family for the Cape; and his work at Feldhausen,
six miles from Cape Town, marked the beginning of southern sidereal
astronomy. The result of his four years' work there was published
in 1847. From 1855 he devoted himself at Collingwood to the
collection and revival of his father's and his own labours. His
"Outlines of Astronomy," published in 1849, and founded on an
earlier "Treatise on Astronomy" of 1833, was an outstanding
success. Herschel's long and happy life, every day of which added
its share to his scientific services, came to an end on May 11,
1871.
_I.--The Wonders of the Milky Way_
There is no science which draws more largely than does astronomy on that
intellectual liberality which is ready to adopt whatever is demonstrated
or concede whatever is rendered highly probable, however new and
uncommon the points of view may be in which objects the most familiar
may thereby become placed. Almost all its conclusions stand in open and
striking contradiction with those of superficial and vulgar observation,
and with what appears to everyone the most positive evidence of his
senses.
There is hardly anything which sets in a stronger light the inherent
power of truth over the mind of man, when opposed by no motives of
interest or passion, than the perfect readiness with which all its
conclusions are assented to as soon as their evidence is clearly
apprehended, and the tenacious hold they acquire over our belief when
once admitted.
If the comparison of the apparent magnitude of the stars with their
number leads to no immediately obvious conclusion, it is otherwise when
we view them in connection with their local distribution over the
heavens. If indeed we confine ourselves to the three or four brightest
classes, we shall find them distributed with a considerable approach to
impartiality over the sphere; a marked preference, however, being
observable, especially in the southern hemisphere, to a zone or belt
passing through _epsilon_ Orionis and _alpha_ Crucis. But if we take in
the whole amount visible to the naked eye we shall perceive a great
increase of numbers as we approach the borders of the Milky Way. And
when we come to telescopic magnitudes we find them crowded beyond
imagination along the extent of that circle and of the branches which it
sends off from it; so that, in fact, its whole light is composed of
nothing but stars of every magnitude from such as are visible to the
naked eye down to the smallest points of light perceptible with the best
telescopes.
These phenomena agree with the supposition that the stars of our
firmament, instead of being scattered indifferently in all directions
through space, form a stratum of which the thickness is small in
comparison with its length and breadth; and in which the earth occupies
a place somewhere about the middle of its thickness and near the point
where it subdivides into two principal laminæ inclined at a small angle
to each other. For it is certain that to an eye so situated the apparent
density of the stars, supposing them pretty equally scattered through
the space they occupy, would be least in the direction of the visual ray
perpendicular to the lamina, and greatest in that of its breadth;
increasing rapidly in passing from one to the other direction, just as
we see a slight haze in the atmosphere thickening into a decided
fog-bank near the horizon by the rapid increase of the mere length of
the visual ray.
Such is the view of the construction of the starry firmament taken by
Sir William Herschel, whose powerful telescopes first effected a
complete analysis of this wonderful zone, and demonstrated the fact of
its entirely consisting of stars.
So crowded are they in some parts of it that by counting the stars in a
single field of his telescope he was led to conclude that 50,000 had
passed under his review in a zone two degrees in breadth during a single
hour's observation. The immense distances at which the remoter regions
must be situated will sufficiently account for the vast predominance of
small magnitudes which are observed in it.
The process of gauging the heavens was devised by Sir William Herschel
for this purpose. It consisted simply in counting the stars of all
magnitudes which occur in single fields of view, of fifteen minutes in
diameter, visible through a reflecting telescope of 18 inches aperture,
and 20 feet focal length, with a magnifying power of 180 degrees, the
points of observation being very numerous and taken indiscriminately in
every part of the surface of the sphere visible in our latitudes.
On a comparison of many hundred such "gauges," or local enumerations, it
appears that the density of starlight (or the number of stars existing
on an average of several such enumerations in any one immediate
neighbourhood) is least in the pole of the Galactic circle [_i.e._, the
great circle to which the course of the Milky Way most nearly conforms:
_gala_ = milk], and increases on all sides down to the Milky Way itself,
where it attains its maximum. The progressive rate of increase in
proceeding from the pole is at first slow, but becomes more and more
rapid as we approach the plane of that circle, according to a law from
which it appears that the mean density of the stars in the galactic
circle exceeds, in a ratio of very nearly 30 to 1, that in its pole, and
in a proportion of more than 4 to 1 that in a direction 15 degrees
inclined to its plane.
As we ascend from the galactic plane we perceive that the density
decreases with great rapidity. So far we can perceive no flaw in this
reasoning if only it be granted (1) that the level planes are continuous
and of equal density throughout; and (2) that an absolute and definite
limit is set to telescopic vision, beyond which, if stars exist, they
elude our sight, and are to us as if they existed not. It would appear
that, with an almost exactly similar law of apparent density in the two
hemispheres, the southern were somewhat richer in stars than the
northern, which may arise from our situation not being precisely in the
middle of its thickness, but somewhat nearer to its northern surface.
_II.--Penetrating Infinite Space_
When examined with powerful telescopes, the constitution of this
wonderful zone is found to be no less various than its aspect to the
naked eye is irregular. In some regions the stars of which it is
composed are scattered with remarkable uniformity over immense tracts,
while in others the irregularity of their distribution is quite as
striking, exhibiting a rapid succession of closely clustering rich
patches separated by comparatively poor intervals, and indeed in some
instances absolutely dark and _completely_ void of any star even of the
smallest telescopic magnitude. In some places not more than 40 or 50
stars on an average occur in a "gauge" field of 15 minutes, while in
others a similar average gives a result of 400 or 500.
Nor is less variety observable in the character of its different
regions in respect of the magnitude of the stars they exhibit, and the
proportional numbers of the larger and smaller magnitudes associated
together, than in respect of their aggregate numbers. In some, for
instance, extremely minute stars, though never altogether wanting, occur
in numbers so moderate as to lead us irresistibly to the conclusion that
in these regions we are _fairly through_ the starry stratum, since it is
impossible otherwise (supposing their light not intercepted) that the
numbers of the smaller magnitudes should not go on increasing _ad
infinitum_.
In such cases, moreover, the ground of the heavens, as seen between the
stars, is for the most part perfectly dark, which again would not be the
case if innumerable multitudes of stars, too minute to be individually
discernible, existed beyond. In other regions we are presented with the
phenomenon of an almost uniform degree of brightness of the individual
stars, accompanied with a very even distribution of them over the ground
of the heavens, both the larger and smaller magnitudes being strikingly
deficient. In such cases it is equally impossible not to perceive that
we are looking through a sheet of stars nearly of a size and of no great
thickness compared with the distance which separates them from us. Were
it otherwise we should be driven to suppose the more distant stars were
uniformly the larger, so as to compensate by their intrinsic brightness
for their greater distance, a supposition contrary to all probability.
In others again, and that not infrequently, we are presented with a
double phenomenon of the same kind--_viz._, a tissue, as it were, of
large stars spread over another of very small ones, the intermediate
magnitudes being wanting, and the conclusion here seems equally evident
that in such cases we look through two sidereal sheets separated by a
starless interval.
Throughout by far the larger portion of the extent of the Milky Way in
both hemispheres the general blackness of the ground of the heavens on
which its stars are projected, and the absence of that innumerable
multitude and excessive crowding of the smallest visible magnitudes, and
of glare produced by the aggregate light of multitudes too small to
affect the eye singly, which the contrary supposition would appear to
necessitate, must, we think, be considered unequivocal indications that
its dimensions, _in directions where those conditions obtain_, are not
only not infinite, but that the space-penetrating power of our
telescopes suffices fairly to pierce through and beyond it.
It is but right, however, to warn our readers that this conclusion has
been controverted, and that by an authority not lightly to be put aside,
on the ground of certain views taken by Olbers as to a defect of perfect
transparency in the celestial spaces, in virtue of which the light of
the more distant stars is enfeebled more than in proportion to their
distance. The extinction of light thus originating proceeding in
geometrical ratio, while the distance increases in arithmetical, a
limit, it is argued, is placed to the space-penetrating power of
telescopes far within that which distance alone, apart from such
obscuration, would assign.
It must suffice here to observe that the objection alluded to, if
applicable to any, is equally so to every part of the galaxy. We are not
at liberty to argue that at one part of its circumference our view is
limited by this sort of cosmical veil, which extinguishes the smaller
magnitudes, cuts off the nebulous light of distant masses, and closes
our view in impenetrable darkness; while at another we are compelled, by
the clearest evidence telescopes can afford, to believe that star-strewn
vistas _lie open_, exhausting their powers and stretching out beyond
their utmost reach, as is proved by that very phenomenon which the
existence of such a veil would render impossible--_viz._, infinite
increase of number and diminution of magnitude, terminating in complete
irresolvable nebulosity.
Such is, in effect, the spectacle afforded by a very large portion of
the Milky Way in that interesting region near its point of bifurcation
in Scorpio, where, through the hollows and deep recesses of its
complicated structure, we behold what has all the appearance of a wide
and indefinitely prolonged area strewed over with discontinuous masses
and clouds of stars, which the telescope at last refuses to analyse.
Whatever other conclusions we may draw, this must anyhow be regarded as
the direction of the greatest linear extension of the ground-plan of the
galaxy. And it would appear to follow also that in those regions where
that zone is clearly resolved into stars well separated and _seen
projected on a black ground_, and where, by consequence, it is certain,
if the foregoing views be correct, that we look out beyond them into
space, the smallest visible stars appear as such not by reason of
excessive distance, but of inferiority of size or brightness.
_III.--Variable, Temporary and Binary Stars_
Wherever we can trace the law of periodicity we are strongly impressed
with the idea of rotatory or orbitual motion. Among the stars are
several which, though in no way distinguishable from others by any
apparent change of place, nor by any difference of appearance in
telescopes, yet undergo a more or less regular periodical increase and
diminution of lustre, involving in one or two cases a complete
extinction and revival. These are called periodic stars. The longest
known, and one of the most remarkable, is the star _Omicron_ in the
constellation Cetus (sometimes called Mira Ceti), which was first
noticed as variable by Fabricius in 1596. It appears about twelve times
in eleven years, remains at its greatest brightness about a fortnight,
being then on some occasions equal to a large star of the second
magnitude, decreases during about three months, till it becomes
completely invisible to the naked eye, in which state it remains about
five months, and continues increasing during the remainder of its
period. Such is the general course of its phases. But the mean period
above assigned would appear to be subject to a cyclical fluctuation
embracing eighty-eight such periods, and having the effect of gradually
lengthening and shortening alternately those intervals to the extent of
twenty-five days one way and the other. The irregularities in the degree
of brightness attained at the maximum are also periodical.
Such irregularities prepare us for other phenomena of stellar variation
which have hitherto been reduced to no law of periodicity--the phenomena
of temporary stars which have appeared from time to time in different
parts of the heavens blazing forth with extraordinary lustre, and after
remaining awhile, apparently immovable, have died away and left no
trace. In the years 945, 1264, and 1572 brilliant stars appeared in the
region of the heavens between Cepheus and Cassiopeia; and we may suspect
them, with Goodricke, to be one and the same star with a period of 312,
or perhaps 156 years. The appearance of the star of 1572 was so sudden
that Tycho Brahe, a celebrated Dutch astronomer, returning one evening
from his laboratory to his dwellinghouse, was surprised to find a group
of country people gazing at a star which he was sure did not exist half
an hour before. This was the star in question. It was then as bright as
Sirius, and continued to increase till it surpassed Jupiter when
brightest, and was visible at midday. It began to diminish in December
of the same year, and in March 1574 had entirely disappeared.
In 1803 it was announced by Sir William Herschel that there exist
sidereal systems composed of two stars revolving about each other in
regular orbits, and constituting which may be called, to distinguish
them from double stars, which are only optically double, binary stars.
That which since then has been most assiduously watched, and has offered
phenomena of the greatest interest, is _gamma Virginis_. It is a star of
the vulgar third magnitude, and its component individuals are very
nearly equal, and, as it would seem, in some slight degree variable. It
has been known to consist of two stars since the beginning of the
eighteenth century, the distance being then between six and seven
seconds, so that any tolerably good telescope would resolve it. When
observed by Herschel in 1780 it was 5.66 seconds, and continued to
decrease gradually and regularly, till at length, in 1836, the two stars
had approached so closely as to appear perfectly round and single under
the highest magnifying power which could be applied to most excellent
instruments--the great refractor of Pulkowa alone, with a magnifying
power of a thousand, continuing to indicate, by the wedge-shaped form of
the disc of the star, its composite nature.
By estimating the ratio of its length to its breadth, and measuring the
former, M. Struve concludes that at this epoch the distance of the two
stars, centre from centre, might be stated at .22 seconds. From that
time the star again opened, and is now again a perfectly easily
separable star. This very remarkable diminution, and subsequent
increase, of distance has been accompanied by a corresponding and
equally remarkable increase and subsequent diminution of relative
angular motion. Thus in 1783 the apparent angular motion hardly amounted
to half a degree per annum; while in 1830 it had decreased to 5 degrees,
in 1834 to 20 degrees, in 1835 to 40 degrees, and about the middle of
1836 to upwards of 70 degrees per annum, or at the rate of a degree in
five days.
This is in entire conformity with the principles of dynamics, which
establish a necessary connection between the angular velocity and the
distance, as well in the apparent as in the real orbit of one body
revolving about another under the influence of mutual attraction; the
former varying inversely as the square of the latter, in both orbits,
whatever be the curve described and whatever the law of the attractive
force.
It is not with the revolutions of bodies of a planetary or cometary
nature round a solar centre that we are concerned; it is that of sun
round sun--each perhaps, at least in some binary systems, where the
individuals are very remote and their period of revolution very long,
accompanied by its train of planets and their satellites, closely
shrouded from our view by the splendour of their respective suns, and
crowded into a space bearing hardly a greater proportion to the enormous
interval which separates them than the distances of the satellites of
our planets from their primaries bear to their distances from the sun
itself.
A less distinctly characterised subordination would be incompatible with
the stability of their systems and with the planetary nature of their
orbits. Unless close under the protecting wing of their immediate
superior, the sweep of their other sun, in its perihelion passage round
their own, might carry them off or whirl them into orbits utterly
incompatible with conditions necessary for the existence of their
inhabitants.
_IV.--The Nebulæ_
It is to Sir William Herschel that we owe the most complete analysis of
the great variety of those objects which are generally classed as
nebulæ. The great power of his telescopes disclosed the existence of an
immense number of these objects before unknown, and showed them to be
distributed over the heavens not by any means uniformly, but with a
marked preference to a certain district extending over the northern pole
of the galactic circle. In this region, occupying about one-eighth of
the surface of the sphere, one-third of the entire nebulous contents of
the heavens are situated.
The resolvable nebulæ can, of course, only be considered as clusters
either too remote, or consisting of stars intrinsically too faint, to
affect us by their individual light, unless where two or three happen to
be close enough to make a joint impression and give the idea of a point
brighter than the rest. They are almost universally round or oval, their
loose appendages and irregularities of form being, as it were,
extinguished by the distance, and only the general figure of the
condensed parts being discernible. It is under the appearance of objects
of this character that all the greater globular clusters exhibit
themselves in telescopes of insufficient optical power to show them
well.
The first impression which Halley and other early discoverers of
nebulous objects received from their peculiar aspect was that of a
phosphorescent vapour (like the matter of a comet's tail), or a gaseous
and, so to speak, elementary form of luminous sidereal matter. Admitting
the existence of such a medium, Sir W. Herschel was led to speculate on
its gradual subsidence and condensation, by the effect of its own
gravity, into more or less regular spherical or spheroidal forms, denser
(as they must in that case be) towards the centre.
Assuming that in the progress of this subsidence local centres of
condensation subordinate to the general tendency would not be wanting,
he conceived that in this way solid nuclei might arise whose local
gravitation still further condensing, and so absorbing the nebulous
matter each in its immediate neighbourhood, might ultimately become
stars, and the whole nebula finally take on the state of a cluster of
stars.
Among the multitude of nebulæ revealed by his telescope every stage of
this process might be considered as displayed to our eyes, and in every
modification of form to which the general principle might be conceived
to apply. The more or less advanced state of a nebula towards its
segregation into discrete stars, and of these stars themselves towards a
denser state of aggregation round a central nucleus, would thus be in
some sort an indication of age.