a tendency to aggregation into streams, let us consider if, in any
other parts of the heavens, similar traces may not be observable. We
traced a stream from Scorpio toward Orion, and so round in a spiral
to the Nubeculæ. Let us now return to Scorpio, and trace the stream
(if any appear) in the contrary direction. Now, although over the
Northern Hemisphere star-streams are not nearly so marked as over the
Southern, yet there appears a decided indication of stream-formation
along Serpens and Corona over the group on the left hand of Boötes
to the Great Bear. A branch of this stream, starting from Corona,
traverses the body of Boötes, Berenice’s Hair, the Sickle in Leo,
the Beehive in Cancer, passing over Castor and Pollux in Gemini,
toward Capella. A branch from the feet of Gemini passes over Canis
Minor, along Hydra (so named doubtless from the obvious tendency to
stream-formation along the length of this constellation), and so to
the right claw of Scorpio.
One other remarkable congeries of stars is to be mentioned. From the
northern part of the Milky Way there will be noticed a projection
toward the North Pole from the head of Cepheus. This projection
seems to merge itself in a complex convolution of stars, forming the
ancient constellation Draco, which doubtless included the ancient
(but probably less ancient) constellation Ursa Minor. After following
the convolutions of Draco, we reach the bright stars Alwaid and
Etanin (Beta and Gamma) of this constellation, and thence the stream
passes to Lyra, where it seems to divide into two, one passing
through Hercules, the other along Aquila, curving into the remarkable
group Delphinus.
The streams here considered include every conspicuous star in the
heavens. But the question will at once suggest itself, whether we
have not been following a merely fanciful scheme, whether all these
apparent streams might not very well be supposed to result from
mere accident. Now, from experiments I have made, I am inclined to
believe that in any chance distribution of points over a surface, the
chance against the occurrence of a single stream as marked as that
which lies (in part) along the back of Grus, or as the curved stream
of bright stars along Scorpio, is very great indeed; I am certain
that the occurrence of _many_ such streams is altogether improbable.
And wherever one observes a tendency to stream-formation in objects
apparently distributed wholly by chance, one is led to suspect,
and thence often to detect, the operation of law. I will take an
illustration, very homely perhaps, but which will serve admirably to
explain my meaning. In soapy water, left in a basin after washing,
there will often be noticed a tendency to the formation of spiral
whorls on the surface. In other cases there may be no definite
spirality, but still a tendency to stream-formation. Now, in this
case, it is easy to see that the curved bottom of the basin has
assisted to generate streams in the water, either circulating in one
direction or opposing and modifying each other’s effects, according
to the accidental character of the disturbance given to the water in
the process of washing.[21] Here, of course, there can be no doubt
of the cause of the observed phenomena; and I believe that in every
case in which even a single marked stream is seen in any congeries
of spots or points, a little consideration will suggest a regulating
cause to which the peculiarity may be referred.
It is hardly necessary to say that, if the stream-formation I
have indicated is considered to be really referable to systematic
distribution, the theory of a stratum of stars distributed with any
approach to uniformity, either as respects magnitude or distance,
must be abandoned. It seems to me to be also quite clear that the
immense extent of the Galaxy, as compared with the distances of the
lucid stars from us, could no longer be maintained. On this last
point we have other evidence, which I will briefly consider.
First, there is the evidence afforded by clusterings in the Milky
Way. I will select one which is well known to every telescopist,
namely, the magnificent cluster on the sword-hand of Perseus. No
doubt can be entertained that this cluster belongs to the galactic
system, that is, that it is not an _external_ cluster: the evidence
from the configuration of the spot and from the position it occupies
is conclusive on this point. Now, within this spot, which shows
no stars to the naked eye, a telescope of moderate power reveals a
multitude of brilliant stars, the brightest of which are of about
the seventh magnitude. Around these there still appears a milky
unresolved light. If a telescope of higher power be applied, more
stars are seen, and around these there still remains a nebulous
light. Increase power until the whole field blazes with almost
unbearable light, yet still there remains an unresolved background.
“The illustrious Herschel,” says Professor Nichol, “penetrated, on
one occasion, into this spot, until he found himself among depths
whose light could not have reached him in much less than 4,000 years;
no marvel that he withdrew from the pursuit, conceiving that such
abysses must be endless.” It is precisely this view that I wish to
controvert. And I think it is no difficult matter to show at least
a probability against the supposition that the milky light in the
spot is removed at a vast distance behind the stars of the seventh
magnitude seen in the same field.
The supposition amounts, in fact, to the highly improbable view that
we are looking here at a range of stars extending in a cylindrical
stratum directly from the eye—a stratum whose section is so very
minute in comparison with its breadth that, whereas the whole
field within which the spot is included is but small, the distance
separating the nearest parts of the group from the furthest is
equivalent to the immense distance supposed to separate the sphere
of seventh magnitude stars from the extreme limits of our Galaxy.
And the great improbability of this view is yet further increased
when it is observed that within this spot there is to be seen a very
marked tendency to the formation of minor streams, around which the
milky light seems to cling. It seems, therefore, wholly improbable
that the cluster really has that indefinite longitudinal extension
suggested by Professor Nichol. In fact, it becomes practically
certain that the milky light comes from orbs really smaller than the
seventh magnitude stars in the same field, and clustering round these
stars in reality as well as in appearance.
The observations applied to this spot may be extended to all clusters
of globular form; and where a cluster is not globular in form,
but exhibits, on examination, either (1) any tendency within its
bounds to stream-formation, or (2) a uniform increase in density as
we proceed from any part of the circumference toward the centre,
it appears wholly inconceivable that the apparent cluster is not
really a cluster, but a long range of stars extending to an enormous
distance directly from the eye of the observer. When, in such a case,
many stars of the higher magnitudes appear within the cluster, we
seem compelled to admit the probability that they belong to it; and,
in any case, we can not assign to the furthest parts of the cluster
a distance greatly exceeding (_proportionally_) that of the nearest
parts.
Of a like character is the evidence afforded by narrow streams and
necks within the Galaxy itself. If we consider the convolutions over
Scorpio, it will seem highly improbable that in each of these we
see, not a real convolution or stream, but the edge of a _roll_ of
stars. For instance, if a spiral roll of paper be viewed from any
point taken at random, the chances are thousands to one against its
appearing as a spiral _curve_, and, of course, the chance against
several such rolls so appearing is very much greater. The fact that
we are assumed to be not very far from the supposed mean plane of the
Milky Way would partly remove the difficulty here considered, if it
were not that the thickness and extent of the stratum, as compared
with the distances of the lucid stars, must necessarily be supposed
very great, on the assumption of any approach to uniformity of
distribution.
Evidence pointing the same way is afforded by circular apertures
in the Galaxy, or indeed by apertures of other forms. Another
peculiarity of these cavities is also noticeable; whereas on the
borders of every one there are many lucid stars, or in some cases two
or three very bright stars, _within_ the cavity there is a marked
paucity of stars. This phenomenon seems to indicate a much closer
connection between the brighter stars and the milky light beyond
than is supposed on the stratum theory. One can hardly conceive the
phenomenon to be wholly accidental.
There are some other points on which I fain would dwell, but space
will not permit me. I will merely note that there are peculiarities
in the distribution of red double and multiple stars, in the position
in which temporary stars have made their appearance, and in the
distribution of nebulæ, which seem very worthy of notice.
One point, however, immediately connected with my subject remains
to be mentioned. I have traced streams of stars _more_ conspicuous
than those forming the Milky Way. We have also evidence of streams
of light yet more delicate and evanescent than the light of our own
Galaxy. In Sir John Herschel’s great work on the southern skies,
he notes the frequent recurrence of “an exceedingly delicate and
uniform dotting, or _stippling_, of the field of view by points of
light too small to admit of any one being steadily or fully examined,
and too numerous for counting, were it possible so to view them.”
In thirty-seven places he detected this remarkable and significant
phenomenon; a phenomenon so faint that he says, “The idea of illusion
has continually arisen subsequently”; an idea well befitting the
modesty of the philosophic observer, but which those who appreciate
Sir John Herschel’s skill as an observer will be very unwilling to
accept. As Professor Nichol remarks, “It is enough to read from
Herschel’s notebook—‘I feel satisfied the stippling is no illusion,
for its dark mottling moves with the stars as I move the tube to and
fro’—to feel convinced that the phenomenon is real.” Now a remarkable
fact connected with those observations is, that when Sir J. Herschel
marked down in a star-chart the places in which he had detected
this nebulous appearance, he found that, “with the exception of
_three_ which appeared outlying and disconnected, they formed several
_distinct but continuous streams_.”
FOOTNOTES:
[20] Field means the actual space covered by the lens.—E. S.
[21] Sometimes a singular regularity of curvature is noticed, and
a spiral is formed closely resembling in configuration some of the
great spiral nebulæ, as drawn by Lord Rosse, so that one is tempted
to see in the centrifugal tendency of the disturbed water, and the
centripetal effects caused by reflection from the basin’s surface,
causes which may in some sense illustrate the laws operating in wider
domains of space.
THE MAGELLANIC CLOUDS—ZODIACAL LIGHT—STAR GROUPS.—AMÉDÉE GUILLEMIN
When we look on the region of the celestial vault which surrounds
the South Pole, we can not help being struck with the contrast
presented by the small quantity of stars which it contains, with the
brilliant zone which borders the Milky Way, from Orion and Argo to
the Centaur, passing by the Southern Cross. One solitary star of the
first magnitude, Achernar, more distant from the pole than are the
beautiful stars of the Centaur and of the Cross, shines in this part
of the sky.
But even this circumstance renders the singular aspect of the two
nebulous spots, which seem two detached pieces of the great galactic
zone, still more striking. These half-stellar, half-nebulous systems,
unequal in magnitude and brightness, but easily seen with the naked
eye on a clear, moonless night, are situated, one, the larger and
more brilliant, between the pole and Canopus, in the constellation
of Doradus; the other, the smaller and less brilliant, ordinarily
visible during the full moon, in Hydrus, between Achernar and the
pole.
Both are known by astronomers and navigators under the name of “Cape
Clouds,” or again, “Magellanic Clouds.” And, to distinguish them, we
have again the Great Cloud (_Nebecula Major_) and the Small Cloud
(_Nebecula Minor_).
The Clouds of Magellan are distinguished from all other nebulæ by
their great apparent dimensions, and by their physical structure;
this last character distinguishes them from most of the branches and
offshoots of the Milky Way, with which, we may also add, they do not
appear connected in any way.
The Great Cloud extends over a space which embraces not less than
forty-two square degrees—about two hundred times the apparent surface
of the lunar disk. The Small Cloud occupies in extent four times less
than the other; according to Humboldt, it is surrounded “with a kind
of desert,” where, it is true, shines the magnificent stellar cluster
of Toucan. If the exterior aspect of these two remarkable nebulæ,
and their situation in a celestial region poor in stars, give to the
southern sky a peculiar appearance, their real structure makes them
one of the wonders of the heavens.
In the Great Cloud, Herschel has counted 582 single stars, among
which one only is of the fifth magnitude; six others are of the order
immediately inferior, and would doubtless be visible to the naked eye
if their light were not effaced by the general glare.
In the Small Cloud, the single stars are proportionally more
numerous, since 200 have been counted, among which three are of the
sixth magnitude, while it only includes thirty-seven of the nebulæ
and seven star-clusters. These immense aggregations, the elements of
which are themselves swarms of suns, remind us of the largest, in
appearance at least, of all the clusters which the eye contemplates
in the depths of the sky—the Milky Way.
In the evenings, about the time of the vernal equinox—in March and
April, when in our climate the twilight is of short duration—if we
examine the horizon toward the west, a little after sunset, we may
perceive a faint light that rises in the form of a cone among the
starry constellations.
This is what astronomers call the Zodiacal Light. Those unfamiliar
with it, or little accustomed to the ordinary aspect of the sky,
might confuse the glimmering either with the Milky Way or with
the ordinary twilight, or even with an aurora. But, with a little
attention, it is impossible to mistake it.
The triangular form of this luminous cone, its elevation and its
inclined position to the horizon, make it a thing apart, and one
eminently deserving particular mention.
As the days lengthen, and with them the duration of twilight, the
Zodiacal Light disappears; it becomes invisible, at least in our
climate. But it may again be seen in the morning, in the east, about
the time of the autumnal equinox, in September and October, when
the dawn has an equally short duration—again, however, to disappear
during the period of long nights and long twilights.
It is needless to add that the sky must be clear and the night
moonless for observations of the Zodiacal Light to be possible.
Among the explanations that have been given, the most probable one
is that which likens the Zodiacal Light to a flattened nebulous ring
surrounding the sun at some distance. It is to be remarked that the
direction of the axis of the cone, or of the pyramid, prolonged
below the horizon, always passes through the sun.
It was believed at first that this direction precisely coincided with
the solar equator; but it seems more certain that it coincides with
the plane of the earth’s orbit, or the ecliptic.
Now, what is the nature of this luminous mass? Must it be considered
as a zone of vapors thrown off by the sun, when in the process of
consolidation, when our central star passed from a nebulous state to
that of a condensed fluid sphere? This was the opinion of Laplace.
Another hypothesis, also connected with the first, is that the
Zodiacal Light is formed of myriads of solid particles, analogous
to the aerolites, possessing a general movement, but traveling
separately around the focus of our solar world. The light of the
ring would be thus produced by the accumulation of this multitude of
brilliant points, reflecting toward us the light borrowed by each of
them from the sun.
This explanation accounts for the intensity of the Zodiacal Light at
different epochs; it would suffice to admit that the condensation of
the particles or the density of the ring is not the same throughout
its extent, and that its movement of circulation round the sun
presents successively different parts to the earth. In this case, it
becomes a question whether this lenticular ring of matter is distinct
from the zone of aerolites.
Lastly, some astronomers regard the Zodiacal Light as a vaporous ring
which belongs to the earth, surrounding it at some distance. But
this is an opinion which appears somewhat wild, and is utterly at
variance with observation.
Are the stars that are visible to the naked eye spread orderless on
the celestial vault? or is there not between those apparently most
closely connected some real or physical connection which requires us
to rank them in natural groups?
These questions have been already partly solved by what is known of
the double and multiple star systems. Soon, exploring the regions of
the sky visible by means of the telescope, we shall have to pass in
review a multitude of stellar associations, in which suns are found
so compact and so numerous, and the form of the groups so regular,
that it is impossible to deny their reciprocal dependence.
But long before the discovery of these islands, these archipelagos
as worlds, scattered with such astonishing profusion over the
infinite, the naked eye had already distinguished a certain number of
groups, the stars composing which were so near together that it was
impossible to doubt their physical connection.
Such, for example, is the group of the Pleiades. Such, again, are
the groups known under the names of the Hyades, of Præsepe, and of
Berenice’s Hair. All are visible to the naked eye, and good eyes
distinguish without difficulty the principal stars of the first-named
groups. The Pleiades are situated in the constellation of the Bull,
which we can distinguish so easily to the northwest of Orion and
Aldebaran.
Of about eighty stars which form the group of the Pleiades, six are
visible without the help of telescopes. Formerly, the Latin poet
tells us, seven were counted, which may be held to prove that one
of them is variable, and has diminished in brightness, or else has
disappeared.
The most brilliant, Alcyone, is of the third magnitude; Electra and
Atlas are of the fourth; Merope, Maïa, and Taygete of the fifth.
Three others again have received particular names, although they are
below the limit of ordinary vision; these are Pleione, Celeno, and
Asterope, from the sixth to the eighth magnitude. All the others are
only visible by the aid of a telescope; but with an ordinary glass
it is possible to distinguish a large number. The Pleiades are known
under the name of the Hen-coop, doubtless because Alcyone appears in
the group as a hen surrounded with her chickens.
The Hyades, which are near the Pleiades, form a less numerous and
more scattered group. The bright light of Aldebaran, which is, as
is known, of the first magnitude, renders them more difficult to
distinguish with the naked eye.
They appear in the rainy season. Hence their name of Hyades, from the
Greek word which signifies to rain.
The connection of the stars which compose this group is not so
striking as in the case of the Pleiades. Nevertheless, it seems
difficult to admit that they are quite independent of each other’s
attraction. In examining the position of these two groups in the
vicinity of the Milky Way, and observing that both are situated
in the prolongation of a branch of the great zone, we are almost
entitled to consider them as two clusters of stars, belonging to the
immense stellar stratum which surrounds us, and in the midst of which
the sun himself is placed.
In Berenice’s Hair, most of the stars are visible to the naked eye,
and are perfectly distinguished in the sky, a little to the east of
the Lion. No very brilliant star in the vicinity inconveniences the
eye by effacing their light.
The next group is situated in the Crab, and is known under the
name of Præsepe: it is visible to the unassisted sight; but it is
impossible to distinguish the separate stars without the help of
a telescope. Nevertheless, an instrument of moderate power easily
separates them.
The groups which we have just described form a transition between
the stars scattered over the celestial vault and the more condensed
clusters, the undefined aspect of which caused them formerly to be
designated under the general name of nebulæ.
Doubtless, if we could place ourselves in space, and contemplate from
a sufficiently distant standpoint the whole of the stars which appear
to us isolated, we should see them condensed into one or several
distinct groups, analogous to those of the Pleiades; while, were we
to penetrate into the midst of one of those compact clusters, we
should see the stars of which it is formed separated and scattered
over the celestial vault in such a way as to give it the aspect of
our own heavens.
THE NEBULÆ AND SWARMS OF SUNS.—J. E. GORE
We will now consider the nebulæ, properly so called, that is to say,
objects which the spectroscope shows to consist of glowing gas. These
are sometimes large and irregular in form, like the great nebula
in the “Sword” of Orion, sometimes with spiral convolutions, and
sometimes of a definite shape, like the planetary and annular nebulæ.
Of the large and irregular nebulæ, one of the most remarkable is that
known as “the great nebula in Orion.” It surrounds the multiple star,
Theta Orionis. It is a curious fact that it escaped the searching eye
of Galileo, although he gave special attention to the constellation
of Orion, for even with a good opera-glass a nebulous gleam is
distinctly visible round the central star of the “Sword.” The nebula
seems to have been discovered by Cysat, a Swiss astronomer, in the
year 1618, and it was sketched by Huygens in 1656. It has been called
the “fish-mouth” nebula, from the fancied resemblance of the centre
portion to the mouth of a fish. A number of small stars are visible
over the surface of the nebula, and at one time Lord Rosse thought it
showed indications of resolution into stars when examined with his
giant telescope; but this is now known to have been a mistake, for
Dr. Huggins finds, with the spectroscope, that it consists of nothing
but glowing gas.
The brightest line in the nebular spectrum—the “chief nebular
line,” as it is called—has not yet been identified with that of any
terrestrial substance.
Mr. W. H. Pickering and Dr. Max Wolf have photographed another
nebula surrounding the star Zeta Orionis—the southern star of the
“Belt,” which seems to be connected with the nebula in the “Sword”;
and Professor Barnard, using the “lens of a cheap oil lantern” of
1½ inches aperture and 3½ inches focal length, has photographed “an
enormous curved nebulosity” stretching over nearly the whole of the
constellation of Orion, and involving the “great nebula.”
Professor Keeler found, with the spectroscope, that the Orion nebula
is apparently receding from the earth at the rate of nearly eleven
miles a second, but this motion may be, in part at least, due to the
sun’s motion in space in the opposite direction. Professor Pickering
considers that the parallax of the nebula is probably not more than
0.″003, which corresponds to a thousand years’ journey for light!