in any attempt to interpret the phenomena of the new star) that when
shining as a star, the new orb, so far as this unusual lustre was
concerned, was of sunlike dimensions. For we cannot tell whether the
surface which gave the strong light was less or greater than, or equal
to, that which is now shining with monochromatic light. Very likely,
if we had been placed where we could have seen the full dimensions of
the planetary nebula as it at present exists, we should have found
only its nuclear part glowing suddenly with increased lustre, which,
after very rapidly attaining its maximum, gradually died out again,
leaving the nebula as it had been before. But that the mass now shining
with monochromatic light is, I will not say enormously large, but
of exceedingly small mean density, so that it is enormously large
compared with the dimensions it would have if its entire substance were
compressed till it had the same mean density as our own sun, must be
regarded as, to all intents and purposes, certain.
We certainly have not here, then, the case of a sun which has grown
old and dead and dark save at the surface, but within whose interior
fire has still remained, only waiting some disturbing cause to enable
it for a while to rush forth. If we could suppose that in such a case
there _could_ be such changes as the spectroscope has indicated—that
the bright lines of the gaseous outbursting matter would, during the
earlier period of the outburst, show on a bright continuous background,
due to the glowing lips of the opening through which the matter had
rushed, but later would shine alone, becoming also fewer in number,
till at last only one was left,—we should find ourselves confronted
with the stupendous difficulty that that single remaining line is the
bright line of the planetary and other gaseous nebulæ. Any hypothesis
accounting for its existence in the spectrum of the faint blue starlike
object into which the star in Cygnus has faded ought to be competent
to explain its existence in the spectrum of those nebulæ. But _this_
hypothesis certainly does not so explain its existence in the nebular
spectrum. The nebulæ cannot be suns which have died out save for the
light of gaseous matter surrounding them, for they are millions, or
rather millions of millions, of times too large. If, for instance, a
nebula, like the one above described as lying near the southernmost
Pointer, were a mass of this kind, having the same mean density as
the sun, and lying only at the distance of the nearest of the stars
from us, then not only would it have the utterly monstrous dimensions
stated by Sir J. Herschel, but it would in the most effective way
perturb the whole solar system. With a diameter exceeding seven times
that of the orbit of Neptune, it would have a volume, and therefore a
mass, exceeding our sun’s volume and mass more than eleven millions
of millions of times. But its distance on this assumption would be
only about two hundred thousand times the sun’s, and its attraction
reduced, as compared with his, on this account only forty thousand
millions of times. So that its attraction on the sun and on the earth
would be greater than his attraction on the earth, in the same degree
that eleven millions are greater than forty thousand—or two hundred
and seventy-five times. The sun, despite his enormous distance from
such a mass, would be compelled to fall very quickly into it, unless
he circuited (with all his family) around it in about one-sixteenth of
a year, which most certainly he does not do. Nor would increasing the
distance at which we assume the star to lie have any effect to save
the sun from being thus perturbed, but the reverse. If we double for
instance our estimate of the nebula’s distance, we increase eightfold
our estimate of its mass, while we only diminish its attraction on
our sun fourfold on account of increased distance; so that now its
attraction on our sun would be one-fourth its former attraction
multiplied by eight, or twice our former estimate. We cannot suppose
the nebula to be much nearer than the nearest star. Again, we cannot
suppose that the light of these gaseous nebulæ comes from some bright
orb within them of only starlike apparent dimensions, for in that case
we should constantly recognize such starlike nucleus, which is not the
case. Moreover, the bright-line spectrum from one of these nebulæ comes
from the whole nebula, as is proved by the fact that if the slit of the
spectroscope be opened it becomes possible to see three spectroscopic
images of the nebula itself, not merely the three bright lines.
So that, if we assume the so-called star in Cygnus to be now like other
objects giving the same monochromatic spectrum—and this seems the only
legitimate assumption—we are compelled to believe that the light now
reaching us comes from a nebulous mass, not from the faintly luminous
envelope of a dead sun. Yet, remembering that when at its brightest
this orb gave a spectrum resembling in general characteristics that of
other stars or suns, and closely resembling even in details that of
stars like Gamma Cassiopeiæ, we are compelled by parity of reasoning
to infer that when the so-called new star was so shining, the greater
part of its light came from a sunlike mass. Thus, then, we are led
to the conclusion that in the case of this body we have a nucleus or
central mass, and that around this central mass there is a quantity of
gaseous matter, resembling in constitution that which forms the bulk
of the other gaseous nebulæ. The denser nucleus ordinarily shines with
so faint a lustre that the continuous spectrum from its light is too
faint to be discerned with the same spectroscopic means by which the
bright lines of the gaseous portion are shown; and the gaseous portion
ordinarily shines with so faint a lustre that its bright lines would
not be discernible on the continuous background of a stellar spectrum.
Through some cause unknown—possibly (as suggested in an article on
the earlier history of this same star in my “Myths and Marvels of
Astronomy”) the rush of a rich and dense flight of meteors upon the
central mass—the nucleus was roused to a degree of heat far surpassing
its ordinary temperature. Thus for a time it glowed as a sun. At the
same time the denser central portions of the nebulous matter were
also aroused to intenser heat, and the bright lines which ordinarily
(and certainly at present) would not stand out bright against the
rainbow-tinted background of a stellar spectrum, showed brightly upon
the continuous spectrum of the new star. Then as the rush of meteors
upon the nucleus and on the surrounding nebulous matter ceased—if
that be the true explanation of the orb’s accession of lustre—or as
the cause of the increase of brightness, whatever that cause may have
been, ceased to act, the central orb slowly returned to its usual
temperature, the nebulous matter also cooling, the continuous spectrum
slowly fading out, the denser parts of the nebulous matter exercising
also a selective absorption (explaining the bands seen in the spectrum
at this stage) which gradually became a continuous absorption—that
is, affected the entire spectrum. Those component gases, also, of the
nebulous portion which had for a while been excited to sufficient heat
to show their bright lines, cooled until their lines disappeared, and
none remained visible except for a while the three usual nebular lines,
and latterly (owing to still further cooling) only the single line
corresponding to the monochromatic light of the fainter gaseous nebulæ.
_STAR-GROUPING, STAR-DRIFT, AND STAR-MIST._
_A Lecture delivered at the Royal Institution on May 6, 1870._
Nearly a century has passed since the greatest astronomer the world has
ever known—the Newton of observational astronomy, as he has justly
been called by Arago—conceived the daring thought that he would gauge
the celestial depths. And because in his day, as indeed in our own,
very little was certainly known respecting the distribution of the
stars, he was forced to found his researches upon a guess. He supposed
that the stars, not only those visible to the naked eye, but all that
are seen in the most powerful telescopes, are suns, distributed with
a certain general uniformity throughout space. It is my purpose to
attempt to prove that—as Sir Wm. Herschel was himself led to suspect
during the progress of his researches—this guess was a mistaken one;
that but a small proportion of the stars can be regarded as real suns;
and that in place of the uniformity of distribution conceived by Sir
Wm. Herschel, the chief characteristic of the sidereal system is
_infinite variety_.
In order that the arguments on which these views are based may be
clearly apprehended, it will be necessary to recall the main results of
Sir Wm. Herschel’s system of star-grouping.
Directing one of his 20-feet reflectors to different parts of the
heavens, he counted the stars seen in the field of view. Assuming that
the telescope really reached the limits of the sidereal system, it is
clear that the number of stars seen in any direction affords a means
of estimating the relative extension of the system in that direction,
provided always that the stars are really distributed throughout the
system with a certain approach to uniformity. Where many stars are
seen, there the system has its greatest extension; where few, there the
limits of the system must be nearest to us.
Sir Wm. Herschel was led by this process of star-grouping to the
conclusion that the sidereal system has the figure of a cloven disc.
The stars visible to the naked eye lie far within the limits of
this disc. Stars outside the relatively narrow limits of the sphere
including all the visible stars, are separately invisible. But where
the system has its greatest extension these orbs produce collectively
the diffused light which forms the Milky Way.
Sir John Herschel, applying a similar series of researches to the
southern heavens, was led to a very similar conclusion. His view of the
sidereal system differs chiefly in this respect from his father’s, that
he considered the stars within certain limits of distance from the sun
to be spread less richly through space than those whose united lustre
produces the milky light of the galaxy.
Now it is clear that if the supposition on which these views are based
is just, the three following results are to be looked for.
In the first place, the stars visible to the naked eye would be
distributed with a certain general uniformity over the celestial
sphere; so that if on the contrary we find certain extensive regions
over which such stars are strewn much more richly than over the rest of
the heavens, we must abandon Sir Wm. Herschel’s fundamental hypothesis
and all the conclusions which have been based upon it.
In the second place, we ought to find no signs of the aggregation
of lucid stars into streams or clustering groups. If we should find
such associated groups, we must abandon the hypothesis of uniform
distribution and all the conclusions founded on it.
Thirdly, and most obviously of all, the lucid stars ought not to be
associated in a marked manner with the figure of the Milky Way. To
take an illustrative instance. When we look through a glass window at
a distant landscape we do not find that the specks in the substance
of the glass seem to follow the outline of valleys, hills, trees, or
whatever features the landscape may present. In like manner, regarding
the sphere of the lucid stars as in a sense the window through which we
view the Milky Way, we ought not to find these stars, which are so near
to us, associated with the figure of the Milky Way, whose light comes
from distances so enormously exceeding those which separate us from the
lucid stars. Here again, then, if there should appear signs of such
association, we must abandon the theory that the sidereal system is
constituted as Sir Wm. Herschel supposed.
It should further be remarked that the three arguments derived from
these relations are independent of each other. They are not as three
links of a chain, any one of which being broken the chain is broken.
They are as three strands of a triple cord. If one strand holds, the
cord holds. It may be shown that all three are to be trusted.
It is not to be expected, however, that the stars as actually seen
should exhibit these relations, since far the larger number are but
faintly visible; so that the eye would look in vain for the signs of
law among them, even though law may be there. What is necessary is that
maps should be constructed on a uniform and intelligible plan, and that
in these maps the faint stars should be made bright, and the bright
stars brighter.
The maps exhibited during this discourse [since published as my
“Library Atlas”] have been devised for this purpose amongst others.
There are twelve of them, but they overlap, so that in effect each
covers a tenth part of the heavens. There is first a north-polar map,
then five maps symmetrically placed around it; again, there is a
south-polar map, and five maps symmetrically placed round that map; and
these five so fit in with the first five as to complete the enclosure
of the whole sphere. In effect, every map of the twelve has five maps
symmetrically placed around it and overlapping it.
Since the whole heavens contain but 5932 stars visible to the naked
eye, each of the maps should contain on the average about 593 stars.
But instead of this being the case, some of the maps contain many more
than their just proportion of stars, while in others the number as
greatly falls short of the average. One recognizes, by combining these
indications, the existence of a roughly circular region, rich in stars,
in the northern heavens, and of another, larger and richer, in the
southern hemisphere.
To show the influence of these rich regions, it is only necessary to
exhibit the numerical relations presented by the maps.
The north-polar map, in which the largest part of the northern rich
region falls, contains no less than 693 lucid stars, of which upwards
of 400 fall within the half corresponding to the rich region. Of the
adjacent maps, two contain upwards of 500 stars, while the remaining
three contain about 400 each. Passing to the southern hemisphere, we
find that the south-polar map, which falls wholly within a rich region,
contains no less than 1132 stars! One of the adjacent maps contains 834
stars, and the four others exhibit numbers ranging from 527 to 595.
It is wholly impossible not to recognize so unequal a distribution as
exhibiting the existence of special laws of stellar aggregation.
It is noteworthy, too, that the greater Magellanic cloud falls in
the heart of the southern rich region. Were there not other signs
that this wonderful object is really associated with the sidereal
system, it might be rash to recognize this relation as indicating the
existence of a physical connection between the Nubecula Major and
the southern region rich in stars. Astronomers have indeed so long
regarded the Nubeculæ as belonging neither to the sidereal nor to the
nebular systems, that they are not likely to recognize very readily
the existence of any such connection. Yet how strangely perverse is
the reasoning which has led astronomers so to regard these amazing
objects. Presented fairly, that reasoning amounts simply to this: The
Magellanic clouds contain stars and they contain nebulæ; therefore
they are neither nebular nor stellar. Can perversity of reasoning be
pushed further? Is not the obvious conclusion this, that since nebulæ
and stars are _seen_ to be intermixed in the Nubeculæ, the nebular and
stellar systems form in reality but one complex system?
As to the existence of star-streams and clustering aggregations, we
have also evidence of a decisive character. There is a well-marked
stream of stars running from near Capella towards Monoceros. Beyond
this lies a long dark rift altogether bare of lucid orbs, beyond which
again lies an extensive range of stars, covering Gemini, Cancer, and
the southern parts of Leo. This vast system of stars resembles a
gigantic sidereal billow flowing towards the Milky Way as towards some
mighty shore-line. Nor is this description altogether fanciful; since
one of the most marked instances of star-drift presently to be adduced
refers to this very region. These associated stars _are_ urging their
way towards the galaxy, and that at a rate which, though seemingly slow
when viewed from beyond so enormous a gap as separates us from this
system, must in reality be estimated by millions of miles in every year.
Other streams and clustering aggregations there are which need not
here be specially described. But it is worth noticing that all the
well-marked streams recognized by the ancients seem closely associated
with the southern rich region already referred to. This is true of the
stars forming the River Eridanus, the serpent Hydra, and the streams
from the water-can of Aquarius. It is also noteworthy that in each
instance a portion of the stream lies outside the rich region, the rest
within it; while all the streams which lie on the same side of the
galaxy tend towards the two Magellanic clouds.
Most intimate signs of association between lucid stars and the galaxy
can be recognized—(i.) in the part extending from Cygnus to Aquila;
(ii.) in the part from Perseus to Monoceros; (iii.) over the ship Argo;
and (iv.) near Crux and the feet of Centaurus.
Before proceeding to the subject of Star-drift, three broad facts may
be stated. They are, I believe, now recognized for the first time, and
seem decisive of the existence of special laws of distribution among
the stars:—
First, the rich southern region, though covering but a sixth part of
the heavens, contains one-third of all the lucid stars, leaving only
two-thirds for the remaining five-sixths of the heavens.
Secondly, if the two rich regions and the Milky Way be considered as
one part of the heavens, the rest as another, then the former part is
three times as richly strewn with lucid stars as the second.
Thirdly, the southern hemisphere contains one thousand more lucid
stars than the northern, a fact which cannot but be regarded as most
striking when it is remembered that the total number of stars visible
to ordinary eyesight in both hemispheres falls short of 6000.
Two or three years ago, the idea suggested itself to me that if
the proper motions of the stars were examined, they would be found
to convey clear information respecting the existence of variety of
structure, and special laws of distribution within the sidereal system.
In the first place, the mere amount of a star’s apparent motion must
be regarded as affording a means of estimating the star’s distance.
The nearer a moving object is, the faster it will seem to move, and
_vice versâ_. Of course in individual instances little reliance can be
placed on this indication; but by taking the average proper motions of
a set of stars, a trustworthy measure may be obtained of their average
distance, as compared with the average distance of another set.
For example, we have in this process the means of settling the question
whether the apparent brightness of a star is indeed a test of relative
nearness. According to accepted theories the sixth-magnitude stars
are ten or twelve times as far off as those of the first magnitude.
Hence their motions should, on the average, be correspondingly small.
Now, to make assurance doubly sure, I divided the stars into two sets,
the first including the stars of the 1st, 2nd, and 3rd, the second
including those of the 4th, 5th, and 6th magnitude. According to
accepted views, the average proper motion for the first set should be
about five times as great as that for the second. I was prepared to
find it about three times as great; that is, not so much greater as
the accepted theories require, but still considerably greater. To my
surprise, I found that the average proper motion of the brighter orders
of stars is barely equal to that of the three lower orders.
This proves beyond all possibility of question that by far the greater
number of the fainter orders of stars (I refer here throughout to lucid
stars) owe their faintness not to vastness of distance, but to real
relative minuteness.
To pass over a number of other modes of research, the actual mapping of
the stellar motions, and the discovery of the peculiarity to which I
have given the name of star-drift, remain to be considered.
In catalogues it is not easy to recognize any instances of community of
motion which may exist among the stars, owing to the method in which
the stars are arranged. What is wanted in this case (as in many others
which yet remain to be dealt with) is the adoption of a plan by which
such relations may be rendered obvious to the eye. The plan I adopted
was to attach to each star in my maps a small arrow, indicating the
amount and direction of that star’s apparent motion in 36,000 years
(the time-interval being purposely lengthened, as otherwise most of the
arrows would have been too small to be recognized). When this was done,
several well-marked instances of community of motion could immediately
be recognized.
It is necessary to premise, however, that before the experiment was
tried, there were reasons for feeling very doubtful whether it would
succeed. A system of stars might really be drifting athwart the
heavens, and yet the drift might be rendered unrecognizable through
the intermixture of more distant or nearer systems having motions of
another sort and seen accidentally in the same general direction.
This was found to be the case, indeed, in several instances. Thus the
stars in the constellation Ursa Major, and neighbouring stars in Draco,
exhibit two well-marked directions of drift. The stars β, γ, δ, ε, and
ζ of the Great Bear, besides two companions of the last-named star,
are travelling in one direction, with equal velocity, and clearly form
one system. The remaining stars in the neighbourhood are travelling in
a direction almost exactly the reverse. But even this relation, thus
recognized in a region of diverse motions, is full of interest. Baron
Mädler, the well-known German astronomer, recognizing the community
of motion between ζ Ursæ and its companions, calculated the cyclic
revolution of the system to be certainly not less than 7000 years. But
when the complete system of stars showing this motion is considered,
we get a cyclic period so enormous, that not only the life of man,
but the life of the human race, the existence of our earth, nay, even
the existence of the solar system, must be regarded as a mere day in
comparison with that tremendous cycle.
Then there are other instances of star-drift where, though two
directions of motion are not intermixed, the drifting nature of the
motion is not at once recognized, because of the various distances at
which the associated stars lie from the eye.
A case of this kind is to be met with in the stars forming the
constellation Taurus. It was here that Mädler recognized a community
of motion among the stars, but he did not interpret this as I do. He
had formed the idea that the whole of the sidereal system must be in
motion around some central point; and for reasons which need not here
be considered, he was led to believe that in whatever direction the
centre of motion may lie, the stars seen in that general direction
would exhibit a community of motion. Then, that he might not have to
examine the proper motions all over the heavens, he inquired in what
direction (in all probability) the centre of motion may be supposed
to lie. Coming to the conclusion that it must lie towards Taurus,
he examined the proper motions in that constellation, and found a
community of motion which led him to regard Alcyone, the chief star of
the Pleiades, as the centre around which the sidereal system is moving.
Had he examined further he would have found more marked instances of
community of motion in other parts of the heavens, a circumstance which
would have at once compelled him to abandon his hypothesis of a central
sun in the Pleiades, or at least to lay no stress on the evidence
derivable from the community of motion in Taurus.