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Nature
(Thursday, January 27, 1870)

Proctor, Richard A.
Are any of the nebulæ star-systems?,   pp. 331-333


Page 332


NA TURE
[.7an. 27, i870
galaxies beyond the sidereal system if we cannot see to
the limits of that system. And I may note in passing
that (independently of Struve's theory) the most powerful
telescopes cannot render visible the most distant stars of
our sidereal scheme ; so that if the nebulae arc really ex-
ternal galaxies, the stars wve see in them must be
enormously greater than those in our galaxies, suppos-
ing Herschel was right in thinking these tolerably uniform
in magnitude.
   Before proceeding to exhibit the evidence which has led
me to the conviction that the nebulae belong to our sidereal
system, I may mention some reasons for believing that if
Sir William Herschel's labours in the sidereal heavens
were to be begun now, not only would he not have been
led to adopt as probable the view on which he formed his
opinions, but he would have rejected it as opposed to
known analogies.
   He had argued that because the planetary system ex-
hibits a definite number of bodies separated by wide
distances, therefore analogy should lead us to regard the
sidereal system as similarly constituted, though on a
much larger scale. This was perfectly just. Despite the
various differences which no one recognised more clearly
than he did, this view was the only one he could safely
adopt for his guidance, ninety years ago.
   But would not he have been the first to reject that view
if he had known what we now know of the solar system?
If he had known that besides the primary planets, there
are hundreds of minute bodies forming a zone between
the orbits of Mars and Jupiter ; that the rings of Saturn
are formed of a multitude of minute satellites; that innu-
merable meteor-systems circle in orbits of every conceiv-
able degree of eccentricity ; that near the sun these
systems grow denser and denser; that the comets of the
solar system must be counted by millions on millions
that, in fine, every conceivable form of matter, every con-
ceivable degree of aggregation, and every conceivable
variety of size, exists within the limits of the solar system,
-would he, then, have been led by analogy to recognise
in the sidereal system only discrete stars and masses
forming into stars ?
  From a careful study of all that Sir William Herschel
has written, I feel certain, that in the case I have ima-
gined, he would have been prepared, even before com-
mencing his labours, to expect precisely that variety of
matter, size, and aggregation, which modern observations,
rightly understood, prove actually to exist within the range
of the sidereal system.
  The Herschels, father and son, discovered about 4,500
nebulae. Other observers have brought up the number to
about 5,400. When these are divided into classes, it
appears that some 4,500 must be looked on as irresolvable
into discrete points of light. But of these the greater
proportion so far resemble resolvable nebulae as to lead
to the belief that increase of optical power alone is want-
ing to resolve them.
  Taking these irresolvable nebulae, however, as we find
them, and marking down their places over the celestial
sphere, we recognise certain peculiarities in their arrange-
ment. In the northern heavens they gather into a clus-
tering group as far as possible from the Milky Way. In
the southern heavens they form into streams, which run
out from a region nearly opposite the northern cluster of
nebulae; but the etr-emities of the streams are the region
where nebulae are most closely crowded.   The Milky
\Vay is almost clear of nebule.
  This withdrawal of the nebule from the Milky Way has
been accepted by many as clearly indicating that there
is no association between them and the sidereal system.
The opinion of the Herschels, if they had been led to
pronounce definitively on this point, would have been dif-
ferent, however ; for the younger Herschel quotes (as
agreeing with it) a remark of his father's to the effect that
the peculiar position of the northern nebular group is not
accidental. If not accidental, it can only be due to some
association between the nebular group and the galaxy.
Every other conceivable explanation will be found to make
the relation merely apparent-that is, accidental, which
neither of the Herschels admit.
  But yet stronger evidence of association exists; evidence
which I do not hesitate to speak of as incontrovertible.
Space will only permit me to treat it very briefly.
  There is a certain wvell-marked stream of nebulae in the
southern heavens leading to awell-marked cluster ofnebulae.
There is an equally well-marked stream of stars leading to
an equally well-marked cluster of stars. The nebular stream
agrees in position with the star-stream, and the proba-
bility is small that this coincidence is accidental. The
nebular cluster agrees in position with the star-cluster
and the probability is still smaller that this second coin-
cidence is accidental. Such are the separate chances. It
will be seen at once, therefore, how small the chance is
that both coincidences are accidental.
  The cluster here referred to is the greater of the cele.
brated Magellanic Clouds. When it is added that the
evidence is repeated point for point in the case of the
lesser Magellanic Cloud, the indications of association
appear overwhelmingly convincing. If the nebulae really
are associated in this manner with fixed stars, the question
which heads this paper is disposed of at once.
  But there is yet further evidence.
  The nebulae pass by insensible gradations from clusters
less and less easily resolvable, to nebulae properly so
called, but still resolvable, and so to irresolvable nebulae.
Now clusters are found not only to aggregate in a general
manner near the Milky \Vay, but in some cases (on which
Sir John Herschel has dwelt with particular force) to
exhibit the clearest possible signs of belonging to that
zone. If they then belong to the Milky Way, can any
good reason be given for believing that the various other
classes of nebulae are not associated with the sidereal
scheme? Where should the line be drawn ?
  Again, some of the nebulae are gaseous, and all the gaseous
nebulae exhibit the same spectrum. Now, two classes of
gaseous nebulae, the planetary and the irregular nebulae,
exhibit a marked preference for the Milky Way, and
therefore we must admit the probability that they, at any
rate, belong to the sidereal scheme. But then a large
proportion of the irresolvable nebulae are also gaseous,
and as they are formed of the same gases, we see good
reason for believing that they also must belong to our
galaxy. This, however, brings in all the nebulae, since the
recent detection by Lieut. Herschel of the same bright
lines in or rather on the continuous spectrum of a star-
cluster, shows the great probability which exists that with
more powerful spectroscopes all the nebulae may be
found to exhibit these bright lines, that is, to contain
these particular gases. I pass over the facts, that many
nebulae are found to be closely associated with stars, and
that if any doubt could remain as to the association being
real and not apparent, it would be removed by a picture of
the nebula M 17, as seen in Mr. Lassell's great reflector at
Malta. The reader will be more interested by the follow-
ing quotation, which I extract (by permission) from a
letter of Sir John Herschel's
  "A remark which the structure of Magellanic Clouds
has often suggested to me has been strongly recalled by
what you say of the inclusion of every variety of nebulous
or clustering forms within the galaxy, viz., that if such be
the case-i.e. if these forms belong to, and form part and
parcel of the Galactic system-then that system includes
within itself miniahtures of itself on an almost infinitely
reduced scale; and what evidence, then, have we that
there exists a universe beyond-unless a sort of argument
from analogy, that the Galaxy with all its contents may
be but one of these miniatures of a more vast universe,
and that there may, in that universe of other systems on
a scale as vast as our galaxy, be the analogues of those
332


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