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Nature
(Thursday, January 27, 1870)

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


Page 331


7an. 27, I870]
NA TURE
ARE ANY OF THE NEBULIE STAR-S VSTE.lIS?
   HIS may seem a bold question, for it is commonly
believed that SirWilliam and Sir John Herschel-the
Ajax and the Achilles of the astronomical host-have
long since proved that many of the nebulae are star-
systems. If we inquire, however, into what the Herschels
have done and said, we shall find that not only have they
not proved this point, but that the younger Herschel, at
any rate, has expressed an opinion rather unfavourable
than otherwise to the theory that the nebulae are galaxies
in any sense resembling our own sidereal system.
  Sir William Herschel, by his noble plan of star-gauging,
proved that the stars aggregate along a certain zone,
which in one direction is double. He argued, therefore,
that presuming a general equality to exist among the stars
and among the distances separating them from each
other, the figure of the sidereal system resembles that of
a cloven disc.
A-Il .ac the nni-
system    from
which he could
form a probable
judgment-- I
mean the plane-
tary system-
presented to him
a number of
bodies,  widely
separated  from
each other and
each a globe of
considerable im-
portance, he rea-
soned from ana-
logy that similar
relations exist in
t h e  sidereal
spaces.   This
being so, his clo-
ven disc theory
of the sidereal
system seemed
satisfactorily es-
tablished.
Then, of course,
those nebulae
which exhibit a
multitude of mi-
nute points of
light very closeto-
gether, and those
other nebulae
thus resolvable
into minute points, yet in other respects resemble those
which are, came naturally to be looked upon as distinct
from the sidereal system. The analogy of this system, in
fact, pointed to them as external star-systems, resembling
it in all important respects.
  Then there were certain other objects, which seemed to
present no analogy either to the sidereal system or to
separate stars. These objects Sir Wm. Herschel con-
sidered to belong to our sidereal system; for he could
not put themn outside its range without looking on them
as objects suigeneris, which would have been to abandon
the argument from analogy. In order to explain their ap-
pearance, he suggested that they might be gaseous bodies,
by whose condensation stars would one day be
formed.
  The value of Sir Wm. Herschel's work is not in the
least affected even if science have to reject every one of
these opinions. He himself held them with a light hand;
he had once held other opinions; and he was gradually
modifying these. Had he seen one sound reason for
rejecting any or all of them he would have done so in-
stantly. For it belonged to the strength of his character
that he was never fettered by his own opinions, as weak
men commonly are.
  Sir John Herschel did for the southern heavens what
his father had done for the northern. He completely sur-
veyed and gauged them. It is commonly believed that
the results of his labours fully confirmed the opinionb
which his father had looked upon as probable.
  Let us see if this is so.
  Sir W. Herschel thought the Milky Way indicated that
the sidereal system has the figure of a cloven disc ; Sir
John Herschel judges rather that the sidereal system has
the figure of a flattened ring. Sir Win. Herschel thought
the stellar nebulae are probably external galaxies; Sir John
gives reasons for believing that they lie within our system,
and Whewell considered that these reasons amount 'to
                                      absolute proof.
  I  nas   neen
further believed
and stated that
the researches of
the elder Struve
go far to confirm
the opinions put
forward by Sir
WV. H erschel as
probable.
  Let us inquire
how far this is
true.
  Struve found
that the numbers
of stars of given
magnitudes ex-
hibit nearly the
same proportion
in different direc-
tions. Thus sup-
posing that in a
given  direction
there are three
times as many
stars of a certain
magnitude as
there are of the
next highest mag-
nitude, then in
other directions,
also, the same
relation is ob-
served. This is
                                      it ve~ry strliking
                                      law; but to male
it serve as a proof of the opinion which Sir \William
Herschel had put forward as probable, it would be neces-
sary that another law should be exhibited. For clearly, if
that opinion were just, it would be easy to calculate what
the relation should be between stars of different mag-
nitudes. Had Struve been able to show that the numbers
actually seen corresponded to the relations thus calculated,
he would have gone far to render that view certain which
Herschel always spoke of as merely an assumption.
  But Struve found no such law of stellar distribution.
On the contrary, he found a law so different, that in order
to force the facts into agreement with Sir William Her-
schel's views about the sidereal system, he had to invent
his famous theory of the extinction of light in traversing
space. Now, according to this theory, we cannot see to
the limits of our sidereal system, even though we could
increase the powers of our telescopes a million-fold ; so
that if the theory is true, the question which heads this
paper is at once disposed of. Obviously, we cannot see
331


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