Of late years we have learned a great deal about nebulæ, by the help
which photography has given to us. Look at this group of stars which
constitutes that beautiful little configuration known as the Pleiades
(Fig. 89). It looks like a miniature representation of the Great
Bear; in fact, it would be far more appropriate to call the Pleiades
the Little Bear than to apply that title to another quite different
constellation, as has unfortunately been done. The Pleiades form a
group containing six or seven stars visible to the ordinary eye, though
persons endowed with exceptionally good vision can usually see a few
more. In an opera-glass the Pleiades becomes a beautiful spectacle,
though in a large telescope the stars appear too far apart to make a
really effective cluster. When Mr. Roberts took a photograph of the
Pleiades he placed a highly sensitive plate in his telescope, and on
that plate the Pleiades engraved their picture with their own light.
He left the plate exposed for hours, and on developing it not only
were the stars seen, but there were also patches of faint light due to
the presence of nebula. It could not be said that the objects on the
plate were fallacious, for another photograph was taken, when the same
appearances were reproduced.
[Illustration: FIG. 89.--The Pleiades.]
When we look at that pretty group of stars which has attracted
admiration during all time, we are to think that some of those stars
are merely the bright points in a vast nebula, invisible to our unaided
eyes or even to our mighty telescopes, though capable of recording
its trace on the photographic plate. Does not this give us a greatly
increased notion of the extent of the universe, when we reflect that
by photography we are enabled to see much which the mightiest of
telescopes had previously failed to disclose?
Of all the nebulæ, now numbering some thousands, there is but a single
one which can be seen without a telescope. It is in the constellation
of Andromeda, and on a clear dark night can just be seen with the
unaided eye as a faint stain of light on the sky. It has happened
before now that persons noticing this nebula for the first time have
thought they had discovered a comet. I would like you to try and find
out this object for yourselves.
If you look at it with an opera-glass it appears to be distinctly
elongated. You can see more of its structure when you view it in larger
instruments, but its nature was never made clear until some beautiful
photographs were taken by Mr. Roberts (Fig. 90). Unfortunately, the
nebula in Andromeda has not been placed in the best position for its
portrait from our point of view. It seems as if it were a rather
flat-shaped object, turned nearly edgewise towards us. To look at the
pattern on a plate, you would naturally hold the plate so as to be
able to look at it squarely. The pattern would not be seen well if
the plate were so tilted that its edge was turned towards you. That
seems to be nearly the way in which we are forced to view the nebula
in Andromeda. We can trace in the photograph some divisions extending
entirely round the nebula, showing that it seems to be formed of a
series of rings; and there are some outlying portions which form part
of the same system. Truly this is a marvellous object. It is impossible
for us to form any conception of the true dimensions of this gigantic
nebula; it is so far off that we have never yet been able to determine
its distance. Indeed, I may take this opportunity of remarking that
no astronomer has yet succeeded in ascertaining the distance of any
nebula. Everything, however, points to the conclusion that they are at
least as far as the stars.
[Illustration: FIG. 90.--The Great Nebula in Andromeda.
(_From Mr. Roberts’ Photograph._)]
It is almost impossible to apply the methods which we use in finding
the distance of a star to the discovery of the distance of the
nebulæ. These flimsy bodies are usually too ill-defined to admit of
being measured with the precision and the delicacy required for the
determination of distance. The measurements necessary for this purpose
can only be made from one star-like point to another similar point. If
we could choose a star in the nebula and determine its distance, then,
of course, we should have the distance of the nebula itself; but the
difficulty is that we have, in general, no means of knowing whether the
star does actually lie in the object. It may, for anything we can tell,
lie billions of miles nearer to us, or billions of miles further off,
and, by merely happening to lie in the line of sight, appear to glimmer
in the nebula itself.
[Illustration: FIG. 91.--To show how Small the Solar System is compared
with a Great Nebula.]
If we have any assurance that the star is surrounded by a mass of
this glowing vapor, then it may be possible to measure that nebula’s
distance. It will occasionally happen that grounds can be found for
believing that a star which appears to be in the glowing gas does
veritably lie therein, and is not merely seen in the same direction.
There are hundreds of stars visible on a good drawing or a good
photograph of the famous object in Andromeda, and doubtless large
numbers of these are merely stars which happen to lie in the same line
of sight. The peculiar circumstances attending the history of one star
seem, however, to warrant us in making the assumption that it was
certainly in the nebula. The history of this star is a remarkable one.
It suddenly kindled from invisibility into brilliancy. How is a change
so rapid in the lustre of a star to be accounted for? In a few days
its brightness had undergone an extraordinary increase. Of course,
this does not tell us for certain that the star lay in the glowing
gas; but the most rational explanation that I have heard offered of
this occurrence is that due, I believe, to my friend Mr. Monck. He has
suggested that the sudden outbreak in brilliancy might be accounted for
on the same principles as those by which we explain the ignition of
meteors in our atmosphere. If a dark star, moving along with terrific
speed through space, were suddenly to plunge into a dense region of
the nebula, heat and light must be evolved in sufficient abundance to
transform the star into a brilliant object. If, therefore, we knew the
distance of this star at the time it was in Andromeda, we should, of
course, learn the distance of that interesting object. This has been
attempted, and it has thus been proved that the Great Nebula must
be very much further from us than is that star of whose distance I
attempted some time ago to give you a notion.
We thus realize the enormous size of the Great Nebula. It appears that
if, on a map of this object, we were to lay down, accurately to scale,
a map of the solar system, putting the sun in the centre and all the
planets around in their true proportions out to the boundary traced by
Neptune, this area, vast though it is, would be a mere speck on the
drawing of the object. Our system would have to be enormously bigger
before it sufficed to cover anything like the area of the sky included
in one of these great objects. Here is a sketch of a nebula (Fig. 91),
and near it I have marked a dot which is to indicate our solar system.
We may feel confident that the Great Nebula is at the very least as
mighty as this proportion would indicate.