Southern Hemisphere, but not of such large apparent dimensions as
those already described. Of these may be mentioned 22 Messier, which
lies about midway between the stars Mu and Sigma Sagittarii. It is
described by Sir John Herschel as a fine globular cluster, with stars
of two magnitudes, namely eleven or twelve, and fifteen or sixteen,
the larger being visibly reddish, and he suggested that it consists
of “two layers, or one shell over another.” Owing to the comparative
brightness of the larger components, this cluster forms a good object
for small telescopes. I saw the brighter stars well with a 3-inch
refractor in the Punjab sky, but, of course, the greater portion of
the cluster has a nebulous appearance in a telescope of this size.
Between Alpha and Beta Scorpii there is a condensed globular cluster.
With small telescopes it very much resembles a telescopic comet, but
with larger instruments its true character is revealed. Sir William
Herschel considered it “the richest and most condensed mass of stars
in the firmament.” In May, 1860, a “temporary star” of the seventh
magnitude suddenly appeared in the centre, almost blotting out the
cluster by its superior light. The star faded away before the end
of June of the same year, and has not been seen with any certainty
since. It has been suggested that this temporary star lay _between_
the cluster and the earth, but it seems to me much more probable that
the outburst took place _in_ the cluster itself, and that it was
possibly caused by a collision between two of the component stars,
or by a swarm of meteors rushing with a high velocity through the
cluster.
The beauty and sublimity of the spectacle presented by these globular
clusters, when viewed with a powerful telescope, is such as can not
be adequately described, and it has been said that when seen for the
first time, “few can refrain from a shout of rapture.” The component
stars, although distinctly visible as points of light, defy all
attempts at counting them, and seem literally innumerable. Placed
like a mass of glittering diamond-dust on the dark background of
the heavens, they impress us forcibly with the idea that if each of
these lucid points is a sun, the thousands which seem massed together
in so small a space must be in reality either relatively close and
individually small, or else the system of suns must be placed at a
distance almost approaching the infinite.
The distance of these globular clusters from the earth is, however,
certainly very great. Attempts to accurately determine their position
in space have not been attended with success. As the component
stars are at practically the same distance from the eye, we have
no comparison stars to measure from, and their exact distance,
therefore, remains unknown. We may, however, estimate their probable
distance with some show of plausibility. We may assume that the stars
of the Hercules cluster would, if concentrated in a point, shine
as a star of about the fourth magnitude. As the components are of
about the twelfth and thirteenth magnitudes, this would imply that
the cluster consists of about 2,500 stars. With the data assumed, we
may therefore conclude that the components of the Hercules cluster
are suns of comparatively small size, separated by considerable
distances, but apparently massed together by the effect of distance.
Among less condensed star clusters there are many interesting
objects. The Pleiades have been already referred to. On a photograph
of this remarkable group, taken at the Paris Observatory, over 2,000
stars can be counted of all degrees of brilliancy, from those
visible without optical aid down to points of light so faint as
to be invisible to the eye in the telescope with which they were
photographed. Here we have a cluster of probably larger size than
that in Hercules, probably at a greater distance from the earth, and
with its larger components of considerably greater mass than our sun.
Near the bright star Pollux, I see a small cluster of stars of
about the seventh and eight magnitudes, which, with a binocular
field-glass, very much resembles the Pleiades as seen with the naked
eye. A smaller cluster (known as 39 Messier) may be seen near the
star Pi Cygni.
The well-known Chi Persei may be also seen with an opera-glass, but
a telescope is necessary to show the component stars to advantage,
and the larger the telescope the greater the number of faint stars in
these wonderful objects.
The cluster known as 35 Messier, a little north of the star Eta
Geminorum, is visible in an opera-glass, but a small telescope
is required to see the component stars. A well-marked clustering
tendency is visible among the brighter stars of the group, two,
three, four, and sometimes five stars being grouped together in
subordinate collections. Admiral Smyth says: “It presents a gorgeous
field of stars from the ninth to the sixteenth magnitude, but with
the centre of the mass less rich than the rest. From the small stars
being inclined to form curves of three or four, and often with a
large one at the root of the curve, it somewhat reminds one of the
bursting of a sky-rocket.” This tendency to “stream” formation in
the components of star clusters is also well marked in a photograph
of the cluster 38 Messier (kindly sent to me by MM. Henry of the
Paris Observatory). It was described by Webb as “a noble cluster
arranged in an oblique cross,” and Smyth says: “The very unusual
shape of this cluster recalls the sagacity of Sir William Herschel’s
speculations upon the subject, and very much favors the idea of an
attractive power lodged in the brightest part. For although the form
is not globular, it is plainly to be seen that there is a tendency
toward sphericity, by the swell of the dimensions as they draw near
the most luminous part, denoting, as it were, a stream or tide of
stars, setting toward the centre.”
Sir W. Herschel, speaking of a compressed cluster in Perseus, says
“the large stars are arranged in lines like interwoven letters,” and
Webb says “it is beautifully bordered by a brighter foreshortened
pentagon.”
Observing with a 3-inch telescope in India, I noticed a beautiful
cluster of stars, about 4° north of Gamma and Upsilon Scorpii,
resembling in shape a bird’s foot, with remarkable streams of stars.
This cluster is visible to the naked eye as a star of about the fifth
magnitude.
Although these loosely associated star clusters do not show such
evidence in favor of family connection as the more closely compacted
globular clusters, still we can hardly escape from the conviction
that their apparent aggregation is really due to some physical bond
of union, and not merely the result of a fortuitous scattering of
stars at different distances in the line of sight.
THE GREAT NEBULA OF ORION.—SIR ROBERT S. BALL
The telescope, ever an ally in the study of the heavens, is in this
part of the science absolutely indispensable. In other branches of
astronomy we can learn something without its aid. Indeed, many great
astronomical discoveries were made long before the telescope was
invented. But ere this memorable event in the history of science
it was impossible for us to know anything of the existence of the
nebulæ. It is indeed true that there is one of these objects which
can be just detected by the naked eye. It lies in the constellation
of Andromeda, where, on a clear and dark night, a faint spot of
light can just be discerned by a good eye. But a mere glimpse gives
us really no adequate notion of the true character of the object.
It might only, so far as the naked eye discloses its nature, be a
cluster of stars like that we have already discerned in Perseus,
or like the similar group that, under the name of the Beehive, is
comparatively familiar in the constellation of Cancer. With the
single exception of the nebula in Andromeda, all the objects so
called are entirely telescopic, yet how important a constituent the
nebulæ form in the contents of the heavens will be shown by a look at
some of the lists of these objects. There are now several thousands
of nebulæ known, and their positions in the sky, as well as the
details of their appearances, are set forth in the catalogues.
The most glorious constellation of stars in the firmament is
undoubtedly that of Orion. This splendid group is seen in the
south during the winter months, and toward the close of January it
is situated in a very convenient position for observing early in
the evening. The group is specially characterized by the number
of unusually bright stars which it includes, and the three stars
in the centre, forming the so-called Belt of Orion, is as well
known a celestial figure as the sky contains. Directly under the
belt are three much smaller stars nearly in a line, which points
straight upward to the middle star of the belt. These three lower
stars are usually known as the sword handle of Orion, this being
the position which they occupied in the fanciful old sketches of
the constellation. The three stars of the sword handle of Orion are
plunged in the Great Nebula. This object can not be seen by the
unassisted eye, though doubtless around the central star a little
haziness is perceptible, and even the slightest telescopic aid will
suffice to indicate that the central star of the sword handle is
attended by a surrounding glow of light, which renders it quite
unlike other stars. This can indeed be sufficiently shown with an
ordinary opera-glass, one glance through which will awaken in the
beholder a keen desire to study the object under more favorable
conditions. But to do justice to the object, telescopes of large
power are desirable.
To realize fully the magnificence of the Great Nebula, the observer
who is being introduced to the object for the first time should not,
strange to say, direct the telescope at the nebula; the instrument
should rather be pointed at the heavens, just a little to the
west of the nebula. The clock driving the equatorial should not be
started, and the observer should take his seat and look through the
eye-piece before the nebula has entered the field. He will see, no
doubt, a few stars on the black background, which gradually pass in
procession across his field of view. This is merely the ordinary
diurnal journey of the heavens, by which all the objects move slowly
from east to west; I ought rather to say _appear_ to move, for, of
course, the motion on the heavens is only apparent, the fact being
that it is the earth which is turning round.
After the observer’s eye for a minute or so has become familiarized
with the dark aspect of the heavens under ordinary circumstances,
he will begin to perceive on the eastern side (it will appear in
the telescope no doubt as on the western side) a faint dawn of
light. Gradually there will steal across his field of view a sort
of ghostlike luminosity that is in marked contrast to the darkness
in the rest of the field; as the seconds move on, this object will
disclose itself until the full splendor of the Great Nebula comes
into view; then the entire field will be filled with the light,
and then it will gradually advance and gradually pass away again
to emphasize the contrast between the brilliance of the nebula and
the darkness of the sky. Unless this method is adopted, the full
interest of a telescopic view of the Great Nebula is not attained,
for when the entire field is full of the glow the beginner will
hardly recognize the nebula. He will be apt to think that the fainter
part of the field he sees is the ordinary groundwork of the sky, and
this illusion can only be dispelled by enabling him to witness the
actual contrast in the way I have described. The central portions
of the nebula are, however, so brilliant and so wonderfully marked
with interesting detail, that even a small instrument will suffice to
reveal much of its beauties.
In the centre of the nebula is the star known to astronomers at
Theta Orionis, the most prominent star of the sword handle. To the
eye this looks like an ordinary star, but the telescope speedily
dispels that notion. Theta Orionis is found to consist of four, or
rather six, stars all so close together that the unaided eye fails
to distinguish them separately. A structure so complex gives to this
star quite a special, indeed a unique, interest, wholly apart from
the marvelous nebula of which it is the focus. We must dwell a little
on the peculiarities of this star. We are familiar with stars which
are called double; there are indeed some ten thousand objects so
designated known to astronomers and duly registered in catalogues.
Many of these double stars are objects of extreme telescopic beauty;
sometimes they offer to our admiration a delightful contrast of
colors; perhaps one will be topaz color and the other bluish, or on
rare occasions a pair of emerald gems will be seen with an invisible
band of mutual connection. Sometimes triple stars are found, in which
three stars are obviously in alliance; but multiple stars of greater
complexity are comparatively rare; and so marvelous a spectacle
as Theta Orionis, in which no fewer than six stars are obviously
an allied group, is almost unique. It is not a little remarkable
that we find the most exquisite multiple star which the sky can
show, beautifully framed or set in the centre of the grandest of
the nebulæ. Of course it might conceivably happen that the apparent
concourse of these objects was fortuitous. The actual phenomenon
could be accounted for by the belief that the Great Nebula was either
very much nearer or very much further than the multiple star, and
that they chanced to lie in the same line of sight, and had no other
connection. But to me it appears that this view is quite at variance
with every reasonable probability; that the most wondrous multiple
star should have happened to lie in line with the very centre of
the most wondrous nebula would have been a coincidence against the
occurrence of which the probabilities were almost infinite. There can
scarcely be any doubt that the multiple star and the Great Nebula are
part of the same system, and that the star is, in truth, placed in
the middle of the nebula, as it actually appears to be.
And now as to the composition of this mysterious object.
The word nebula means, of course, a little cloud, but the expression
is apt to be a misleading one. In a sense no doubt they are little,
inasmuch as the patch of the sky which a nebula covers would be small
compared with one of our ordinary clouds. Indeed, a nebula which
covered as large an apparent part of the sky as the size of the
moon would be ranked as a large object of its class, while even the
greatest of them is perhaps not more than ten or twelve times as
great. Nor is the word cloud, as applied to nebula, an appropriate
one. What we mean by a cloud is only a vast mass of watery vapor
raised by the sun from the sea, and poised aloft until such time as
it shall be again dispersed into invisible water, or until it shall
descend to the earth as rain. Such clouds are, of course, within
the limits of our atmosphere, and are rarely more than a few miles
above the earth’s surface. The light which renders clouds visible
only comes from reflected sunbeams, and consequently at night
clouds become invisible, though the astronomer is often only too
unpleasantly made acquainted with their presence by the opacity with
which they shut out the stars from his view.
Utterly different in all respects are the nebulæ. They are not masses
of watery vapor. It may no doubt possibly be that water in some form
is there, but it is not water which we see. We are looking at some
gaseous material of a bluish hue. The light with which it glows is
no reflected sunlight. The nebula is indeed indebted to no foreign
source for that weird—I had almost said ghostlike—radiance which
it gives forth. The light comes from the nebula itself. But how,
it may well be asked, should a purely gaseous substance be able to
radiate forth light? It is easy for us to comprehend how stars or
suns or comparatively solid bodies can, in virtue of their tremendous
temperature, glow with heat like red-hot or white-hot iron. It is
true that flame is gas in an incandescent state, but in flame a
vehement chemical union of oxygen with some other substance is in
progress, and this is the source of the heat and the light that
flame gives forth. We can not regard the Great Nebula in Orion as
originating in anything resembling flame.
We can, however, in our physical laboratories arrange an experiment
which seems to throw some light on the composition of the nebula.
Into a glass tube a small quantity of hydrogen gas is admitted, the
air having been previously extracted. Then, by means of two wires,
one at each end of the tube, an electric current is transmitted
through the gas. Here there is no combustion; the gas is merely the
vehicle by which the electricity flows from one pole to the other.
In doing so the gas instantly begins to glow with an intense bluish
light, and a very beautiful effect is produced, which can be renewed
or terminated at will by simply making or breaking the electric
current. It would seem as if the gas we see in the nebula were in a
condition somewhat analogous to the gas in the tube. I do not mean
that the passage of electricity through the nebula is the source of
its luminosity. There is, indeed, no ground for such a supposition.
It is the property of electricity when passing through a conductor
to warm that conductor; thus we know that if a powerful current be
transmitted through a wire of the most infusible of all metals,
platinum, the wire will not only get warm, but it may become red
hot, white hot, and even melt under the influence of the heat which
is generated. In those beautiful incandescent electric lamps which
are now happily coming into extensive use a current of electricity
flows through a filament of carbon, and kindles that exquisite
incandescence which is maintained while the current flows. It would
appear that so long as the electricity is flowing through the glass
tube its action on the gas is to impart a very high temperature. It
is in consequence of this temperature that the gas glows. Now we can
offer a reasonable account of the luminosity of the Great Nebula in
Orion. The particles of gaseous or vaporous material of which it is
formed are of an extremely high temperature, sufficient to enable
them to glow with the brilliancy which renders them visible.
It is now almost twenty years since a marvelous accession to our
knowledge of such objects as the Great Nebula in Orion was made
by Dr. Huggins. I have used our gas hydrogen as an illustration
in describing the character of the nebula, but I have now to add
that the presence of hydrogen is no mere fiction but a substantial
verity. Truly we here open up one of the most marvelous chapters
which science has to disclose. The chemist can analyze the different
substances on the earth with his test tubes, and he can tell the
elements of which they are composed. But in this old-fashioned
chemistry it was at least reasonable for the chemist to demand a
portion of the substance he was expected to analyze. Unless he were
provided with a sample, how could it be possible for him to grind
it up or submit it to the various operations of his laboratory? In
these modern days the chemist can perform operations of which his
predecessors never even dreamed. No doubt the old method is still
used—nay, is indeed at this moment cultivated with greater skill and
means than in any previous age—but side by side with the old method,
and as an invaluable supplement thereto, the new method of chemical
research, called spectrum analysis, has been created, and has already
conducted to many profoundly interesting discoveries in the most
varied branches of science.
In the application of the spectroscopic method it is not
indispensably necessary that we actually have a fragment of the
substance; all we require is a beam of light which that substance can
be made to yield when heated to a sufficiently high temperature.
When a beam of the nebular light is transmitted through the prisms,
it declares at once that the object from which that light has come
is totally different from a star like the sun. Instead of the
beautifully colored band, decked in all the glowing hues of the
rainbow, the nebular beam is seen to be composed simply of six or
seven widely separated strips. It is important to test the character
of the light in these strips. Fortunately this can be done in a way
that is completely satisfactory. We can produce artificial lights
from known sources, and observe them through the spectroscope
simultaneously with the light of the nebula.
There are in the composition of this globe some sixty or seventy
different elementary substances, and under suitable conditions
each one of these substances can afford a perfectly characteristic
spectrum. Thus the way of making the comparison with the nebula is
to try the different elements one after another, until one can be
discovered which pours forth a light that behaves under the prism
as does the light from the nebula. Pursuing this inquiry, Dr.
Huggins found that when hydrogen gas was ignited to incandescence by
the passage of electricity, it emitted light which, after passage
through the prisms, came to coincidence with one of the lines in the
spectrum of nebula; and the hydrogen character of two of the other
lines has been since demonstrated. It was thus established that
hydrogen is one of the constituents of the Great Nebula in Orion.
Further confirmation of this important discovery was forthcoming
when the photographs of the spectrum of the Great Nebula were
subsequently obtained. On these photographs lines were present which
are constituted by light of such a nature as to be wholly invisible
to the eye, though perceptible on the photographic plate. It is of
the greatest interest to discover that these invisible rays from
the nebula are also indicative of the presence of hydrogen. Thus we
obtain a beautiful confirmation of the fact that the nebula is partly
composed of glowing hydrogen.
There are, however, some remaining lines, the character of which has
not yet been ascertained.
It would be a little premature to assert that there must be some
substance in the Great Nebula not at present known to us on the
earth. This would be, no doubt, one interpretation of the facts. We
must, however, admit the possibility of another explanation. It is
frequently found that the lines yielded by an incandescent material
vary to some extent when the physical conditions of temperature and
of pressure are modified. It is, therefore, not impossible that
the unknown lines in the spectrum of the Great Nebula may be due to
some element known to us, but which has not yet been tested under
the conditions which would make it yield the particular rays we are
speaking of.
The composition of a nebula as disclosed to us by these researches
is very instructive. Here we are looking at an object which seems to
lie at the very limits of the visible universe—an object so remote
that our attempts to fathom its distance are quite unsuccessful; yet
in this inconceivably distant part of our system we find at least one
ingredient which we know well on the earth. Previous to actual trial
no one would have expected, I think, to find the Great Nebula largely
constituted from such a familiar element as hydrogen. This gas enters
into the composition of water, and is thus an element of extreme
abundance on the earth. That an element so common with us here should
also be abundant in these awfully distant regions of the universe is
one of the most astonishing facts that modern science has revealed.