STAR CLUSTERS AND NEBULAE.
Interesting Sidereal Objects--Stars not Scattered uniformly--Star
Clusters--Their Varieties--The Cluster in Perseus--The Globular
Cluster in Hercules--The Milky Way--A Cluster of Minute Stars--The
Magellanic Clouds--Nebulae distinct from Clouds--Number of known
Nebulae--The Constellation of Orion--The Position of the Great
Nebula--The Wonderful Star th Orionis--The Drawing of the
Great Nebula in Lord Rosse's Telescope--Photographs of this
Wonderful Object--The Great Nebula in Andromeda--The Annular Nebula
in Lyra--Resemblance to Vortex Rings--Planetary Nebulae--Drawings of
Several Remarkable Nebulae--Nature of Nebulae--Spectra of
Nebulae--Their Distribution; the Milky Way.
We have already mentioned Saturn as one of the most glorious telescopic
spectacles in the heavens. Setting aside the obvious claims of the sun
and of the moon, there are, perhaps, two other objects visible from
these latitudes which rival Saturn in the splendour and the interest of
their telescopic picture. One of these objects is the star cluster in
Hercules; the other is the great nebula in Orion. We take these objects
as typical of the two great classes of bodies to be discussed in this
chapter, under the head of Star Clusters and Nebulae.
The stars, which to the number of several millions bespangle the sky,
are not scattered uniformly. We can see that while some regions are
comparatively barren, others contain stars in profusion. Sometimes we
have a small group, like the Pleiades; sometimes we have a stupendous
region of the heavens strewn over with stars, as in the Milky Way. Such
objects are called star clusters. We find every variety in the clusters;
sometimes the stars are remarkable for their brilliancy, sometimes for
their enormous numbers, and sometimes for the remarkable form in which
they are grouped. Sometimes a star cluster is adorned with
brilliantly-coloured stars; sometimes the luminous points are so close
together that their separate rays cannot he disentangled; sometimes the
stars are so minute or so distant that the cluster is barely
distinguishable from a nebula.
Of the clusters remarkable at once both for richness and brilliancy of
the individual stars, we may mention the cluster in the Sword-handle of
Perseus. The position of this object is marked on Fig. 83, page 415. To
the unaided eye a hazy spot is visible, which in the telescope expands
into two clusters separated by a short distance. In each of them we have
innumerable stars, crowded together so as to fill the field of view of
the telescope. The splendour of this object may be appreciated when we
reflect that each one of these stars is itself a brilliant sun, perhaps
rivalling our own sun in lustre. There are, however, regions in the
heavens near the Southern Cross, of course invisible from northern
latitudes, in which parts of the Milky Way present a richer appearance
even than the cluster in Perseus.
The most striking type of star cluster is well exhibited in the
constellation of Hercules. In this case we have a group of minute stars
apparently in a roughly globular form. Fig. 96 represents this object as
seen in Lord Rosse's great telescope, and it shows three radiating
streaks, in which the stars seem less numerous than elsewhere. It is
estimated that this cluster must contain from 1,000 to 2,000 stars, all
concentrated into an extremely small part of the heavens. Viewed in a
very small telescope, this object resembles a nebula. The position of
the cluster in Hercules is shown in a diagram previously given (Fig. 88,
page 420). We have already referred to this glorious aggregation of
stars as one of the three especially interesting objects in the heavens.
[Illustration: PLATE D.
MILKY WAY NEAR MESSIER II.
_Photographed by E.E. Barnard, 29th June, 1892._]
The Milky Way forms a girdle which, with more or less regularity,
sweeps completely around the heavens; and when viewed with the
telescope, is seen to consist of myriads of minute stars. In some places
the stars are much more numerous than elsewhere. All these stars are
incomparably more distant than the sun, which they surround, so it is
evident that our sun and, of course, the system which attends him lie
actually inside the Milky Way. It seems tempting to pursue the thought
here suggested, and to reflect that the whole Milky Way may, after all,
be merely a star cluster, comparable in size with some of the other star
clusters which we see, and that, viewed from a remote point in space,
the Milky Way would seem to be but one of the many clusters of stars
containing our sun as an indistinguishable unit.
[Illustration: Fig. 96.--The Globular Cluster in Hercules.]
In the southern hemisphere there are two immense masses which are
conspicuously visible to the naked eye, and resemble detached portions
of the Milky Way. They cannot be seen by observers in our latitude, and
are known as the Magellanic clouds or the two nubeculae. Their structure,
as revealed to an observer using a powerful telescope, is of great
complexity. Sir John Herschel, who made a special study of these
remarkable objects, gives the following description of them: "The
general ground of both consists of large tracts and patches of
nebulosity in every stage of resolution, from light irresolvable, in a
reflector of eighteen inches aperture, up to perfectly separated stars
like the Milky Way, and clustering groups sufficiently insulated and
condensed to come under the designation of irregular and in some cases
pretty rich clusters. But besides these there are also nebulae in
abundance and globular clusters in every state of condensation." It can
hardly be doubted that the two nubeculae, which are, roughly speaking,
round, or, rather, oval, are not formed accidentally by a vast number of
very different objects being ranged at various distances along the same
line of sight, but that they really represent two great systems of
objects, widely different in constitution, which here are congregated in
each other's neighbourhood, whereas they generally do not co-exist close
to each other in the Milky Way, with which the mere naked-eye view would
otherwise lead us to associate the Magellanic clouds.
When we direct a good telescope to the heavens, we shall occasionally
meet with one of the remarkable celestial objects which are known as
nebulae. They are faint cloudy spots, or stains of light on the black
background of the sky. They are nearly all invisible to the naked eye.
These celestial objects must not for a moment be confounded with clouds,
in the ordinary meaning of the word. The latter exist only suspended in
the atmosphere, while nebulae are immersed in the depths of space. Clouds
shine by the light of the sun, which they reflect to us; nebulae shine
with no borrowed light; they are self-luminous. Clouds change from hour
to hour; nebulae do not change even from year to year. Clouds are far
smaller than the earth; while the smallest nebula known to us is
incomparably greater than the sun. Clouds are within a few miles of the
earth; the nebulae are almost inconceivably remote.
Immediately after Herschel and his sister had settled at Slough he
commenced his review of the northern heavens in a systematic manner. For
observations of this kind it is essential that the sky be free from
cloud, while even the light of the moon is sufficient to obliterate the
fainter and more interesting objects. It was in the long and fine
winter nights, when the stars were shining brilliantly and the pale path
of the Milky Way extended across the heavens, that the labour was to be
done. The telescope being directed to the heavens, the ordinary diurnal
motion by which the sun and stars appear to rise and set carries the
stars across the field of view in a majestic panorama. The stars enter
slowly into the field of view, slowly move across it, and slowly leave
it, to be again replaced by others. Thus the observer, by merely
remaining passive at the eye-piece, sees one field after another pass
before him, and is enabled to examine their contents. It follows, that
even without moving the telescope a long narrow strip of the heavens is
brought under review, and by moving the telescope slightly up and down
the width of this strip can be suitably increased. On another night the
telescope is brought into a different position, and another strip of the
sky is examined; so that in the course of time the whole heavens can be
carefully scrutinised.
Herschel stands at the eye-piece to watch the glorious procession of
celestial objects. Close by, his sister Caroline sits at her desk, pen
in hand, to take down the observations as they fall from her brother's
lips. In front of her is a chronometer from which she can note the time,
and a contrivance which indicates the altitude of the telescope, so that
she can record the exact position of the object in connection with the
description which her brother dictated. Such was the splendid scheme
which this brother and sister had arranged to carry out as the object of
their life-long devotion. The discoveries which Herschel was destined to
make were to be reckoned not by tens or by hundreds, but by thousands.
The records of these discoveries are to be found in the "Philosophical
Transactions of the Royal Society," and they are among the richest
treasures of those volumes. It was left to Sir John Herschel, the only
son of Sir William, to complete his father's labour by repeating the
survey of the northern heavens and extending it to the southern
hemisphere. He undertook with this object a journey to the Cape of Good
Hope, and sojourned there for the years necessary to complete the great
work.
[Illustration: Fig. 97.--The Constellation of Orion, showing the
Position of the Great Nebula.]
As the result of the gigantic labours thus inaugurated and continued by
other observers, there are now about eight thousand nebulae known to us,
and with every improvement of the telescope fresh additions are being
made to the list. They differ from one another as eight thousand pebbles
selected at random on a sea-beach might differ--namely, in form, size,
colour, and material--but yet, like the pebbles, bear a certain generic
resemblance to each other. To describe this class of bodies in any
detail would altogether exceed the limits of this chapter; we shall
merely select a few of the nebulae, choosing naturally those of the most
remarkable character, and also those which are representatives of the
different groups into which nebulae may be divided.
[Illustration: PLATE XIV.
THE GREAT NEBULA IN ORION.]
We have already stated that the great nebula in the constellation of
Orion is one of the most interesting objects in the heavens. It is alike
remarkable whether we consider its size or its brilliancy, the care with
which it has been studied, or the success which has attended the efforts
to learn something of its character. To find this object, we refer to
Fig. 97 for the sketch of the chief stars in this constellation, where
the letter A indicates the middle one of the three stars which form the
sword-handle of Orion. Above the handle will be seen the three stars
which form the well-known belt so conspicuous in the wintry sky. The
star A, when viewed attentively with the unaided eye, presents a
somewhat misty appearance. In the year 1618 Cysat directed a telescope
to this star, and saw surrounding it a curious luminous haze, which
proved to be the great nebula. Ever since his time this object has been
diligently studied by many astronomers, so that very many observations
have been made of the great nebula, and even whole volumes have been
written which treat of nothing else. Any ordinary telescope will show
the object to some extent, but the more powerful the telescope the more
are the curious details revealed.
[Illustration: Fig. 98.--The Multiple star (th Orionis) in the
Great Nebula of Orion.]
In the first place, the object which we have denoted by A (th
Orionis, also called the trapezium of Orion) is in itself the most
striking multiple star in the whole heavens. It consists really of six
stars, represented in the next diagram (Fig. 98). These points are so
close together that their commingled rays cannot be distinguished
without a telescope. Four of them are, however, easily seen in quite
small instruments, but the two smaller stars require telescopes of
considerable power. And yet these stars are suns, comparable, it may be,
with our sun in magnitude.
It is not a little remarkable that this unrivalled group of six suns
should be surrounded by the renowned nebula; the nebula or the multiple
star would, either of them alone, be of exceptional interest, and here
we have a combination of the two. It seems impossible to resist drawing
the conclusion that the multiple star really lies in the nebula, and not
merely along the same line of vision. It would, indeed, seem to be at
variance with all probability to suppose that the presentation of these
two exceptional objects in the same field of view was merely accidental.
If the multiple star be really in the nebula, then this object affords
evidence that in one case at all events the distance of a nebula is a
quantity of the same magnitude as the distance of a star. This is
unhappily almost the entire extent of our knowledge of the distances of
the nebulae from the earth.
The great nebula of Orion surrounds the multiple star, and extends out
to a vast distance into the neighbouring space. The dotted circle drawn
around the star marked A in Fig. 97 represents approximately the extent
of the nebula, as seen in a moderately good telescope. The nebula is of
a faint bluish colour, impossible to represent in a drawing. Its
brightness is much greater in some places than in others; the central
parts are, generally speaking, the most brilliant, and the luminosity
gradually fades away as the edge of the nebula is approached. In fact,
we can hardly say that the nebula has any definite boundary, for with
each increase of telescopic power faint new branches can be seen. There
seems to be an empty space in the nebula immediately surrounding the
multiple star, but this is merely an illusion, produced by the contrast
of the brilliant light of the stars, as the spectroscopic examination of
the nebula shows that the nebulous matter is continuous between the
stars.
The plate of the great nebula in Orion which is here shown (Plate XIV.)
represents, in a reduced form, the elaborate drawing of this object,
which has been made with the Earl of Rosse's great reflecting telescope
at Parsonstown.[40] A telescopic view of the nebula shows two hundred
stars or more, scattered over its surface. It is not necessary to
suppose that these stars are immersed in the substance of the nebula as
the multiple star appears to be; they may be either in front of it, or,
less probably, behind it, so as to be projected on the same part of the
sky.
[Illustration: PLATE XV.
PHOTOGRAPH OF THE NEBULA 31 M ANDROMEDAE
EXPOSURE 4 HOURS, ENLARGED 3 TIMES.
TAKEN BY MR. ISAAC ROBERTS, 29 DECEMBER, 1882.]
A considerable number of drawings of this unique object have been made
by other astronomers. Among these we must mention that executed by
Professor Bond, in Cambridge, Mass., which possesses a faithfulness in
detail that every student of this object is bound to acknowledge. Of
late years also successful attempts have been made to photograph the
great nebula. The late Professor Draper was fortunate enough to obtain
some admirable photographs. In England Mr. Common was the first to take
most excellent photographs of the nebula, and superb photographs of the
same object have also been obtained by Dr. Roberts and Mr. W.E. Wilson,
which show a vast extension of the nebula into regions which it was not
previously known to occupy.
The great nebula in Andromeda, which is faintly visible to the unaided
eye, is shown in Plate XV., which has been copied with permission from
one of the astonishing photographs that Dr. Isaac Roberts has obtained.
Two dark channels in the nebula cannot fail to be noticed, and the
number of faint stars scattered over its surface is also a point to
which attention may be drawn. To find this object we must look out for
Cassiopeia and the Great Square of Pegasus, and then the nebula will be
easily perceived in the position shown on p. 413. In the year 1885 a new
star of the seventh magnitude suddenly appeared close to the brightest
part of the nebula, and declined again to invisibility after the lapse
of a few months.
The nebula in Lyra is the most conspicuous ring nebula in the heavens,
but it is not to be supposed that it is the only member of this class.
Altogether, there are about a dozen of these objects. It seems difficult
to form any adequate conception of the nature of such a body. It is,
however, impossible to view the annular nebulae without being, at all
events, reminded of those elegant objects known as vortex rings. Who has
not noticed a graceful ring of steam which occasionally escapes from the
funnel of a locomotive, and ascends high into the air, only dissolving
some time after the steam not so specialised has disappeared? Such
vortex rings can be produced artificially by a cubical box, one open
side of which is covered with canvas, while on the opposite side of the
box is a circular hole. A tap on the canvas will cause a vortex ring to
start from the hole; and if the box be filled with smoke, this ring will
be visible for many feet of its path. It would certainly be far too much
to assert that the annular nebulae have any real analogy to vortex rings;
but there is, at all events, no other object known to us with which they
can be compared.
The heavens contain a number of minute but brilliant objects known as
the planetary nebulae. They can only be described as globes of glowing
bluish-coloured gas, often small enough to be mistaken for a star when
viewed through a telescope. One of the most remarkable of these objects
lies in the constellation Draco, and can be found half-way between the
Pole Star and the star g Draconis. Some of the more recently
discovered planetary nebulae are extremely small, and they have indeed
only been distinguished from small stars by the spectroscope. It is also
to be noticed that such objects are a little out of the stellar focus in
the refracting telescope in consequence of their blue colour. This
remark does not apply to a reflecting telescope, as this instrument
conducts all the rays to a common focus.
There are many other forms of nebulae: there are long nebulous rays;
there are the wondrous spirals which have been disclosed in Lord Rosse's
great reflector; there are the double nebulae. But all these various
objects we must merely dismiss with this passing reference. There is a
great difficulty in making pictorial representations of such nebulae.
Most of them are very faint--so faint, indeed, that they can only be
seen with close attention even in powerful instruments. In making
drawings of these objects, therefore, it is impossible to avoid
intensifying the fainter features if an intelligible picture is to be
made. With this caution, however, we present Plate XVI., which exhibits
several of the more remarkable nebulae as seen through Lord Rosse's great
telescope.
[Illustration: Fig. 99.--The Nebula N.G.C., 1,499.
(_By E.E. Barnard, Lick Observatory, September 21, 1895._)]
The actual nature of the nebulae offers a problem of the greatest
interest, which naturally occupied the mind of the first assiduous
observer of nebulae, William Herschel, for many years. At first he
assumed all nebulae to be nothing but dense aggregations of stars--a very
natural conclusion for one who had so greatly advanced the optical power
of telescopes, and was accustomed to see many objects which in a small
telescope looked nebulous become "resolved" into stars when scrutinised
with a telescope of large aperture. But in 1864, when Sir William
Huggins first directed a telescope armed with a spectroscope to one of
the planetary nebulae, it became evident that at least some nebulae were
really clouds of fiery mist and not star clusters.
We shall in our next chapter deal with the spectra of the fixed stars,
but we may here in anticipation remark that these spectra are
continuous, generally showing the whole length of spectrum, from red to
violet, as in the sun's spectrum, though with many and important
differences as to the presence of dark and bright lines. A star cluster
must, of course, give a similar spectrum, resulting from the
superposition of the spectra of the single stars in the cluster. Many
nebulae give a spectrum of this kind; for instance, the great nebula in
Andromeda. But it does not by any means follow from this that these
objects are only clusters of ordinary stars, as a continuous spectrum
may be produced not only by matter in the liquid or solid state, or by
gases at high pressure, but also by gases at lower pressure but high
temperature under certain conditions. A continuous spectrum in the case
of a nebula, therefore, need not indicate that the nebula is a cluster
of bodies comparable in size and general constitution with our sun. But
if a spectrum of bright lines is given by a nebula, we can be certain
that gases at low pressure are present in the object under examination.
And this was precisely what Sir William Huggins discovered to be the
case in many nebulae. When he first decided to study the spectra of
nebulae, he selected for observation those objects known as planetary
nebulae--small, round, or slightly oval discs, generally without central
condensation, and looking like ill-defined planets. The colour of their
light, which often is blue tinted with green, is remarkable, since this
is a colour very rare among single stars. The spectrum was found to be
totally different to that of any star, consisting merely of three or
four bright lines. The brightest one is situated in the bluish-green
part of the spectrum, and was at first thought to be identical with a
line of the spectrum of nitrogen, but subsequent more accurate measures
have shown that neither this nor the second nebular line correspond to
any dark line in the solar spectrum, nor can they be produced
experimentally in the laboratory, and we are therefore unable to ascribe
them to any known element. The third and fourth lines were at once seen
to be identical with the two hydrogen lines which in the solar spectrum
are named F and g.
[Illustration: PLATE E.
NEBULAE IN THE PLEIADES.
_From a Photograph by Dr. Isaac Roberts._]
Spectrum analysis has here, as on so many other occasions, rendered
services which no telescope could ever have done. The spectra of nebulae
have, after Huggins, been studied, both visually and photographically,
by Vogel, Copeland, Campbell, Keeler, and others, and a great many very
faint lines have been detected in addition to those four which an
instrument of moderate dimensions shows. It is remarkable that the red
C-line of hydrogen, ordinarily so bright, is either absent or
excessively faint in the spectra of nebulae, but experiments by Frankland
and Lockyer have shown that under certain conditions of temperature and
pressure the complicated spectrum of hydrogen is reduced to one green
line, the F-line. It is, therefore, not surprising that the spectra of
gaseous nebulae are comparatively simple, as the probably low density of
the gases in them and the faintness of these bodies would tend to reduce
the spectra to a small number of lines. Some gaseous nebulae also show
faint continuous spectra, the place of maximum brightness of which is
not in the yellow (as in the solar spectrum), but about the green. It is
probable that these continuous spectra are really an aggregate of very
faint luminous lines.
A list of all the nebulae known to have a gaseous spectrum would now
contain about eighty members. In addition to the planetary nebulae, many
large and more diffused nebulae belong to this class, and this is also
the case with the annular nebula in Lyra and the great nebula of Orion.
It is needless to say that it is of special interest to find this grand
object enrolled among the nebulae of a gaseous nature. In this nebula
Copeland detected the wonderful D3 line of helium at a time when
"helium" was a mere name, a hypothetical something, but which we now
know to be an element very widely distributed through the universe. It
has since been found in several other nebulae. The ease with which the
characteristic gaseous spectrum is recognised has suggested the idea of
sweeping the sky with a spectroscope in order to pick up new planetary
nebulae, and a number of objects have actually been discovered by
Pickering and Copeland in this manner, as also more recently by
Pickering by examining spectrum photographs of various regions of the
sky. Most of these new objects when seen through a telescope look like
ordinary stars, and their real nature could never have been detected
without the spectroscope.
When we look up at the starry sky on a clear night, the stars seem at
first sight to be very irregularly distributed over the heavens. Here
and there a few bright stars form characteristic groups, like Orion or
the Great Bear, while other equally large tracts are almost devoid of
bright stars and only contain a few insignificant ones. If we take a
binocular, or other small telescope, and sweep the sky with it, the
result seems to be the same--now we come across spaces rich in stars;
now we meet with comparatively empty places. But when we approach the
zone of the Milky Way, we are struck with the rapid increase of the
number of stars which fill the field of the telescope; and when we reach
the Milky Way itself, the eye is almost unable to separate the single
points of light, which are packed so closely together that they produce
the appearance to the naked eye of a broad, but very irregular, band of
dim light, which even a powerful telescope in some places can hardly
resolve into stars. How are we to account for this remarkable
arrangement of the stars? What is the reason of our seeing so few at the
parts of the heavens farthest from the Milky Way, and so very many in or
near that wonderful belt? The first attempt to give an answer to these
questions was made by Thomas Wright, an instrument maker in London, in a
book published in 1750. He supposed the stars of our sidereal system to
be distributed in a vast stratum of inconsiderable thickness compared
with its length and breadth. If we had a big grindstone made of glass,
in which had become uniformly imbedded a vast quantity of grains of sand
or similar minute particles, and if we were able to place our eye
somewhere near the centre of this grindstone, it is easy to see that we
should see very few particles near the direction of the axle of the
grindstone, but a great many if we looked towards any point of the
circumference. This was Wright's idea of the structure of the Milky Way,
and he supposed the sun to be situated not very far from the centre of
this stellar stratum.
[Illustration: PLATE F.
o CENTAURI.
_From a Drawing in the Publications of Harvard College Observatory._]
If the Milky Way itself did not exist--and we had simply the fact to
build on that the stars appeared to increase rapidly in number towards a
certain circle (almost a great circle) spanning the heavens--then the
disc theory might have a good deal in its favour. But the telescopic
study of the Milky Way, and even more the marvellous photographs of its
complicated structure produced by Professor Barnard, have given the
death blow to the old theory, and have made it most reasonable to
conclude that the Milky Way is really, and not only apparently, a mighty
stream of stars encircling the heavens. We shall shortly mention a few
facts which point in this direction. A mere glance is sufficient to show
that the Milky Way is not a single belt of light; near the constellation
Aquila it separates into two branches with a fairly broad interval
between them, and these branches do not meet again until they have
proceeded far into the southern hemisphere. The disc theory had, in
order to explain this, to assume that the stellar stratum was cleft in
two nearly to the centre. But even if we grant this, how can we account
for the numerous more or less dark holes in the Milky Way, the largest
and most remarkable of which is the so-called "coal sack" in the
southern hemisphere? Obviously we should have to assume the existence of
a number of tunnels, drilled through the disc-like stratum, and by some
strange sympathy all directed towards the spot where our solar system is
situated. And the many small arms which stretch out from the Milky Way
would have to be either planes seen edgeways or the convexities of
curved surfaces viewed tangentially. The improbability of these various
assumptions is very great. But evidence is not wanting that the
relatively bright stars are crowded together along the same zone where
the excessively faint ones are so closely packed. The late Mr. Proctor
plotted all the stars which occur in Argelander's great atlas of the
northern hemisphere, 324,198 in number, on a single chart, and though
these stars are all above the tenth magnitude, and thus superior in
brightness to that innumerable host of stars of which the individual
members are more or less lost in the galactic zone, and on the
hypothesis of uniform distribution ought to be relatively near to us,
the chart shows distinctly the whole course of the Milky Way by the
clustering of these stars. This disposes sufficiently of the idea that
the Milky Way is nothing but a disc-like stratum seen projected on the
heavenly sphere; after this it is hardly necessary to examine Professor
Barnard's photographs and see how fairly bright and very faint regions
alternate without any attempt at regularity, in order to become
convinced that the Milky Way is more probably a stream of stars
clustered together, a stream or ring of incredibly enormous dimensions,
inside which our solar system happens to be situated. But it must be
admitted that it is premature to attempt to find the actual figure of
this stream or to determine the relative distance of the various
portions of it.
[Illustration: PLATE XVI.
NEBULAE
OBSERVED WITH LORD ROSSE'S GREAT TELESCOPE.]