SYSTEMS OF STARS
Many stars are seen comparatively close together. This may plainly arise
from two reasons. Firstly, the stars may happen to be almost in the same
line of sight; that is to say, seen in nearly the same direction; and
though one star may be ever so much nearer to us than the other, the
result will give all the appearance of a related pair. A seeming
arrangement of two stars in this way is known as a "double," or double
star; or, indeed, to be very precise, an "optical double." Secondly, in
a pair of stars, both bodies may be about the same distance from us, and
actually connected as a system like, for instance, the moon and the
earth. A pairing of stars in this way, though often casually alluded to
as a double star, is properly termed a "binary," or binary system.
But collocations of stars are by no means limited to two. We find,
indeed, all over the sky such arrangements in which there are three or
more stars; and these are technically known as "triple" or "multiple"
stars respectively. Further, groups are found in which a great number of
stars are closely massed together, such a massing together of stars
being known as a "cluster."
The Pole Star (Polaris) is a double star, one of the components being of
a little below the second magnitude, and the other a little below the
ninth. They are so close together that they appear as one star to the
naked eye, but they may be seen separate with a moderately sized
telescope. The brighter star is yellowish, and the faint one white. This
brighter star is found _by means of the spectroscope_ to be actually
composed of three stars so very close together that they cannot be seen
separately even with a telescope. It is thus a triple star, and the
three bodies of which it is composed are in circulation about each
other. Two of them are darker than the third.
The method of detecting binary stars by means of the spectroscope is an
application of Doppler's principle. It will, no doubt, be remembered
that, according to the principle in question, we are enabled, from
certain shiftings of the lines in the spectrum of a luminous body, to
ascertain whether that body is approaching us or receding from us. Now
there are certain stars which always appear single even in the largest
telescopes, but when the spectroscope is directed to them a spectrum
_with two sets of lines_ is seen. Such stars must, therefore, be double.
Further, if the shiftings of the lines, in a spectrum like this, tell us
that the component stars are making small movements to and from us which
go on continuously, we are therefore justified in concluding that these
are the orbital revolutions of a binary system greatly compressed by
distance. Such connected pairs of stars, since they cannot be seen
separately by means of any telescope, no matter how large, are known as
"spectroscopic binaries."
In observations of spectroscopic binaries we do not always get a double
spectrum. Indeed, if one of the components be below a certain
magnitude, its spectrum will not appear at all; and so we are left in
the strange uncertainty as to whether this component is merely faint or
actually dark. It is, however, from the shiftings of the lines in the
spectrum of the other component that we see that an orbital movement is
going on, and are thus enabled to conclude that two bodies are here
connected into a system, although one of these bodies resolutely refuses
directly to reveal itself even to the all-conquering spectroscope.
Mizar, that star in the handle of the Plough to which we have already
drawn attention, will be found with a small telescope to be a fine
double, one of the components being white and the other greenish.
Actually, however, as the American astronomer, Professor F.R. Moulton,
points out, these stars are so far from each other that if we could be
transferred to one of them we should see the other merely as an ordinary
bright star. The spectroscope shows that the brighter of these stars is
again a binary system of two huge suns, the components revolving around
each other in a period of about twenty days. This discovery made by
Professor E.C. Pickering, the _first_ of the kind by means of the
spectroscope, was announced in 1889 from the Harvard Observatory in the
United States.
A star close to Vega, known as [e] (Epsilon) Lyræ (see Plate XIX., p.
292), is a double, the components of which may be seen separately with
the naked eye by persons with very keen eyesight. If this star, however,
be viewed with the telescope, the two companions will be seen far apart;
and it will be noticed that each of them is again a double.
By means of the spectroscope Capella is shown to be really composed of
two stars (one about twice as bright as the other) situated very close
together and forming a binary system. Sirius is also a binary system;
but it is what is called a "visual" one, for its component stars may be
_seen_ separately in very large telescopes. Its double, or rather
binary, nature, was discovered in 1862 by the celebrated optician Alvan
G. Clark, while in the act of testing the 18-inch refracting telescope,
then just constructed by his firm, and now at the Dearborn Observatory,
Illinois, U.S.A. The companion is only of the tenth magnitude, and
revolves around Sirius in a period of about fifty years, at a mean
distance equal to about that of Uranus from the sun. Seen from Sirius,
it would shine only something like our full moon. It must be
self-luminous and not a mere planet; for Mr. Gore has shown that if it
shone only by the light reflected from Sirius, it would be quite
invisible even in the Great Yerkes Telescope.
Procyon is also a binary, its companion having been discovered by
Professor J.M. Schaeberle at the Lick Observatory in 1896. The period of
revolution in this system is about forty years. Observations by Mr. T.
Lewis of Greenwich seem, however, to point to the companion being a
small nebula rather than a star.
The star [ê] (Eta) Cassiopeiæ (see Plate XIX., p. 292), is easily seen
as a fine double in telescopes of moderate size. It is a binary system,
the component bodies revolving around their common centre of gravity in
a period of about two hundred years. This system is comparatively near
to us, _i.e._ about nine light years, or a little further off than
Sirius.
In a small telescope the star Castor will be found double, the
components, one of which is brighter than the other, forming a binary
system. The fainter of these was found by Belopolsky, with the
spectroscope, to be composed of a system of two stars, one bright and
the other either dark or not so bright, revolving around each other in a
period of about three days. The brighter component of Castor is also a
spectroscopic binary, with a period of about nine days; so that the
whole of what we see with the naked eye as Castor, is in reality a
remarkable system of four stars in mutual orbital movement.
Alpha Centauri--the nearest star to the earth--is a visual binary, the
component bodies revolving around each other in a period of about
eighty-one years. The extent of this system is about the same as that of
Sirius. Viewed from each other, the bodies would shine only like our sun
as seen from Neptune.
Among the numerous binary stars the orbits of some fifty have been
satisfactorily determined. Many double stars, for which this has not yet
been done, are, however, believed to be, without doubt, binary. In some
cases a parallax has been found; so that we are enabled to estimate in
miles the actual extent of such systems, and the masses of the bodies in
terms of the sun's mass.
Most of the spectroscopic binaries appear to be upon a smaller scale
than the telescopic ones. Some are, indeed, comparatively speaking,
quite small. For instance, the component stars forming [b] Aurigæ are
about eight million miles apart, while in [z] Geminorum, the distance
between the bodies is only a little more than a million miles.
Spectroscopic binaries are probably very numerous. Professor W.W.
Campbell, Director of the Lick Observatory, estimates, for instance,
that, out of about every half-a-dozen stars, one is a spectroscopic
binary.
It is only in the case of binary systems that we can discover the masses
of stars at all. These are ascertained from their movements with regard
to each other under the influence of their mutual gravitative
attractions. In the case of simple stars we have clearly nothing of the
kind to judge by; though, if we can obtain a parallax, we may hazard a
guess from their brightness.
Binary stars were incidentally discovered by Sir William Herschel. In
his researches to get a stellar parallax he had selected a number of
double stars for test purposes, on the assumption that, if one of such a
pair were much nearer than the other, it might show a displacement with
regard to its neighbour as a direct consequence of the earth's orbital
movement around the sun. He, however, failed entirely to obtain any
parallaxes, the triumph in this being, as we have seen, reserved for
Bessel. But in some of the double stars which he had selected, he found
certain alterations in the relative positions of the bodies, which
plainly were not a consequence of the earth's motion, but showed rather
that there was an actual circling movement of the bodies themselves
under their mutual attractions. It is to be noted that the existence of
such connected pairs had been foretold as probable by the Rev. John
Michell, who lived a short time before Herschel.
The researches into binary systems--both those which can be seen with
the eye and those which can be observed by means of the spectroscope,
ought to impress upon us very forcibly the wide sway of the law of
gravitation.
Of star clusters about 100 are known, and such systems often contain
several thousand stars. They usually cover an area of sky somewhat
smaller than the moon appears to fill. In most clusters the stars are
very faint, and, as a rule, are between the twelfth and sixteenth
magnitudes. It is difficult to say whether these are actually small
bodies, or whether their faintness is due merely to their great distance
from us, since they are much too far off to show any appreciable
parallactic displacement. Mr. Gore, however, thinks there is good
evidence to show that the stars in clusters are really close, and that
the clusters themselves fill a comparatively small space.
One of the finest examples of a cluster is the great globular one, in
the constellation of Hercules, discovered by Halley in 1714. It contains
over 5000 stars, and upon a clear, dark night is visible to the naked
eye as a patch of light. In the telescope, however, it is a wonderful
object. There are also fine clusters in the constellations of Auriga,
Pegasus, and Canes Venatici. In the southern heavens there are some
magnificent examples of globular clusters. This hemisphere seems,
indeed, to be richer in such objects than the northern. For instance,
there is a great one in the constellation of the Centaur, containing
some 6000 stars (see Plate XXI., p. 306).
[Illustration: PLATE XXI. THE GREAT GLOBULAR CLUSTER IN THE SOUTHERN
CONSTELLATION OF CENTAURUS
From a photograph taken at the Cape Observatory, on May 24th, 1903. Time
of exposure, 1 hour.
(Page 306)]
Certain remarkable groups of stars, of a nature similar to clusters,
though not containing such faint or densely packed stars as those we
have just alluded to, call for a mention in this connection. The best
example of such star groups are the Pleiades and the Hyades (see Plate
XX., p. 296), Coma Berenices, and Præsepe (or the Beehive), the
last-named being in the constellation of Cancer.
Stars which alter in their brightness are called _Variable Stars_, or
"variables." The first star whose variability attracted attention is
that known as Omicron Ceti, namely, the star marked with the Greek
letter [o] (Omicron) in the constellation of Cetus, or the Whale, a
constellation situated not far from Taurus. This star, the variability
of which was discovered by Fabricius in 1596, is also known as Mira, or
the "Wonderful," on account of the extraordinary manner in which its
light varies from time to time. The star known by the name of Algol,[32]
popularly called the "Demon Star"--whose astronomical designation is [b]
(Beta) Persei, or the star second in brightness in the constellation of
Perseus--was discovered by Goodricke, in the year 1783, to be a variable
star. In the following year [b] Lyræ, the star in Lyra next in order of
brightness after Vega, was also found by the same observer to be a
variable. It may be of interest to the reader to know that Goodricke was
deaf and dumb, and that he died in 1786 at the early age of twenty-one
years!
It was not, however, until the close of the nineteenth century that much
attention was paid to variable stars. Now several hundreds of these are
known, thanks chiefly to the observations of, amongst others, Professor
S.C. Chandler of Boston, U.S.A., Mr. John Ellard Gore of Dublin, and Dr.
A.W. Roberts of South Africa. This branch of astronomy has not, indeed,
attracted as much popular attention as it deserves, no doubt because the
nature of the work required does not call for the glamour of an
observatory or a large telescope.
The chief discoveries with regard to variable stars have been made by
the naked eye, or with a small binocular. The amount of variation is
estimated by a comparison with other stars. As in many other branches of
astronomy, photography is now employed in this quest with marked
success; and lately many variable stars have been found to exist in
clusters and nebulæ.
It was at one time considered that a variable star was in all
probability a body, a portion of whose surface had been relatively
darkened in some manner akin to that in which sun spots mar the face of
the sun; and that when its axial rotation brought the less illuminated
portions in turn towards us, we witnessed a consequent diminution in the
star's general brightness. Herschel, indeed, inclined to this
explanation, for his belief was that all the stars bore spots like those
of the sun. It appears preferably thought nowadays that disturbances
take place periodically in the atmosphere or surroundings of certain
stars, perhaps through the escape of imprisoned gases, and that this may
be a fruitful cause of changes of brilliancy. The theory in question
will, however, apparently account for only one class of variable star,
namely, that of which Mira Ceti is the best-known example. The scale on
which it varies in brightness is very great, for it changes from the
second to the ninth magnitude. For the other leading type of variable
star, Algol, of which mention has already been made, is the best
instance. The shortness of the period in which the changes of brightness
in such stars go their round, is the chief characteristic of this latter
class. The period of Algol is a little under three days. This star when
at its brightest is of about the second magnitude, and when least bright
is reduced to below the third magnitude; from which it follows that its
light, when at the minimum, is only about one-third of what it is when
at the maximum. It seems definitely proved by means of the spectroscope
that variables of this kind are merely binary stars, too close to be
separated by the telescope, which, as a consequence of their orbits
chancing to be edgewise towards us, eclipse each other in turn time
after time. If, for instance, both components of such a pair are bright,
then when one of them is right behind the other, we will not, of course,
get the same amount of light as when they are side by side. If, on the
other hand, one of the components happens to be dark or less luminous
and the other bright, the manner in which the light of the bright star
will be diminished when the darker star crosses its face should easily
be understood. It is to the second of these types that Algol is supposed
to belong. The Algol system appears to be composed of a body about as
broad as our sun, which regularly eclipses a brighter body which has a
diameter about half as great again.
Since the companion of Algol is often spoken of as a _dark_ body, it
were well here to point out that we have no evidence at all that it is
entirely devoid of light. We have already found, in dealing with
spectroscopic binaries, that when one of the component stars is below a
certain magnitude[33] its spectrum will not be seen; so one is left in
the glorious uncertainty as to whether the body in question is
absolutely dark, or darkish, or faint, or indeed only just out of range
of the spectroscope.
It is thought probable by good authorities that the companion of Algol
is not quite dark, but has some inherent light of its own. It is, of
course, much too near Algol to be seen with the largest telescope. There
is in fact a distance of only from two to three millions of miles
between the bodies, from which Mr. Gore infers that they would probably
remain unseparated even in the largest telescope which could ever be
constructed by man.
The number of known variables of the Algol type is, so far, small; not
much indeed over thirty. In some of them the components are believed to
revolve touching each other, or nearly so. An extreme example of this is
found in the remarkable star V. Puppis, an Algol variable of the
southern hemisphere. Both its components are bright, and the period of
light variation is about one and a half days. Dr. A. W. Roberts finds
that the bodies are revolving around each other in actual contact.
_Temporary stars_ are stars which have suddenly blazed out in regions of
the sky where no star was previously seen, and have faded away more or
less gradually.
It was the appearance of such a star, in the year 134 B.C., which
prompted Hipparchus to make his celebrated catalogue, with the object of
leaving a record by which future observers could note celestial changes.
In 1572 another star of this kind flashed out in the constellation of
Cassiopeia (see Plate XIX., p. 292), and was detected by Tycho Brahe. It
became as bright as the planet Venus, and eventually was visible in the
day-time. Two years later, however, it disappeared, and has never since
been seen. In 1604 Kepler recorded a similar star in the constellation
of Ophiuchus which grew to be as bright as Jupiter. It also lasted for
about two years, and then faded away, leaving no trace behind. It is
rarely, however, that temporary stars attain to such a brilliance; and
so possibly in former times a number of them may have appeared, but not
have risen to a sufficient magnitude to attract attention. Even now,
unless such a star becomes clearly visible to the naked eye, it runs a
good chance of not being detected. A curious point, worth noting, with
regard to temporary stars is that the majority of them have appeared in
the Milky Way.
These sudden visitations have in our day received the name of _Novæ_;
that is to say, "New" Stars. Two, in recent years, attracted a good deal
of attention. The first of these, known as Nova Aurigæ, or the New Star
in the constellation of Auriga, was discovered by Dr. T.D. Anderson at
Edinburgh in January 1892. At its greatest brightness it attained to
about the fourth magnitude. By April it had sunk to the twelfth, but
during August it recovered to the ninth magnitude. After this last
flare-up it gradually faded away.
The startling suddenness with which temporary stars usually spring into
being is the groundwork upon which theories to account for their origin
have been erected. That numbers of dark stars, extinguished suns, so to
speak, may exist in space, there is a strong suspicion; and it is just
possible that we have an instance of one dark stellar body in the
companion of Algol. That such dark stars might be in rapid motion is
reasonable to assume from the already known movements of bright stars.
Two dark bodies might, indeed, collide together, or a collision might
take place between a dark star and a star too faint to be seen even with
the most powerful telescope. The conflagration produced by the impact
would thus appear where nothing had been seen previously. Again, a
similar effect might be produced by a dark body, or a star too faint to
be seen, being heated to incandescence by plunging in its course through
a nebulous mass of matter, of which there are many examples lying about
in space.
The last explanation, which is strongly reminiscent of what takes place
in shooting stars, appears more probable than the collision theory. The
flare-up of new stars continues, indeed, only for a comparatively short
time; whereas a collision between two bodies would, on the other hand,
produce an enormous nebula which might take even millions of years to
cool down. We have, indeed, no record of any such sudden appearance of a
lasting nebula.
The other temporary star, known as Nova Persei, or the new star in the
constellation of Perseus, was discovered early in the morning of
February 22, 1901, also by Dr. Anderson. A day later it had grown to be
brighter than Capella. Photographs which had been taken, some three days
previous to its discovery, of the very region of the sky in which it had
burst forth, were carefully examined, and it was not found in these. At
the end of two days after its discovery Nova Persei had lost one-third
of its light. During the ensuing six months it passed through a series
of remarkable fluctuations, varying in brightness between the third and
fifth magnitudes. In the month of August it was seen to be surrounded by
luminous matter in the form of a nebula, which appeared to be gradually
spreading to some distance around. Taking into consideration the great
way off at which all this was taking place, it looked as if the new star
had ejected matter which was travelling outward with a velocity
equivalent to that of light. The remarkable theory was, however, put
forward by Professor Kapteyn and the late Dr. W.E. Wilson that there
might be after all no actual transmission of matter; but that perhaps
the real explanation was the gradual _illumination_ of hitherto
invisible nebulous matter, as a consequence of the flare-up which had
taken place about six months before. It was, therefore, imagined that
some dark body moving through space at a very rapid rate had plunged
through a mass of invisible nebulous matter, and had consequently become
heated to incandescence in its passage, very much like what happens to a
meteor when moving through our atmosphere. The illumination thus set up
temporarily in one point, being transmitted through the nebulous wastes
around with the ordinary velocity of light, had gradually rendered this
surrounding matter visible. On the assumptions required to fit in with
such a theory, it was shown that Nova Persei must be at a distance from
which light would take about three hundred years in coming to us. The
actual outburst of illumination, which gave rise to this temporary star,
would therefore have taken place about the beginning of the reign of
James I.
Some recent investigations with regard to Nova Persei have, however,
greatly narrowed down the above estimate of its distance from us. For
instance, Bergstrand proposes a distance of about ninety-nine light
years; while the conclusions of Mr. F.W. Very would bring it still
nearer, _i.e._ about sixty-five light years.
The last celestial objects with which we have here to deal are the
_Nebulæ_. These are masses of diffused shining matter scattered here and
there through the depths of space. Nebulæ are of several kinds, and have
been classified under the various headings of Spiral, Planetary, Ring,
and Irregular.
A typical _spiral_ nebula is composed of a disc-shaped central portion,
with long curved arms projecting from opposite sides of it, which give
an impression of rapid rotatory movement.
The discovery of spiral nebulæ was made by Lord Rosse with his great
6-foot reflector. Two good examples of these objects will be found in
Ursa Major, while there is another fine one in Canes Venatici (see Plate
XXII., p. 314), a constellation which lies between Ursa Major and
Boötes. But the finest spiral of all, perhaps the most remarkable nebula
known to us, is the Great Nebula in the constellation of Andromeda, (see
Plate XXIII., p. 316)--a constellation just further from the pole than
Cassiopeia. When the moon is absent and the night clear this nebula can
be easily seen with the naked eye as a small patch of hazy light. It is
referred to by Al Sufi.
[Illustration: PLATE XXII. SPIRAL NEBULA IN THE CONSTELLATION OF CANES
VENATICI
From a photograph by the late Dr. W.E. Wilson, D.Sc., F.R.S.
(Page 314)]
Spiral nebulæ are white in colour, whereas the other kinds of nebula
have a greenish tinge. They are also by far the most numerous; and the
late Professor Keeler, who considered this the normal type of nebula,
estimated that there were at least 120,000 of such spirals within the
reach of the Crossley reflector of the Lick Observatory. Professor
Perrine has indeed lately raised this estimate to half a million, and
thinks that with more sensitive photographic plates and longer exposures
the number of spirals would exceed a million. The majority of these
objects are very small, and appear to be distributed over the sky in a
fairly uniform manner.
_Planetary_ nebulæ are small faint roundish objects which, when seen in
the telescope, recall the appearance of a planet, hence their name. One
of these nebulæ, known astronomically as G.C. 4373, has recently been
found to be rushing through space towards the earth at a rate of between
thirty and forty miles per second. It seems strange, indeed, that any
gaseous mass should move at such a speed!
What are known as _ring_ nebulæ were until recently believed to form a
special class. These objects have the appearance of mere rings of
nebulous matter. Much doubt has, however, been thrown upon their being
rings at all; and the best authorities regard them merely as spiral
nebulæ, of which we happen to get a foreshortened view. Very few
examples are known, the most famous being one in the constellation of
Lyra, usually known as the Annular Nebula in Lyra. This object is so
remote from us as to be entirely invisible to the naked eye. It contains
a star of the fifteenth magnitude near to its centre. From photographs
taken with the Crossley reflector, Professor Schaeberle finds in this
nebula evidences of spiral structure. It may here be mentioned that the
Great Nebula in Andromeda, which has now turned out to be a spiral, had
in earlier photographs the appearance of a ring.
There also exist nebulæ of _irregular_ form, the most notable being the
Great Nebula in the constellation of Orion (see Plate XXIV., p. 318). It
is situated in the centre of the "Sword" of Orion (see Plate XX., p.
296). In large telescopes it appears as a magnificent object, and in
actual dimensions it must be much on the same scale as the Andromeda
Nebula. The spectroscope tells us that it is a mass of glowing gas.
The Trifid Nebula, situated in the constellation of Sagittarius, is an
object of very strange shape. Three dark clefts radiate from its centre,
giving it an appearance as if it had been torn into shreds.
The Dumb-bell Nebula, a celebrated object, so called from its likeness
to a dumb-bell, turns out, from recent photographs taken by Professor
Schaeberle, which bring additional detail into view, to be after all a
great spiral.
There is a nest, or rather a cluster of nebulæ in the constellation of
Coma Berenices; over a hundred of these objects being here gathered into
a space of sky about the size of our full moon.
[Illustration: PLATE XXIII. THE GREAT NEBULA IN THE CONSTELLATION OF
ANDROMEDA
From a photograph taken at the Yerkes Observatory.
(Page 314)]
The spectroscope informs us that spiral nebulæ are composed of
partially-cooled matter. Their colour, as we have seen, is white. Nebulæ
of a greenish tint are, on the other hand, found to be entirely in a
gaseous condition. Just as the solar corona contains an unknown element,
which for the time being has been called "Coronium," so do the gaseous
nebulæ give evidence of the presence of another unknown element. To this
Sir William Huggins has given the provisional name of "Nebulium."
The _Magellanic Clouds_ are two patches of nebulous-looking light, more
or less circular in form, which are situated in the southern hemisphere
of the sky. They bear a certain resemblance to portions of the Milky
Way, but are, however, not connected with it. They have received their
name from the celebrated navigator, Magellan, who seems to have been one
of the first persons to draw attention to them. "Nubeculæ" is another
name by which they are known, the larger cloud being styled _nubecula
major_ and the smaller one _nubecula minor_. They contain within them
stars, clusters, and gaseous nebulæ. No parallax has yet been found for
any object which forms part of the nubeculæ, so it is very difficult to
estimate at what distance from us they may lie. They are, however,
considered to be well within our stellar universe.
Having thus brought to a conclusion our all too brief review of the
stars and the nebulæ--of the leading objects in fine which the celestial
spaces have revealed to man--we will close this chapter with a recent
summation by Sir David Gill of the relations which appear to obtain
between these various bodies. "Huggins's spectroscope," he says, "has
shown that many nebulæ are not stars at all; that many well-condensed
nebulæ, as well as vast patches of nebulous light in the sky, are but
inchoate masses of luminous gas. Evidence upon evidence has accumulated
to show that such nebulæ consist of the matter out of which stars
(_i.e._ suns) have been and are being evolved. The different types of
star spectra form such a complete and gradual sequence (from simple
spectra resembling those of nebulæ onwards through types of gradually
increasing complexity) as to suggest that we have before us, written in
the cryptograms of these spectra, the complete story of the evolution of
suns from the inchoate nebula onwards to the most active sun (like our
own), and then downward to the almost heatless and invisible ball. The
period during which human life has existed upon our globe is probably
too short--even if our first parents had begun the work--to afford
observational proof of such a cycle of change in any particular star;
but the fact of such evolution, with the evidence before us, can hardly
be doubted."[34]
[32] The name Al gûl, meaning the Demon, was what the old Arabian
astronomers called it, which looks very much as if they had already
noticed its rapid fluctuations in brightness.
[33] Mr. Gore thinks that the companion of Algol may be a star of the
sixth magnitude.
[34] Presidential Address to the British Association for the Advancement
of Science (Leicester, 1907), by Sir David Gill, K.C.B., LL.D., F.R.S.,
&c. &c.
[Illustration: PLATE XXIV. THE GREAT NEBULA IN THE CONSTELLATION OF
ORION
From a photograph taken at the Yerkes Observatory.
(Page 316)]