similar in appearance to the great nebula in Orion. It surrounds the
famous variable star Eta Argûs. It is sometimes spoken of as the
“keyhole” nebula, owing to a curious opening of that shape near its
centre. It was carefully drawn by Sir John Herschel at the Cape of
Good Hope in the years 1834-38. It lies in a very brilliant portion
of the Milky Way, and Sir John Herschel thus describes it: “It is
not easy for language to convey a full impression of the beauty and
sublimity of the spectacle which the nebula offers as it enters the
field of view of a telescope, fixed in right ascension, by the
diurnal motion, ushered in as it is by so glorious and innumerable
a procession of stars, to which it forms a sort of climax, and in
a part of the heavens otherwise full of interest,” and he adds:
“In no part of its extent does this nebula show any appearance of
resolvability into stars, being, in this respect, analogous to the
nebula of Orion. It has, therefore, nothing in common with the Milky
Way, on the ground of which we see it projected, and may therefore
be, and not improbably is, placed at an immeasurable distance behind
that stratum.” Sir John Herschel’s conclusion as to its physical
constitution has been fully confirmed by the spectroscope, which
shows it to consist of luminous gas. As in the Orion nebula, there
are numerous stars scattered over it. Some of these may possibly have
a physical connection with the nebula, while others may belong to the
Milky Way. The nebula is of great extent, covering an apparent space
about five times the area of the full moon, and its real dimensions
must be enormous. It was photographed by Mr. Russell, director of
the Sydney Observatory, in July, 1890, and the photograph shows that
“one of the brightest and most conspicuous parts of the nebula”—the
swan-shaped form near the centre of Herschel’s drawing—has “wholly
disappeared,” and its place is now occupied by “a great, dark oval.”
Mr. Russell first missed the vanished portion of the nebula in
the year 1871, while examining it with a telescope of 11½ inches
aperture, and the photograph now confirms the disappearance, which
is very remarkable, and shows that changes are actually in progress
in these wonderful nebulæ, changes which may be detected after a
comparatively short interval of time.
Smaller than the nebula in Argo, but somewhat similar in general
appearance, is that known as 30 Doradus, which forms one of the
numerous and diverse objects which together constitute the greater
Magellanic Cloud. Sir John Herschel drew it carefully at the Cape
of Good Hope, and describes it as “one of the most singular and
extraordinary objects which the heavens present,” and he says “it is
unique even in the system to which it belongs, there being no other
object in either nubecula to which it bears the least resemblance.”
It is sometimes called the “looped nebula,” from the curious openings
it contains. One of these is somewhat similar to the “key-hole”
opening in the Argo nebula. Near its centre is a small cluster of
stars, and scattered over the nebula are many faint stars, of which
Sir John Herschel gives a catalogue of 105, ranging from the ninth to
the seventeenth magnitude. I do not know whether this nebula has been
examined with the spectroscope, but its appearance would suggest that
it is gaseous. It is remarkable as being the only object of its class
which is found outside the zone of the Milky Way.
Among the nebulæ of irregular shape, although its spectrum is said to
be not gaseous, may be mentioned that known as the “trifid nebula,”
or 20 Messier. It lies closely north of the star 4 Sagittarii in a
magnificent region of the heavens. In the drawing made by Sir John
Herschel at the Cape of Good Hope, the principal portion consists of
three masses of nebulous matter separated by dark “lanes” or “rifts.”
Near the junction of the three “rifts” is a triple star. A beautiful
drawing of this nebula has also been made by Trouvelot. It agrees
fairly well with that of Sir John Herschel, but shows more detail.
Among other gaseous nebulæ may be mentioned that called by Sir John
Herschel the “dumb-bell” nebula. It lies a little south of the sixth
magnitude star 14 Vulpeculæ, and was discovered by Messier in 1779,
while observing Bode’s comet of that year. In small telescopes it
has the appearance of a dumb-bell, or hour-glass, but in larger
telescopes the outline is filled in with fainter nebulous light,
giving to the whole an elliptical form. Several faint stars have
been seen in it, but these probably belong to the Milky Way, as Dr.
Huggins finds the spectrum gaseous. Dr. Roberts has photographed it,
and he thinks that “the nebula is probably a globular mass of nebular
matter which is undergoing the process of condensation into stars,
and the faint protrusions of nebulosity in the _south following_
and _north preceding_ ends are the projections of a broad ring of
nebulosity which surrounds the globular mass. This ring, not being
sufficiently dense to obscure the light of the central region of the
globular mass, is dense enough to obscure those parts of it that are
hidden by the increased thickness of the nebulosity, thus producing
the ‘dumb-bell’ appearance. If these inferences are true, we may
proceed yet a step, or a series of steps, further, and predict that
the consummation of the life-history of this nebula will be its
reduction to a globular cluster of stars.”
Among the gaseous nebulæ may also be included those known as “annular
nebulæ.” These are very rare objects, only a few being known in the
whole heavens. The most remarkable is that known as 57 Messier, which
lies between the stars Beta and Gamma Lyræ, south of the bright star
Vega. It was discovered by Darquier, at Toulouse, in 1779, while
following Bode’s comet of that year. Lord Rosse thought it resolvable
into stars, and so did Chacornac and Secchi, but no stars are
perceptible with the great American telescopes, and Dr. Huggins finds
it to be gaseous. The central portion is not absolutely dark, but
contains some faint nebulous light. Examined with the great telescope
of the Lick Observatory, Professor Barnard finds that the opening of
the ring is filled in with fainter light “about midway in brightness
between the brightness of the ring and the darkness of the adjacent
sky. The aperture was more nearly circular than the outer boundary
of the nebula, so that the ends of the ring were thicker than the
sides.” The entire nebula was of a milky color. A central star,
noticed by some observers, was usually seen by Professor Barnard, but
was never a conspicuous object. He found the extreme dimensions of
the nebula about 81″ in length by about 59″ in width, or more than
double the apparent area of Jupiter’s disk. It has been beautifully
photographed by Dr. Roberts, and he says “the photograph shows the
nebula and the interior of the ring more elliptical than the drawings
and descriptions indicate; and the star of the _following_ side is
nearer to the ring than the distance given. The nebulosity on the
_preceding_ and _following_ ends of the ring protrudes a little, and
is less dense than on the _north_ and _south_ sides. This probably
suggested the filamentous appearance which Lord Rosse shows. Some
photographs of the nebula have been taken between 1887 and 1891, and
the central star is strongly shown on some of them, but on others it
is scarcely visible, which points to the star being variable.” On a
photograph taken by MM. Androyer and Montaugerand of the Toulouse
Observatory, with an exposure of nine hours (in multiple exposures),
about 4,800 stars are visible on and near the nebula in an area of
three square degrees.
Another object of the annular class will be found a little to the
southwest of the star Lambda Scorpii. It is thus described by Sir
John Herschel: “A delicate, extremely faint, but perfectly well
defined, annulus. The field crowded with stars, two of which are
on the nebula. A beautiful, delicate ring of a faint, ghost-like
appearance, about 40″ in diameter in a field of about 150 stars,
eleven and twelve magnitude and under.”
Near the stars 44 and 51 Ophiuchi is another object of the annular
class, which Sir John Herschel describes as “exactly round, pretty
faint, 12″ diameter, well terminated, but a little cottony at the
edge, and with a decided darkness in the middle, equal to a tenth
magnitude star at the most. Few stars in the field, a beautiful
specimen of the planetary annular class of nebula.”
The Planetary Nebulæ form an interesting class. They were so named
by Sir William Herschel from their resemblance to the disks of the
planets, but, of course, much fainter. They are generally of uniform
brightness, without any nucleus or brighter part in the centre. There
are numerous examples of this class, one of the most remarkable
being that known as 97 Messier, which is situated about two degrees
southeast of Beta Ursæ Majoris—the southern of the two “pointers” in
the Plow. It is of considerable apparent size, and even supposing its
distance to be not greater than that of 61 Cygni, its real dimensions
must be enormous. Lord Rosse observed two openings in the centre
with a star in each opening, and from this appearance he called it
the “owl nebula.” One of the stars seems to have disappeared since
1850, and a photograph recently taken by Dr. Roberts confirms the
disappearance.
Another fine object of the planetary class is one which lies close
to the pole of the ecliptic. Webb saw it “like a considerable star
out of focus.” Smyth found it pale blue in color. Dr. Huggins finds
a gaseous spectrum, the first discovery of the kind made. Professor
Holden, observing it with the great Lick telescope, finds its
structure extraordinary. He says it “is apparently composed of rings
overlying each other, and it is difficult to resist the conviction
that these are arranged in space in the form of a true helix,” and he
ranks it in a new class which he calls “helical nebulæ.”
A somewhat similar nebula lies a little to the west of the star Nu
Aquarii. Secchi believed it to be in reality a cluster of small
stars, but Dr. Huggins finds its spectrum gaseous. A small nebula
on each side gives it an appearance somewhat similar to the planet
Saturn, with the rings seen edgewise. The great Lick telescope shows
it as a wonderful object—“a central ring lies upon an oval of much
fainter nebulosity.” Professor Holden says “the color is a pale
blue,” and he compares the appearance of the central ring “to that of
a footprint left in the wet sand on a sea beach.”
About two degrees south of the star Mu Hydræ is another planetary
nebula, which Smyth describes as resembling the planet Jupiter in
“size, equable light and color.” Webb saw it of “a steady, pale blue
light,” and Sir John Herschel, at the Cape of Good Hope, speaks of
its color as “a decided blue—at all events, a good sky-blue,” a
color which seems characteristic of these curious objects. Although
Sir William Herschel, with his large telescopes, failed to resolve
it into stars, Secchi thought he saw it breaking up into stars with
a “sparkling ring.” Dr. Huggins, however, finds the spectrum to be
gaseous, so that the luminous points seen by Secchi could not have
been stellar.
Sir John Herschel, in his _Cape Observations_, describes a planetary
nebula which lies between the stars Pi Centauri and Delta Crucis.
He says it is “perfectly round, very planetary, color fine blue ...
very like Uranus, only about half as large again, and blue.... It
is of the most decided independent blue color when in the field by
itself, and with no lamplight and no bright star. About 10′ north of
it is an orange-colored star, eighth magnitude. When this is brought
into view, the blue color of the nebula becomes intense ... color, a
beautiful rich blue, between Prussian blue and verditer green.”
There are some rare objects called “nebulous stars.” The star Epsilon
Orionis—the centre star of Orion’s Belt—is involved in a great
nebulous atmosphere. The triple star Iota Orionis is surrounded by a
nebulous haze. The star Beta in Canes Venatici is a 4½ magnitude star
surrounded by a nebulous atmosphere.
The term elliptical nebulæ has been applied to those of an elliptical
or elongated shape. This form is probably due in many cases to the
effect of perspective, their real shape being circular, or nearly so.
Perhaps the most remarkable object of this class is the well-known
“nebula in Andromeda,” known to astronomers as 31 Messier. It can be
just seen with the naked eye, on a clear moonless night, as a hazy
spot of light near the star Nu Andromedæ, and it is curious that it
is not mentioned by the ancients, although it must have been very
visible to their keen eyesight in the clear Eastern skies. It was,
however, certainly seen so far back as 905 A. D., and it is referred
to as a familiar object by the Persian astronomer, Al-Sûfi, who wrote
a description of the heavens about the middle of the Tenth Century.
Tycho Brahe and Bayer failed to notice it, but Simon Marius saw it
in December, 1612, and described it “as a light seen from a great
distance through half-transparent horn plates.” It was also observed
by Bullialdus, in 1664, while following the comet of that year. It
has frequently been mistaken for a comet by amateur observers in
recent years. Closely northwest of the great nebula is a smaller
one discovered by Le Gentil in 1749, and another to the south,
detected by Miss Caroline Herschel in 1783. The great nebula is of
an elliptical shape and considerable apparent size. The American
astronomer, Bond, using a telescope of 15 inches aperture, traced
it to a length of about four degrees, and a width of two and a half
degrees. A beautiful photograph taken by Dr. Roberts in December,
1888, shows an extension of nearly two degrees in length, and about
half a degree in width, or considerably larger than the apparent
size of the full moon. Bond could not see any symptom of resolution
into stars, but noticed two dark rifts or channels running nearly
parallel to the length of the nebula. In Dr. Roberts’s photograph
these rifts are seen to be really dark intervals between consecutive
nebulous rings into which the nebula is divided. Dr. Roberts says:
“A photograph which I took with the 20-inch reflector on October 10,
1887, revealed for the first time the true character of the great
nebula, and one of the features exhibited was that the dark bands,
referred to by Bond, formed parts of divisions between symmetrical
rings of nebulous matter surrounding the large diffuse centre of the
nebula. Other photographs were taken in 1887, November 15; 1888,
October 1; 1888, October 2; 1888, December 29; besides several
others taken since, upon all of which the rings of nebulosity are
identically shown, and thus the photographs confirm the accuracy of
each other, and the objective reality of the details shown of the
structure of the nebula.” Dr. Roberts adds: “These photographs throw
a strong light on the probable truth of the _Nebular Hypothesis_, for
they show what appears to be the progressive evolution of a gigantic
stellar system.”
The largest telescopes have hitherto completely failed to resolve
this wonderful object into stars. Dr. Huggins, however, finds that
the spectrum is _not_ gaseous, so that if the nebula really consists
of stellar points, they must be of very small dimensions.
The question may be asked, What is the probable size and distance of
this wonderful nebula? and could it be an external universe?
The temporary star which appeared near the nucleus of the nebula in
August, 1885, was of the seventh magnitude. I find that our sun,
if placed at the distance indicated by a parallax of 1/200th of a
second, would be reduced to a star of about the eleventh magnitude,
or four magnitudes fainter than the temporary star appeared to
us. That is to say, the star would have been—with the assumed
distance—about forty times brighter than the sun. With any greater
distance, the star would have been proportionately brighter, compared
with the sun. This seems improbable, and tends to the conclusion
that the nebula is _not_ an external galaxy, but a member of our
own sidereal system, a system which probably includes all the stars
and nebulæ visible in our largest telescopes. Dr. Common, indeed,
suggests that it may be comparatively near our system. He says:
“It is difficult to imagine that such an enormous object, as the
Andromeda nebula must be, is not very near to us; perhaps it may be
found to be the nearest celestial object of all beyond the Solar
System. It is one that offers the best chance of the detection of
parallax, as it seems to be projected on a crowd of stars, and there
are well-defined points that might be taken as fiducial points for
measurement,” and he adds: “Apart from the great promise this nebula
seems to give of determining parallax, there is a fair presumption
that in the course of time the rotation of the outer portion may
perhaps be detected by observation of the positions of the two outer
detached portions in relation to the neighboring stars.”
The spiral nebulæ are wonderful objects, and were discovered by the
late Lord Rosse with his great six-foot telescope. Their character
has been fully confirmed by photographs taken by Dr. Roberts. One
of the most remarkable of these extraordinary objects is that known
as 51 Messier. It lies about three degrees southwest of the bright
star Eta Ursæ Majoris—the star at the end of the Great Bear’s tail.
It was discovered by Messier while comet-hunting on October 13,
1773. Telescopes of moderate power merely show two nebulæ nearly
in contact, but Lord Rosse saw it as a wonderful spiral, and his
drawing agrees fairly well with a photograph taken by Dr. Roberts in
April, 1889. The nebula has also been photographed by Dr. Common. Dr.
Roberts says: “The photograph shows both nuclei of the nebula to be
stellar, surrounded by dense nebulosity, and the convolutions of the
spiral in this as in other spiral nebulæ are broken up into star-like
condensations with nebulosity around them. Those stars that do not
conform to the trends of the spiral have nebulous trails attached
to them, and seem as if they had broken away from the spirals.” A
tendency to a spiral structure in the smaller nebula is also visible
on the original negative. Dr. Huggins finds that the spectrum is
_not_ gaseous.
The nebula known as 99 Messier is of the spiral form. It lies on the
borders of Virgo and Coma Berenices, near the star 6 Comæ. In large
telescopes it somewhat resembles a “Catherine wheel.” D’Arrest and
Key thought it resolvable into stars. It has been photographed by M.
Von Gothard.
Among the clusters and nebulæ, we may class the Magellanic Clouds,
or Nubeculæ in the Southern Hemisphere, as they consist of stars,
clusters, and nebulæ.
Among the so-called nebulæ are many objects which, when examined
with telescopes of adequate power, are seen to be resolved into
myriads of small stars; their comparative isolation from surrounding
objects impresses us forcibly with the idea that they form, as it
were, families of stars connected by some physical bond of union. Of
these clusters, as they are called, we have naked-eye examples in
the Pleiades and the “Bee-Hive” in Cancer. Others may be partially
seen with a good opera-glass or binocular, but most of them require
telescopes of considerable power to view them to advantage. They
are of various forms and of all degrees of condensation. Some are
comparatively large and irregular, others small and compressed, with
the component stars densely crowded. Many are of such uniform shape
as to have received the name of globular clusters. These have been
aptly termed “balls of stars,” and are among the most interesting
objects in the stellar heavens.
The most remarkable object of this class visible in the Northern
Hemisphere is that known as 13 Messier. It lies between the tolerably
bright stars Zeta and Eta Herculis, nearer the latter star. It may be
seen with an opera-glass as a hazy-looking star of about the sixth
magnitude, with a star on each side of it. Examined with a powerful
telescope, it is resolved into numerous small stars. Sir William
Herschel estimated them at 14,000, but the real number is probably
much less. Assuming the average magnitude of the components at twelve
and a half, I find that an aggregation of 14,000 stars of this
brightness would shine as a star of about the second magnitude, or a
little fainter.
Another object of the globular class, but less resolvable, is that
known as 92 Messier, which lies between the stars Eta and Iota in
Hercules, nearer the latter. Sir William Herschel’s telescopes showed
it as seven or eight minutes of arc in diameter. It is considerably
brighter at the centre. The larger components are easily visible in
moderate-sized telescopes, but even Lord Rosse’s giant instrument
failed to resolve the central blaze. There is no doubt, however,
that it consists wholly of small stars, as the unerring eye of
the spectroscope shows a stellar spectrum, similar to that of the
neighboring 13 Messier.
[Illustration: Fig. 17.—The Region of Boötes and Hercules]
Another fine example of the globular class is 5 Messier, which lies
closely north, preceding the fifth magnitude star, 5 Serpentis. It is
considerably compressed at the centre. Sir William Herschel counted
200 stars, but failed to resolve the central nebulosity. Messier, its
discoverer, found it visible with a telescope only one foot long.
Another fine object is 3 Messier, in Boötes. Admiral Smyth describes
it as “a brilliant and beautiful globular aggregation of not less
than 1,000 small stars.” It is beyond the power of small telescopes,
but it was resolved by Buffham, even in the centre, with a 9-inch
reflector.
Numerous fine examples of the globular class are found in the
Southern Hemisphere, which indeed seems to be richer in these
marvelous objects than the northern sky. Of these the most
interesting are those known as Omega Centauri and 47 Toucani. Omega
Centauri, from its great apparent size—about two-thirds of the
moon’s diameter—and its visibility to the naked eye, may perhaps be
considered as the most remarkable object of its kind in the heavens.
It shines as a hazy star of the fourth magnitude, and I have often
so seen it in the Punjab sky. Its large size and globular form are
clearly visible in a binocular field-glass, but, of course, its
component stars are far beyond the reach of such an instrument. Sir
John Herschel, observing it with his large telescope at the Cape of
Good Hope, found it a “truly astonishing object. All clearly resolved
into stars of two magnitudes, viz., thirteen and fifteen, the larger
lying in lines and ridges over the smaller;... the larger form rings
like lace-work on it.” If we take the average magnitude of the
components at thirteen and a half, the apparent brightness of the
cluster would imply that it contains about 15,000 stars.
[Illustration: The Great Nebula in the Constellation Cygnus]
The other wonderful cluster is that known as 47 Toucani, which lies
close to the smaller Magellanic Cloud. It is smaller in apparent
size than Omega Centauri, but Dr. Gould, observing it at Cordoba,
speaks of it as “one of the most impressive and perhaps the grandest
of its kind in either hemisphere,” and he estimates its magnitude at
four and a half, as seen with the naked eye. It is thus described by
Sir John Herschel: “A most magnificent globular cluster. It fills
the field with its outskirts, but within its more compressed part I
can insulate a tolerably defined, circular space, of 90″ diameter,
wherein the compression is much more decided, and the stars seem to
run together, and this part, I think, has a pale pinkish or rose
color, which contrasts, evidently, with the white light of the rest.
The stars are equal, fourteen magnitude, immensely numerous and
compressed.... Condensation in three distinct stages.... A stupendous
object.” Sir John Herschel’s drawing of this cluster reminds one of a
swarm of bees, and perhaps suggested to Tennyson the lines:
“Clusters and beds of worlds, and bee-like swarms
Of suns and starry streams.”