similar objects are visible through the telescope. In Sir John
Herschel's catalogue of nebulæ and clusters, are found 4,053
irresolvable nebulæ, and with every increase of the aperture of
telescopes, new nebulæ, invisible in smaller instruments, are found.
Notwithstanding their irresolvability it is probable, however, that many
among them have a stellar structure, which their immense distance
prevents us from recognizing, and are not therefore true nebulæ. The
giant telescope of Lord Rosse has shown nebulæ so remote that it has
been estimated that it takes their light 30 million years to reach the
Earth.
The nebulæ are very far from being uniformly distributed in space. In
some regions they are rare, while in others they are numerous and
crowded together, forming many small, irregular groups, differing in
size and in richness of aggregation. The grouping of the nebulæ does
not occur at random in any part of the heavens, as might naturally be
supposed, but, on the contrary, it is chiefly confined to certain
regions. Outside of these regions nebulæ are rare and are separated
from each other by immense intervals; so that these isolated objects
appear as if they were lost wanderers from the great nebulous systems.
The regions where the nebulæ congregate in great number are very
extensive, and in a general view there are two vast systems of nebular
agglomeration, occupying almost opposite points of the heavens, whose
centres are not very distant from the poles of the Milky-way. In the
northern hemisphere, the nebulous system is much richer and more
condensed than in the southern hemisphere. The northern nebulæ are
principally contained in the constellations Ursa Minor and Major, in
Draco, Canes Venatici, Bootes, Leo Major and Minor, Coma Berenices, and
Virgo. In this region, which occupies about ⅛ of the whole surface of
the heavens, ⅓ of the known nebulæ are assembled. The southern
nebulæ are more evenly distributed and less numerous, with the
exception of two comparatively small, but very remarkable centres of
condensation which, together with many star-clusters, constitute the
Magellanic clouds.
These two vast nebular groups are by no means regular in outline, and
send various branches toward each other. They are separated by a wide
and very irregular belt, comparatively free from nebulæ, which
encircles the celestial sphere, and whose medial line approximately
coincides with that of the galactic belt. The Milky-way, so rich in
star-clusters, is very barren in nebulæ; but it is a very remarkable
fact, nevertheless, that almost all the brightest, largest, and most
complicated nebulæ of the heavens are situated either within it, or in
its immediate vicinity. Such are the great nebulæ in Orion and
Andromeda; the nebula of _ζ_ Orionis; the Ring nebula in Lyra; the
bifurcate nebula in Cygnus; the Dumb-bell nebula in Vulpecula; the Fan,
Horse-shoe, Trifid and Winged nebulæ in Sagittarius; the great nebula
around _η_ Argus Navis, and the Crab nebula in Taurus.
Aside from the discovery of some of the largest nebulæ by different
observers, and their subsequent arrangement in catalogues by Lacaille
and Messier, very little had been done towards the study of these
objects before 1779, when Sir W. Herschel began to observe them with the
earnestness of purpose which was one of the distinctive points of the
character of this great man. He successively published three catalogues
in 1786, 1789, and 1802, in which the position of 2,500 nebulous objects
was given. This number was more than doubled before 1864, when Sir John
Herschel published his catalogue of 5,079 nebulæ and star-clusters. To
this long list must be added several hundred similar objects, since
discovered by D'Arrest, Stephan, Gould and others. But, as has been
shown above, among the so-called nebulæ are many star-clusters which do
not properly belong to the same class of objects, it being sometimes
impossible in the present state of our knowledge to know whether a
nebulous object belongs to one class or to the other.
The nebulæ exhibit a great variety of forms and appearances, and, in
accordance with their most typical characters, they are usually divided
into several classes, which are: the Nebulous stars, the Circular, or
Planetary, the Elliptical, the Annular, the Spiral and Irregular
nebulæ.
[PLATE XV.--THE GREAT NEBULA IN ORION.
From a study made in the years 1875-76]
The so-called nebulous stars consist of a faint nebulosity, usually
circular, surrounding a bright and sharp star, which generally occupies
its centre. The nebulosity surrounding these stars varies in brightness
as well as in extent, and while, in general, its light gradually fades
away, it sometimes terminates quite suddenly. Such nebulosities are
usually brighter and more condensed towards the central star. The stars
thus surrounded do not seem, however, to be distinguished from others by
any additional peculiarity. Some nebulæ of this kind are round, with
one star in the centre; others are oval and have two stars, one at each
of their foci. The nebulous star, _τ_ Orionis, represented at the upper
part of Plate XV., above the great nebula, has a bright star at its
centre and two smaller ones on the side. The association of double stars
with nebulæ is very remarkable, and may in some cases indicate a mutual
relation between them.
The so-called planetary nebulæ derive their name from their likeness to
the planets, which they resemble in a more or less equable distribution
of light and in their round or slightly oval form. While some of them
have edges comparatively sharp and well defined, the outlines of others
are more hazy and diffused. These nebulæ, which are frequently of a
bluish tint, are comparatively rare objects, and most of those known
belong to the southern hemisphere. When seen through large telescopes,
however, they present a different aspect, and their apparent uniformity
changes. The largest of these objects, No. 2,343 of the General
Catalogue, is situated in the Great Bear, close to the star _β_. Its
apparent diameter is, according to Sir J. Herschel, 2', 40", and "its
light is equable, except at the edge, where it is a little hazy." In a
study which I made of this object in 1876, with a refractor 6⅓ inches
in aperture, I found it decidedly brighter on the preceding side, where
the brightest part is crescent shaped. In Lord Rosse's telescope its
disk is transformed into a luminous ring with a fringed border, and two
small star-like condensations are found within. Another planetary
nebula, near _χ_ Andromedæ, has also shown an annular structure in
Rosse's telescope.
The elliptical nebulæ, as their name implies, are elongated, elliptical
objects; but while some of them are only slightly elongated ovals,
others form ellipses whose eccentricity is so great that they appear
almost linear. In all these objects the light is more or less condensed
towards the centre; but while in some of them the condensation is
gradual and slight, in others it is so great and sudden that the centre
of the nebula appears as a large diffused star, somewhat resembling the
nucleus of a comet. From the general appearance of these objects, it is
not unlikely that some of them are either flattish, nebulous disks, like
the planetary nebulæ, or nebulous rings, seen more or less sidewise.
The condensation of light at their centres does not appear to be
stellar, but nebulous like the rest, and it is a remarkable fact that
very few, if any, of these objects are resolvable into stars.
Several elliptical nebulæ are remarkable for having a star at or near
each of their foci, or at each of their extremities. Such are the
elliptical nebulæ in Draco, Centaurus, and Sagittarius, Nos. 4,419,
3,706 and 4,395 of the General Catalogue, the last of which is in the
vicinity of the triple star _μ_ Sagitarii. Each of these nebulæ has a
star at each of its foci, while No. 1, in Cetus, has a star at each of
its extremities.
Among the most remarkable elliptic nebulæ may be mentioned Nos. 1,861
and 2,373 of Sir J. Herschel's catalogue, both situated in the
constellation Leo. The first is one of Lord Rosse's spiral nebulæ, and
the last, which is a very elongated object, is formed of concentric oval
rings, which are especially visible towards its central part. The
constellation Draco is particularly remarkable for the number of
elliptical nebulæ found within its boundaries. Among them are Nos.
3,939, 4,058, 4,064, 4,087, 4,415, etc., which are quite remarkable
objects of their class. No. 4,058, of which I have made a study, is
bright, and has a decided lenticular form with a condensation in the
centre. Its following edge is better defined than the preceding. In Lord
Rosse's telescope this object exhibits a narrow, dark, longitudinal, gap
in its interior.
By far the largest and the finest object of this class is the great
nebula in Andromeda. Although this object belongs rather to the class of
irregular nebulæ, yet it is generally considered as an elliptic nebula,
since its complicated structure, being less prominent, was not
recognized until 1848, when it was perceived by George P. Bond, Director
of the Harvard College Observatory. This, the first nebula discovered,
was found in 1612 by Simon Marius. It is situated in the constellation
Andromeda, in the vicinity of the star _ν_, and almost in a line with
the stars _μ_ and _β_ of the same constellation. It is visible to the
naked eye, and appears as a faint comet-like object. It is represented
at the upper left hand corner of Plate XIII., on the border of the
Milky-way, as it appears to the naked eye.
The nebula in Andromeda is one of the brightest in the heavens, and is
closely attended by two smaller nebulæ. Perhaps it would be rather more
correct to say that it has three centres of condensation, as the two
small nebulæ referred to are entirely involved in the same faint and
extensive nebulosity. Its general form is that of an irregular oval,
upwards of one degree in breadth and two and a half degrees in length.
Its brightest and most prominent part, which alone was seen by the
earlier observers, consists in a very elongated lenticular mass, which
gradually condenses towards its centre into a blazing, star-like
nucleus, surrounded by a brilliant nebulous mass. At a little distance
to the south of this central condensation is found one of the lesser
centres of condensation noted above, which is globular in appearance,
with a bright, star-like nucleus like the former. The other centre of
condensation is found to the north-west of the centre of the principal
mass, and is quite elongated, with a centre of condensation towards its
southern extremity, but it is not so bright as the others. Close to the
western edge of the bright lenticular mass first described, and making a
very slight angle with its longer axis, are found two narrow and nearly
rectilinear dark rifts, running almost parallel to each other, and both
terminating in a slender point in the south. These dark rifts, which are
almost totally devoid of nebulous matter, are quite rare in nebulæ, and
afford a good opportunity to watch the changes which this part of the
nebulæ may undergo.
This nebula has never been positively resolved into stars, although
Prof. Geo. Bond and others have strongly suspected its resolvability. In
a study which I have made of it, with the same instrument employed by
Bond, and also with the great Washington telescope, I detected a decided
mottled appearance in several places, which might be attributable to a
beginning of resolvability; but I do not consider this a conclusive
indication that the nebula is resolvable. The continuous spectrum given
by this nebula, showing that it is not in the gaseous state which its
appearance seems to indicate, warrants the conclusion, however, that it
will ultimately be found to be resolvable. This object, being situated
on the edge of the Galaxy and involved in its diffused light, has a
great number of small stars belonging to this belt projected upon it.
During my observations I have mapped out 1,323 of these stars, none of
which seems to be in physical connection with the nebula.
Among the circular and elliptical nebulæ a few exhibit a very
remarkable structure, being apparently perforated, and forming either
round, slightly oval, or elongated rings of great beauty. These Annular
nebulæ are among the rarest objects in the heavens. In Scorpio, two
such nebulæ are found involved in the Milky-way, and also one in
Cygnus. One of those in Scorpio has two stars involved within its ring,
at the extremities of its smallest interior diameter. A very elongated
nebula in the vicinity of the fine triple star _γ_ Andromedæ is also
annular, and has two stars symmetrically placed at the extremities of
its greatest interior axis. Another elongated annular nebula is also
found north of _η_ Pegasi.
The grandest and most remarkable of the annular nebulæ is found in the
constellation Lyra, about midway between the two stars _β_ and _γ_. It
is slightly elliptical in form, and according to Prof. E. S. Holden, its
major axis is 77".3 and the minor 58". From a study and several drawings
which I have made of this object, with instruments of various apertures,
I have found it decidedly brighter towards its outer border, at the
extremities of its minor axis, than at the ends of the major axis. On
very favorable occasions, some of its brightest parts have appeared
decidedly, but very faintly mottled, and I have recognized three small
centres of condensation. Its interior, in which Professor Holden has
detected a very faint star, is quite strongly nebulous. In Lord Rosse's
telescope, this nebula is completely surrounded by wisps and appendages
of all sorts of forms, which I have failed to trace, however, both with
the refractor of the Harvard College Observatory and with that of the
Naval Observatory at Washington; Rosse, Secchi and Chacornac, have seen
this nebula glittering as if it were a "heap of star dust," although its
spectrum indicates that it is gaseous.
The nebula No. 1,541, in Camelopardus, of which I have also made a study
and a drawing, is closely allied to the class of annular nebulæ. This
object, which is quite bright, has a remarkable appearance. It consists
principally of somewhat more than half of an oval ring, surrounding a
bright, nebulous mass which condenses around a star; this mass being
separated from the imperfect ring by a dark interval. Upon the bright
portion of the ring, and on opposite points, are found two bright stars,
between which lies the star occupying the central mass. The central mass
extends at some distance outside of the ring on its open side. Several
stars are involved in this object.
The Spiral nebulæ are very curious and complicated objects, but they
are visible only in the largest telescopes. Prominent above all is the
double spiral nebula No. 3,572, in Canes Venatici, which is not far from
_η_ Ursa Majoris. In Lord Rosse's telescope, this object presents a
wonderful spiral disposition, looking somewhat like one of the
fire-works called pin-wheels, and forming long, curved wisps, diverging
from two bright centres. The spectrum of this object, however, is not
that of a gas. In the constellation Virgo, Rosse has detected another
such nebula. In Cepheus, Triangulum, and Ursa Major, are found other
spiral nebulæ of smaller size. Lord Rosse has recognized 40 spiral
nebulæ and suspected a similar structure in 30 others.
The class of the Irregular nebulæ, which will be now considered,
differs greatly in character from the others, and includes the largest,
the brightest and the most extraordinary nebulæ in the heavens. The
nebulæ of this class differ from those belonging to the other classes
by a want of symmetry in their form and in the distribution of their
light, as well as by their capricious shapes, and their very complicated
structure. Another and perhaps the principal difference between them and
the objects above described, consists in the remarkable fact already
stated, that they are not, except in rare cases, to be found in the
regions where the other nebulæ abound. On the contrary, they are found
in or very near the Milky-way, precisely where the other nebulæ are the
most rare. This fact, recognized by Sir J. Herschel, led him to consider
them as "outlying, very distant, and as it were detached fragments of
the great stratum of the Galaxy." It seems very probable that the reason
why these objects differ so greatly from the other nebulæ in size,
brightness and complication of structure, is simply because they are
much nearer to us than are most of the others. They are perhaps nebulous
members of our Galaxy. The same remark which has been made of
star-clusters may be applied to nebulæ. The nearer they are to us, the
larger, the brighter and the more complicated they will appear, while
the farther they are removed, the more simple and regular and round they
will appear, only their brightest and deepest parts being then visible.
The Crab Nebula of Lord Rosse, near _ζ_ Tauri, No. 1,157, is one of the
interesting objects of this class. It has curious appendages streaming
off from an oval, luminous mass, which give it a distant resemblance to
the animal from which it derives its name. The Bifid nebula in Cygnus,
Nos. 4,400 and 4,616, is another object of this class. It consists of a
long, narrow, crooked streak, forking out at several places, and passing
through _χ_ Cygni. Observers, having failed to recognize the connection
existing between its different centres of brightness, have made distinct
nebulæ of this extended object.
The Dumb-bell nebula in Vulpecula, No. 4,532, is a bright and curious
object, with a general resemblance to the instrument from which it
derives its name. Lord Rosse's telescope has shown many stars in it,
projected on a nebulous background, and Prof. Bond seems to have thought
that it showed traces of resolvability, although in the study which I
made of this nebula with the same instrument used by the latter
observer, I failed to perceive any such traces. Dr. Huggins finds its
spectrum gaseous.
The star-cluster, No. 4,400, in Scutum Sobieskii, which is described by
Sir J. Herschel as a loose cluster of at least 100 stars, I have found
to be involved in an extensive, although not very bright, nebula, which
would seem to have escaped his scrutiny. In a study and drawing of this
nebula made in 1876, its general form is that of an open fan, with the
exception that the handle is wanting, with deeply indented branches on
the preceding side, where the brightest stars of the cluster are
grouped. From its peculiar form, this object might appropriately be
called the Fan nebula.
The Omega or Horse-shoe nebula, in Sagittarius, No. 4,403, of which I
have made a study and two drawings, one with a refractor 6⅓ inches in
aperture, and the other conjointly with Prof. Holden, with the great
telescope of the Naval Observatory, is a bright and very complicated
object. Its general appearance in small instruments, with low power, is
that of a long, narrow pisciform mass of light, from which proceeds on
the preceding side, the great double loop from which it derives its
name. But in the great Washington refractor its structure becomes very
complicated, forming various bright nebulous masses and wisps of great
extension. Prof. Holden, who has made a careful, comparative study of
the published drawings of this object, thinks there are reasons to
believe that its western branch has moved relatively to the stars found
within its loop. The spectrum of this nebula is gaseous.
The Trifid nebula, No. 4,355, in the same constellation, is also a very
remarkable object, although it is not so bright as the last. This
nebula, which I have studied with the refractor of the Cambridge
Observatory, consists of four principal masses of light, separated by a
wide and irregular gap branching out in several places. These masses,
which are brighter along the dark gap, gradually fade away externally. A
group of stars, two of which are quite bright, is found near the centre
of the nebula, on the inner edge of the following mass, and close to the
principal branch of the dark channel. A little to the north, and
apparently forming a part of this nebula, is a globular-looking nebula,
having a pale yellow star at its centre. Prof. Holden's studies on this
nebula show that the triple star, which was centrally situated in the
dark gap from 1784 to 1833, was found involved in the border of the
nebulous mass following it, from 1839 to 1877; the change, he thinks,
is attributable either to the proper motion of the group of stars or to
that of the nebula itself.
In the same vicinity is found the splendid and very extensive nebula No.
4,361, in which is involved a loose, but very brilliant star-cluster.
This nebula and cluster, which I have studied and drawn with a 6⅓ inch
telescope, is very complicated in structure, and divided by a dark
irregular gap into three principal masses of light, condensing at one
point around a star, and at others forming long, bright, gently-curved
branches, which give to this object a strong resemblance to the wings of
a bird when extended upwards in the action of flying. From this
peculiarity this object might appropriately be called the Winged nebula.
Its spectrum is that of a gas.
The variable star _η_ Argus is completely surrounded by the great
nebula of the same name, No. 2,197, first delineated by Sir J. Herschel,
during his residence at the Cape of Good Hope, in 1838. This object,
which covers more than ⁴⁄₇ of a square degree, is divided into
three unequal masses, separated by dark oval spots, comparatively free
from nebulosity, and is suspected to have undergone changes since
Herschel's time.
In the same field with the double star, _ζ_ Orionis, the most easterly
of the three bright stars in the belt of Orion, is found another
irregular nebula of the Trifid type. From the drawings which I have made
of this object, it appears to be composed of three principal unequal
masses, separated by a wide, irregular, dark channel, two of the masses
being quite complicated in structure, and forming curved, nebulous
streams of considerable length and breadth. This nebula, like the next
to be described, seems to be connected with the Galaxy by the great
galactic loop described in another section.
By far the most conspicuous irregular nebula visible from our northern
States, is the great nebula in Orion, No. 1,179, represented on Plate
XV. This object, visible to the naked eye, is the brightest and the most
wonderful nebula in the heavens. It is situated a little to the south of
the three bright stars in the belt of Orion, and may be readily detected
surrounding the star _θ_, situated between and in a line with two faint
stars, the three being in a straight line which points directly towards
_ε_, the middle star of the three in Orion's belt. The area occupied by
this nebula is about equal to that occupied by the Moon.