of a pentagonal form, from which issue many luminous appendages of
various shapes and lengths. This principal mass is divided into
secondary masses, separated by darkish, irregular intervals. These
secondary masses in their turn appear mottled and fleecy. Towards the
lower part of the pentagonal mass is found a roundish dark space,
comparatively devoid of nebulosity, in which are involved four bright
stars forming a trapezium, and several fainter ones. The four bright
stars of the trapezium constitute the quadruple star _θ_ Orionis, from
which the nebula has received its name. The cloud-like pentagonal form
is brightest on the north-west of the trapezium, and is surrounded on
three sides by long, soft, curved wisps, fading insensibly into the
outer nebulous mass in which they are involved. On the east a broad,
wavy wing spreads out, and sends an important branch southward.
South-east of the trapezium are found several curious dark spaces,
comparatively devoid of nebulosity, especially those on the east, which
give to this nebula a singular character. Close to the north-eastern
part of the nebula, or rather in contact with it, is found a small,
curiously-shaped nebula, condensing around a bright star into a blazing
nucleus. From this centre it continues northward in a narrow diffused
stream, which spreads out in passing over the stars _c^1_ and _c^2_; and
after having sent short branches northward, it curves back to the south
and joins the main nebula on the west of its starting point, having thus
formed a great loop which is not shown on the Plate. The nebula also
forms a loop towards the south, which is partly shown on Plate XV., a
small branch of which, passing through _τ_ Orionis, the nebulous star
shown at the top of the Plate, and extending southward, is not here
represented.
On ordinary nights the nebula in Orion is a splendid object, and
inspires the observer with amazement; but this is as nothing compared
with the grand and magnificent sight which it presents during the very
rare moments when our atmosphere is perfectly clear and steady. I have
seen this nebula but once under these favorable circumstances, and I was
surprised by the grandeur of the scene. Then could be detected features
to be seen at no other time, and its fleecy, floculent, cloud-like
masses glittered with such intensity that it seemed as if thousands of
stars were going to blaze out the next moment. Although I observed the
nebula under such favorable conditions, and with the fifteen-inch
refractor of the Cambridge Observatory, yet I was disappointed in my
expectations, and distinguished no new stars or points of light, and
nothing more than a very bright mass, finely divided into minute blazing
cloudlets. Although I failed to resolve this nebula into stars, yet Lord
Rosse, Bond and Secchi thought they had caught glimpses of star dust.
Its spectrum, however, proves to be mainly that of incandescent gases,
probably hydrogen and nitrogen. In the curved wisps found in this
nebula, Lord Rosse and others saw indications of a spiral structure.
Several bright stars are found scattered over this nebula, and besides
those forming the trapezium, there are three in a row, a little to the
south-east of that group, which are quite bright and remarkable. Among
the stars involved in this nebula, few show signs of having a physical
connection with it, although it seems probable that the group of the
trapezium is so connected. Some of these stars are variable. The small
stars represented on this Plate, as on others of the series, are
somewhat exaggerated in size, as was unavoidable with any process of
reproduction which could be adopted.
In 1811, W. Herschel was led to suspect that some changes had occurred
in this nebula, but changes in such complicated and delicate objects are
not easily ascertained, since, for the most part, we have for comparison
with our later observations only coarse drawings made by hands unskilled
in delineation.
Although comparatively rare, double and multiple nebulæ may be found in
the sky. When this occurs, their constituents most commonly belong to
the class of spherical nebulæ. Sometimes the components are separated
and distinct, at other times one of them is projected upon the other,
either really or by the effect of perspective. Sometimes one is round
and the other elongated. It is probable that while some of these nebulæ
are physically associated and form a system, others appear to be so only
because they happen to be almost in a line with the observer. A double
nebula in Draco, Nos. 4,127 and 4,128, which I have drawn, is a fair
type of those which are separated. The first is a globular nebula, and
the last an oval one, with a star at its centre. The double nebula, Nos.
858 and 859, in Taurus, which I have also studied, is a type of the
cases in which one nebula is partly projected on another. In this
instance both the nebulæ are globular.
The nebulæ in general show very little color in their light, which is
ordinarily whitish and pale. Some, however, present a decided bluish or
greenish tint. The great nebula in Orion has a greenish cast, and we
have seen that some planetary nebulæ are bluish.
It has been a question whether nebulæ are changing. It has already been
stated that Prof. Holden believes there is ground to suspect that the
Trifid and Horse-shoe nebulæ have undergone some changes. A nebula near
_ε_ Tauri has been lost and found again several times. Two other
nebulæ in the same constellation have presented curious variations.
One, near a star of the tenth magnitude, exhibited variations of
brightness like those of the star itself, and for a time disappeared.
The other, near _ζ_ Tauri, increased in brightness for three months,
after which it disappeared. In 1859 Tempel discovered a nebula in the
Pleiades, which has shown some fluctuations. In 1875 I made a long study
of this object, and drew it carefully a dozen times, but I was not able
to see any changes in it within the two or three months during which my
observations were continued. But on Nov. 24, 1876, it was found of a
different color, being purplish and very faint. On Dec. 23, 1880, it was
found just as bright and visible as when I drew it in 1875, and on Oct.
20, 1881, it appeared faint and purplish again, as in 1876. On this last
night, and on those which followed it, it was impossible for me to trace
the nebulosity as far as in 1875. I consider this as due to a variation
in the light of this object, which in 1875 was bright enough to be well
seen while the Moon after her First Quarter was within ten or fifteen
degrees from the Pleiades.
From the observations of M. Laugier, it appears that some nebulæ have a
proper motion, comparable to that of stars. From the displacement of the
lines of their spectra by their motion in the line of sight, Dr. Huggins
found that no nebula observed by him has a proper motion surpassing 25
miles per second. The Ring nebula in Lyra appears to move from us at the
rate of 3 miles per second, and that in Orion recedes about 17 miles per
second.
The important question arises, are all the irresolvable nebulæ in the
heavens to be considered as so many star-clusters, differing only from
them by the minuteness of their components, or their immense distance
from us; or are they cosmical clouds, composed of luminous vapors,
similar to the matter composing the heads and tails of comets?
Originally, W. Herschel, with many astronomers, thought that all these
objects were stellar aggregations, too distant to be resolved into
stars; but he subsequently modified his opinion, and accepted the idea
that some of them are of a gaseous nature.
No direct proof that the nebulæ are gaseous could be obtained, however,
before the spectroscope was known. The attempt to analyze the light of
the nebulæ with this instrument was made in 1864, by Dr. Huggins, who
directed his spectroscope to the planetary nebula, No. 4,373, in Draco.
Its spectrum was found to consist of three bright, distinct lines, the
brightest of which corresponded with the strongest nitrogen line, and
the feeblest with the hydrogen C line. Besides these lines, it gave also
a very faint, continuous spectrum, apparently due to a central point of
condensation. By this observation, the gaseous nature of a nebula was
for the first time demonstrated. Dr. Huggins thus analyzed 70 nebulæ,
of which one-third gave a gaseous spectrum, consisting of several bright
lines, the brightest of which invariably corresponded with the lines of
nitrogen. The others gave a continuous spectrum, with the red end
usually deficient. These results indicate that if some of the so-called
nebulæ are due to an aggregation of stars, either too minute or too
remote in space to be individually resolved, others are in a gaseous
state. Yet the faint, continuous spectrum, given by some nebulæ, in
addition to their gaseous spectra, seems to show that these nebulæ have
some stars or matter in a different state, either involved in them or
projected on their surface.
The idea of diffused matter distributed here and there in space, and
gradually condensing into stars, is by no means new. As early as 1572,
Tycho Brahé proposed such an hypothesis, to explain the sudden
apparition of a new star in Cassiopeia, which he considered as formed by
the recent agglomeration of the "celestial matter" diffused in space.
Kepler adopted the same idea to explain the new star which appeared in
Ophiuchus, in 1604. Halley, Lacaille, Mairan and others, entertained the
same opinion. The hypothesis of a self-luminous, nebulous matter
diffused in space, and forming here and there immense masses, has been
proposed from the origin of the telescope, and was adopted by Sir
William Herschel, who in his grand speculations on the universe
considered the nebulæ as immense masses of phosphorescent vapors,
gradually condensing around one or several centres into stars or
clusters of stars. The evidence afforded by the spectroscope seems to be
in favor of such an hypothesis, and shows us that gaseous agglomerations
exist in space.
According to our modern conception, the visible universe is but an
infinitely small portion of the infinite universe perceived by our mind.
The great blazing centre around which our little, non-luminous globe
pursues its endless journey, is only an humble member of a cluster
comprising four hundred equally powerful suns, as they are believed to
be, although they appear to us as little twinkling stars. The nearest of
these stars is 221,000 times as far from the Sun as the Sun is from the
Earth, and yet this entire cluster is only one among the several hundred
Star-clusters composing the great galactic nebula in which we are
involved, comprising thirty or fifty millions of such suns. Among the
4,000 irresolvable nebulæ in the sky, perhaps over one-half are
supposed to be galaxies, like our own galaxy, composed of star-clusters,
and millions of stars. Besides these remote galaxies, vast
agglomerations of yet uncondensed, nebulous matter exist in space, and
form the nebulæ proper, in which the genesis of suns is slowly
elaborated. Although the visible universe is limited by the penetrating
power of our instruments, yet we see in imagination the infinite
universe stretching farther and farther; but we know not whether this
invisible universe is totally devoid of matter, or whether it also is
filled with millions and millions of suns and galaxies.
APPENDIX
KEY TO THE PLATES
PLATE I.--GROUP OF SUN-SPOTS AND VEILED SPOTS.
_Observed June_ 17, 1875, _at 7h. 30m. A. M._
The background shows the sun's visible surface, or _photosphere_, as
seen with a telescope of high power at the most favorable moments,
composed of innumerable light markings, or granules, separated by a
network of darker gray. The granules, each some hundreds of miles in
width, are thought to be the flame-like summits of the radial filaments
or columns of gas and vapor which compose the photospheric shell. The
two principal sun-spots of the group here represented show the
characteristic dark _umbra_ in the centre, overhung by the thatch-like
_penumbra_, composed of whitish gray filaments. The penumbral filaments
are not supposed to differ in their nature from those constituting the
ordinary photosphere, save that they are seen here elongated and
violently disturbed by the force of gaseous currents. Both spots are
traversed partly or wholly by bright overlying _faculæ_, or so-called
_luminous bridges_, depressed portions of which, in the left-hand spot,
form the _gray and rosy veils_ commonly attendant upon this class of
spots. In each of these spots, also, the inner ends of projecting
penumbral filaments have fallen so far within the umbra as to appear
much darker than the rest. At the right of the upper portion of the
left-hand spot, is a mass of white facular clouds, honey-combed by dark
spaces, through which are seen traces of the undeveloped third spot of
the triple group first observed. If seen upon the sun's limb, this would
have presented the appearance of a _lateral spot_. Above the right-hand
spot is a small black "dot," or incipient spot, without distinct
penumbra. The irregular dark rift below the two large spots and
connecting them is a spot of the crevasse type, with very slight umbra,
a still better example of which is seen in a westward
prolongation of the penumbra of the left-hand spot. In the upper
left-hand corner of the Plate are seen several small _faculæ_,
appearing as irregular whitish streaks amongst the granules. In the
pear-shaped darkening of the solar surface below and at their left, is
seen a veiled spot, two of which attended this group.
_Approximate scale, 2500 miles--1 inch._
PLATE II.--SOLAR PROTUBERANCES.
_Observed May_ 5, 1873, _at 9h. 40m. A. M._
A view of an upheaval of the _chromosphere_, or third outlying envelope
of the sun, as observed with the tele-spectroscope, or telescope with
spectroscope attached.
The _method of the observation_ requires a word of explanation. Save on
the rare occasions of a total solar eclipse, no direct telescopic view
of the solar prominences or flames is possible, owing to the fact that
the intense white light from the sun's main disk entirely obscures
the feeble pink light of the chromosphere. A few years ago Messrs.
Jannsen and Lockyer found that a spectroscope of high dispersive power
so weakens the spectrum of ordinary sun-light as to show the spectrum of
bright lines given by the chromosphere, on any clear day. The telescope
is adjusted so that a portion of the sun's limb, usually near a group of
active sun-spots, shall be presented before the opened slit of the
spectroscope. The light of the chromosphere thus admitted along with
some diffused sun-light from the earth's atmosphere, produces a spectrum
of intensely bright lines, widely separated, on the fainter background
of the strongly dispersed spectrum of sun-light. The most prominent of
these bright lines are those known as the C line (_scarlet_), F line
(_blue_), which with several others are due to the hydrogen present in
the chromosphere, the D{3} line (_orange_) ascribed to a little known
substance called "helium", and occasionally the sodium lines D{1}, D{3},
(_yellow_). By adjusting the slit upon the scarlet C line, the
appearances represented in Plate II. were observed as through an
atmosphere of scarlet light: in the D or F lines identical appearances
may be seen, but somewhat less clearly defined, as through yellow or
blue light respectively. Hence the solar times, as here observed with
the spectroscope in the hydrogen C line, are seen through a portion only
(the scarlet rays) of the light coming from but one substance (hydrogen)
of the companion incandescent substances present in the chromosphere.
The color of the collective chromospheric light is seen directly with
the telescope during an eclipse (See Plate III.) to be a delicate rosy
pink.
_Description of the Plate._--The black background represents the general
darkness of the eye-piece to the spectroscope. The broad red stripe
stretching from top to bottom of the Plate is a portion of the red band
of the spectrum, magnified about 100 times as compared with the actual
spectroscopic view. The upper and lower edges of the cross-section of
dusky red correspond with the edges of the slit, opened widely enough to
admit a view of the chromospheric crest and of the whole height of the
protuberances at once. With a narrower opening of the slit this
background would have been nearly black, its reddish cast increasing
with the amount of opening and consequent admission of diffused
sun-light. Rising above the lower edge of the opening is seen a small
outer segment of the chromosphere, which, as a portion of the sun's
eastern limb, should be imagined as moving directly towards the
beholder. The seams and rifts by which its surface is broken, as well as
the distorted forms of the huge protuberances show the chromosphere to
be in violent agitation. Some of the most characteristic shapes of the
_eruptive protuberances_ are presented, as also _cloud-like_ forms
overtopping the rest. In the immediate foreground the bases of two
towering columns appear deeply depressed below the general horizon of
the segment observed, showing an extraordinary velocity of motion of the
whole uplifted mass toward the observer. The highest of these
protuberances was 126,000 miles in height at the moment of observation.
The triple protuberance at the left with two drooping wings and a tall
swaying spire tipped with a very bright flame, shows by its more
brilliant color the higher temperature (and possibly compression) to
which its gases have been subjected. The irregular black bands behind
this protuberance indicate the presence there of less condensed and
cooler clouds of the same gases. The dimmer jets of dame rising from the
chromosphere are either vanishing protuberances, or, as in the case of
the smallest jet shown at the extreme right of the horizon, are the tops
of protuberances just coming into view.
_Approximate scale, 6000 miles--1 inch._
PLATE III.--TOTAL ECLIPSE OF THE SUN.
_Observed July_ 29, 1878, _at Creston, Wyoming Territory._
A telescopic view of the sun's _corona_ or extreme outer atmosphere and
of the _solar flames_ or _prominences_ during a total eclipse. At the
moment of observation the dark disk of the moon, while still hiding the
sun's main body, had passed far enough eastward to allow the rosy pink
chromospheric prominences to be seen on its western border. On all sides
of the sun's hidden disk, the _corona_ shows its pale greenish light
extending in halo-like rays and streamers, and two very remarkable wings
stretch eastward and westward very nearly in the plane of the ecliptic
and in the direction of the positions of Mercury and Venus respectively
at the time of observation. The full extent of these wings could not be
shown in the Plate without reducing its scale materially, since the
westerly wing extended no less than twelve times the sun's diameter, and
the easterly wing nearly as far, or over ten million miles. A circlet of
bright light immediately bordering the moon's disk is the so-called
_inner corona_, next to which the wings and streamers arc brightest,
thence shading off imperceptibly into the twilight sky of the eclipse.
Other noteworthy peculiarities of the corona, as observed during this
eclipse, are the varying angles at which the radiating streamers are
seen to project, the comparatively dark intervals between them, and the
curved, wisp-like projections seen upon the wings. An especially
noticeable gap appears where the most westerly of the upward streamers
abruptly cuts off the view of the long wing. The largest and brightest
of the curving streamers on the westerly wing coincides with the highest
flame-like protuberance. To some observers of this eclipse the upward
and downward streamers seemed pointed at their outer extremities and
less regular in form.
_Approximate scale, 135,000 miles--1 inch._
PLATE IV.--AURORA BOREALIS.
_As observed March_ 1, 1872, _at 9h. 25m. P. M._
The view presents the rare spectacle of an aurora spanning the sky from
east to west in concentric arches. The Polar Star is nearly central in
the background, the constellation of the Great Bear on the right and
Cassiopeia's chair on the left. The large star at some distance above
the horizon on the right is Arcturus. The almost black inner segment of
the aurora resting upon the horizon, has its summit in the magnetic
meridian, which was in this case a little west of north, its arc being
indented by the bases of the ascending streamers. Both streamers and
arches were, when observed, tremulous with upward pulsations and there
was also a wave-like movement of the streamers from west to east. The
prevailing color of this aurora is a pale whitish green and the
complementary red appears especially at the west end of the auroral
arch. The summits of the streamers are from four hundred to live hundred
miles above the earth and the aurora is therefore a phenomenon of the
terrestrial atmosphere rather than of astronomical observation proper.
PLATE V.--THE ZODIACAL LIGHT.
_Observed February_ 20, 1876.
An observation of the cone of light whose axis lies along the Zodiac,
whence it derives its name. It is drawn as seen in the west, with its
base in the constellation Pisces, and its apex near the familiar group
of the Pleiades in the constellation Taurus. The first bright star above
the horizon in the base of the cone is the planet _Venus_ and at some
distance above is the reddish disk of _Mars_, the two being in rare
companionship as evening stars. Above the constellation Pisces, two
bright stars of Aries lie just outside the cone at the right. The
nearest bright star above these at the right is _Beta_, the leading star
of the constellation Triangula. Further at the right the three prominent
stars nearly in a line are, in ascending order, _Delta_, _Beta_ and
_Gamma_ of the constellation Andromeda. Above these at the left, the
brightest star of a quadrangular group of four is the remarkable
variable star _Algol_ (_Beta_) of the constellation Perseus, which
changes from the second to the fourth magnitude in a period of less than
three days. At the left and a little above the Pleiades is the ruddy
star _Aldebaran_, one of the Hyades and chief star in the constellation
Taurus. These are the principal stars visible in this portion of the sky
at the time of the observation. Their relative positions are represented
as seen in the sky and not by the common method of star-atlases, which
allows for the change from a spherical to a plane surface. Their
magnitude in the order of brightness is indicated only approximately.
PLATE VI.--MARE HUMORUM.
_From a study made in 1875._