Chapter III: Nebulæ

Classification of Nebulæ.

370. _Planetary Nebulæ._--Many nebulæ (328) present a well-defined circular disk, like that of a planet, and are therefore called _planetary_ nebulæ. Specimens of planetary nebulæ are shown in Fig. 429.

371. _Circular and Elliptical Nebulæ._--While many nebulæ are circular in form, others are elliptical. The former are called _circular_ nebulæ, and the latter _elliptical_ nebulæ. Elliptical nebulæ have been discovered of every degree of eccentricity. Examples of various circular and elliptical nebulæ are given in Fig. 430.

372. _Annular Nebulæ._--Occasionally ring-shaped nebulæ have been observed, sometimes with, and sometimes without, nebulous matter within the ring. They are called _annular_ nebulæ. They are both circular and elliptical in form. Several specimens of this class of nebulæ are given in Fig. 431.

373. _Nebulous Stars._--Sometimes one or more minute stars are enveloped in a nebulous haze, and are hence called _nebulous stars_. Several of these nebulæ are shown in Fig. 432.

374. _Spiral Nebulæ._--Very many nebulæ disclose a more or less spiral structure, and are known as _spiral_ nebulæ. They are illustrated in Fig. 433. There are, however, a great variety of spiral forms. We shall have occasion to speak of these nebulæ again (381-383).

375. _Double and Multiple Nebulæ._--Many _double_ and _multiple_ nebulæ have been observed, some of which are represented in Fig. 434.

Fig. 435 shows what appears to be a double annular nebula. Fig. 436 gives two views of a double nebula. The change of position in the components of this double nebula indicates a motion of revolution similar to that of the components of double stars.

Irregular Nebulæ.

376. _Irregular Forms._--Besides the more or less regular forms of nebulæ which have been classified as indicated above, there are many of very irregular shapes, and some of these are the most remarkable nebulæ in the heavens. Fig. 437 shows a curiously shaped nebula, seen by Sir John Herschel in the southern heavens; and Fig. 438, one in _Taurus_, known as the _Crab_ nebula.

377. _The Great Nebula of Andromeda._--This is one of the few nebulæ that are visible to the naked eye. We see at a glance that it is not a star, but a mass of diffused light. Indeed, it has sometimes been very naturally mistaken for a comet. It was first described by Marius in 1614, who compared its light to that of a candle shining through horn. This gives a very good idea of the impression it produces, which is that of a translucent object illuminated by a brilliant light behind it. With a small telescope it is easy to imagine it to be a solid like horn; but with a large one the effect is more like fog or mist with a bright body in its midst. Unlike most of the nebulæ, its spectrum is a continuous one, similar to that from a heated solid, indicating that the light emanates, not from a glowing gas, but from matter in the solid or liquid state. This would suggest that it is really an immense star-cluster, so distant that the highest telescopic power cannot resolve it; yet in the largest telescopes it looks less resolvable, and more like a gas, than in those of moderate size. If it is really a gas, and if the spectrum is continuous throughout the whole extent of the nebula, either it must shine by reflected light, or the gas must be subjected to a great pressure almost to its outer limit, which is hardly possible. If the light is reflected, we cannot determine whether it comes from a single bright star, or a number of small ones scattered through the nebula.

With a small telescope this nebula appears elliptical, as in Fig. 439. Fig. 440 shows it as it appeared to Bond, in the Cambridge refractor.

378. _The Great Nebula of Orion._--The nebula which, above all others, has occupied the attention of astronomers, and excited the wonder of observers, is the _great nebula of Orion_, which surrounds the middle star of the three which form the sword of Orion. A good eye will perceive that this star, instead of looking like a bright point, has a hazy appearance, due to the surrounding nebula. This object was first described by Huyghens in 1659, as follows:--

"There is one phenomenon among the fixed stars worthy of mention, which, so far as I know, has hitherto been noticed by no one, and indeed cannot be well observed except with large telescopes. In the sword of Orion are three stars quite close together. In 1656, as I chanced to be viewing the middle one of these with the telescope, instead of a single star, twelve showed themselves (a not uncommon circumstance). Three of these almost touched each other, and with four others shone through a nebula, so that the space around them seemed far brighter than the rest of the heavens, which was entirely clear, and appeared quite black; the effect being that of an opening in the sky, through which a brighter region was visible."

The representation of this nebula in Fig. 441 is from a drawing made by Bond. In brilliancy and variety of detail it exceeds any other nebula visible in the northern hemisphere. In its centre are four stars, easily distinguished by a small telescope with a magnifying power of forty or fifty, together with two smaller ones, requiring a nine-inch telescope to be well seen. Besides these, the whole nebula is dotted with stars.

In the winter of 1864-65 the spectrum of this nebula was examined independently by Secchi and Huggins, who found that it consisted of three bright lines, and hence concluded that the nebula was composed, not of stars, but of glowing gas. The position of one of the lines was near that of a line of nitrogen, while another seemed to coincide with a hydrogen line. This would suggest that the nebula is a mixture of hydrogen and nitrogen gas; but of this we cannot be certain.

379. _The Nebula in Argus._--There is a nebula (Fig. 442) surrounding the variable star _Eta Argus_ (355), which is remarkable as exhibiting variations of brightness and of outline.

In many other nebulæ, changes have been suspected; but the indistinctness of outline which characterizes most of these objects, and the very different aspect they present in telescopes of different powers, render it difficult to prove a change beyond a doubt.

380. _The Dumb-Bell Nebula._--This nebula was named from its peculiar shape. It is a good illustration of the change in the appearance of a nebula when viewed with different magnifying powers. Fig. 443 shows it as it appeared in Herschel's telescope, and Fig. 444 as it appears in the great Parsonstown reflector (20).

Spiral Nebulæ.

381. _The Spiral Nebula in Canes Venatici._--The great spiral nebula in the constellation _Canes Venatici_, or the _Hunting-Dogs_, is one of the most remarkable of its class. Fig. 445 shows this nebula as it appeared in Herschel's telescope, and Fig. 446 shows it as it appears in the Parsonstown reflector.

382. _Condensation of Nebulæ._--The appearance of the nebula just mentioned suggests a body rotating on its axis, and undergoing condensation at the same time.

It is now a generally received theory that nebulæ are the material out of which stars are formed. According to this theory, the stars originally existed as nebulæ, and all nebulæ will ultimately become condensed into stars.

383. _Other Spiral Nebulæ._--Fig. 447 represents a spiral nebula of the _Great Bear_. This nebula seems to have several centres of condensation. Fig. 448 is a view of a spiral nebula in _Cepheus_, and Fig. 449 of a singular spiral nebula in the _Triangle_. This also appears to have several points of condensation. Figs. 450 and 451 represent oval and elliptical nebulæ having a spiral structure.

_THE MAGELLANIC CLOUDS._

384. _Situation and General Appearance of the Magellanic Clouds._--The _Magellanic clouds_ are two nebulous-looking bodies near the southern pole of the heavens, as shown in the right-hand portion of Fig. 452. In the appearance and brightness of their light they resemble portions of the Milky-Way.

The larger of these clouds is called the _Nubecula Major_. It is visible to the naked eye in strong moonlight, and covers a space about two hundred times the surface of the moon. It is shown in Fig. 453. The smaller cloud is called the _Nubecula Minor_. It has only about a fourth the extent of the larger cloud, and is considerably less brilliant. It is visible to the naked eye, but it disappears in full moonlight. This cloud is shown in Fig. 454. The region around this cloud is singularly bare of stars; but the magnificent cluster of _Toucan_, already described (346), is near, and is shown a little to the right of the cloud in the figure.

385. _Structure of the Nubeculæ._--Fig. 455 shows the structure of these clouds as revealed by a powerful telescope. The general ground of both consists of large tracts and patches of nebulosity in every stage of resolution,--from that which is irresolvable with eighteen inches of reflecting aperture, up to perfectly separated stars, like the Milky-Way and clustering groups. There are also nebulæ in abundance, both regular and irregular, globular clusters in every state of condensation, and objects of a nebulous character quite peculiar, and unlike any thing in other regions of the heavens. In the area occupied by the _nubecula major_ two hundred and seventy-eight nebulæ and clusters have been enumerated, besides fifty or sixty outliers, which ought certainly to be reckoned as its appendages, being about six and a half per square degree; which very far exceeds the average of any other part of the nebulous heavens. In the _nubecula minor_ the concentration of such objects is less, though still very striking. The nubeculæ, then, combine, each within its own area, characters which in the rest of the heavens are no less strikingly separated; namely, those of the galactic and the nebular system. Globular clusters (except in one region of small extent) and nebulæ of regular elliptic forms are comparatively rare in the Milky-Way, and are found congregated in the greatest abundance in a part of the heavens the most remote possible from that circle; whereas in the nubeculæ they are indiscriminately mixed with the general starry ground, and with irregular though small nebulæ.

THE NEBULAR HYPOTHESIS.

386. _The Basis of the Nebular Hypothesis._--We have seen that the
planets all revolve around the sun from west to east in nearly the
same plane, and that the sun rotates on his axis from west to east.
The planets, so far as known, rotate on their axes from west to
east; and all the moons, except those of Uranus and Neptune, revolve
around their planets from west to east. These common features in the
motion of the sun, moons, and planets, point to the conclusion that
they are of a common origin.

387. _Kant's Hypothesis._--Kant, the celebrated German philosopher,
seems to have the best right to be regarded as the founder of the
modern nebular hypothesis. His reasoning has been concisely stated
thus: "Examining the solar system, we find two remarkable features
presented to our consideration. One is, that six planets and nine
satellites [the entire number then known] move around the sun in
circles, not only in the same direction in which the sun himself
revolves on his axis, but very nearly in the same plane. This common
feature of the motion of so many bodies could not by any reasonable
possibility have been a result of chance: we are therefore forced to
believe that it must be the result of some common cause originally
acting on all the planets.

"On the other hand, when we consider the spaces in which the planets
move, we find them entirely void, or as good as void; for, if there
is any matter in them, it is so rare as to be without effect on the
planetary motions. There is, therefore, no material connection now
existing between the planets through which they might have been
forced to take up a common direction of motion. How, then, are we to
reconcile this common motion with the absence of all material
connection? The most natural way is to suppose that there was once
some such connection, which brought about the uniformity of motion
which we observe; that the materials of which the planets are formed
once filled the whole space between them. There was no formation in
this chaos, the formation of separate bodies by the mutual
gravitation of parts of the mass being a later occurrence. But,
naturally, some parts of the mass would be more dense than others,
and would thus gather around them the rare matter which filled the
intervening spaces. The larger collections thus formed would draw
the smaller ones into them, and this process would continue until a
few round bodies had taken the place of the original chaotic mass."

Kant, however, failed to account satisfactorily for the motion of
the sun and planets. According to his system, all the bodies formed
out of the original nebulous mass should have been drawn to a common
centre so as to form one sun, instead of a system of revolving
bodies like the solar system.

388. _Herschel's Hypothesis._--The idea of the gradual transmutation
of nebulæ into stars seems to have been suggested to Herschel, not
by the study of the solar system, but by that of the nebulæ
themselves. Many of these bodies he believed to be immense masses of
phosphorescent vapor; and he conceived that these must be gradually
condensing, each around its own centre, or around the parts where it
is most dense, until it should become a star, or a cluster of stars.
On classifying the nebulæ, it seemed to him that he could see this
process going on before his eyes. There were the large, faint,
diffused nebulæ, in which the condensation had hardly begun; the
smaller but brighter ones, which had become so far condensed that
the central parts would soon begin to form into stars; yet others,
in which stars had actually begun to form; and, finally,
star-clusters in which the condensation was complete. The
spectroscopic revelations of the gaseous nature of the true nebulæ
tend to confirm the theory of Herschel, that these masses will all,
at some time, condense into stars.

389. _Laplace's Hypothesis._--Laplace was led to the nebular
hypothesis by considering the remarkable uniformity in the direction
of the rotation of the planets. Believing that this could not have
been the result of chance, he sought to investigate its cause. This,
he thought, could be nothing else than the atmosphere of the sun,
which once extended so far out as to fill all the space now occupied
by the planets. He begins with the sun, surrounded by this immense
fiery atmosphere. Since the sum total of rotary motion now seen in
the planetary system must have been there from the beginning, he
conceives the immense vaporous mass forming the sun and his
atmosphere to have had a slow rotation on its axis. As the intensely
hot mass gradually cooled, it would contract towards the centre. As
it contracted, its velocity of rotation would, by the laws of
mechanics, constantly increase; so that a time would arrive, when,
at the outer boundary of the mass, the centrifugal force due to the
rotation would counterbalance the attractive force of the central
mass. Then those outer portions would be left behind as a revolving
ring, while the next inner portions would continue to contract until
the centrifugal and attractive forces were again balanced, when a
second ring would be left behind; and so on. Thus, instead of a
continuous atmosphere, the sun would be surrounded by a series of
concentric revolving rings of vapor. As these rings cooled, their
denser materials would condense first; and thus the ring would be
composed of a mixed mass, partly solid and partly vaporous, the
quantity of solid matter constantly increasing, and that of vapor
diminishing. If the ring were perfectly uniform, this condensation
would take place equally all around it, and the ring would thus be
broken up into a group of small planets, like the asteroids. But if,
as would more likely be the case, some portions of the ring were
much denser than others, the denser portions would gradually attract
the rarer portions, until, instead of a ring, there would be a
single mass composed of a nearly solid centre, surrounded by an
immense atmosphere of fiery vapor. This condensation of the ring of
vapor around a single point would not change the amount of rotary
motion that had existed in the ring. The planet with its atmosphere
would therefore be in rotation; and would be, on a smaller scale,
like the original solar mass surrounded by its atmosphere. In the
same way that the latter formed itself first into rings, which
afterwards condensed into planets, so the planetary atmospheres, if
sufficiently extensive, would form themselves into rings, which
would condense into satellites. In the case of Saturn, however, one
of the rings was so uniform throughout, that there was no denser
portion to attract the rest around it; and thus the ring of Saturn
retained its annular form.

Such is the celebrated nebular hypothesis of Laplace. It starts, not
with a purely nebulous mass, but with the sun, surrounded by an
immense atmosphere, out of which the planets were formed by gradual
condensation. Fig. 456 represents the condensing mass according to
this theory.

390. _The Modern Nebular Hypothesis._--According to the nebular
hypothesis as held at the present time, the sun, planets, and
meteoroids originated from a purely nebulous mass. This nebula first
condensed into a nebulous star, the star being the sun, and its
surrounding nebulosity being the fiery atmosphere of Laplace. The
original nebula must have been put into rotation at the beginning.
As it contracted and became condensed through the loss of heat by
radiation into space, and under the combined attraction of gravity,
cohesion, and affinity, its speed of rotation increased; and the
nebulous envelop became, by the centrifugal force, flattened into a
thin disk, which finally broke up into rings, out of which were
formed the planets and their moons. According to Laplace, the rings
which were condensed into the planets were thrown off in succession
from the equatorial region of the condensing nebula; and so the
outer planets would be the older. According to the more modern idea,
the nebulous mass was first flattened into a disk, and subsequently
broken up into rings, in such a way that there would be no marked
difference in the ages of the planets. The sun represents the
central portion of the original nebula, and the comets and
meteoroids its outlying portion. At the sun the condensation is
still going on, and the meteoroids appear to be still gradually
drawn in to the sun and planets.

The whole store of energy with which the original solar nebula was
endowed existed in it in the potential form. By the condensation and
contraction this energy was gradually transformed into the kinetic
energy of molar motion and of heat; and the heat became gradually
dissipated by radiation into space. This transformation of potential
energy into heat is still going on at the sun, the centre of the
condensing mass, by the condensation of the sun itself, and by the
impact of meteors as they fall into it.

It has been calculated, that, by the shrinking of the sun to the
density of the earth, the transformation of potential energy into
heat would generate enough heat to maintain the sun's supply, at the
present rate of dissipation, for seventeen million years. A
shrinkage of the sun which would generate all the heat he has poured
into space since the invention of the telescope could not be
detected by the most powerful instruments yet constructed.

The least velocity with which a meteoroid could strike the sun would
be two hundred and eighty miles a second; and it is easy to
calculate how much heat would be generated by the collision. It has
been shown, that, were enough meteoroids to fall into the sun to
develop its heat, they would not increase his mass appreciably
during a period of two thousand years.

The sun's heat is undoubtedly developed by contraction and the fall
of meteoroids; that is to say, by the transformation of the
potential energy of the original nebula into heat.

It must be borne in mind that the nebular hypothesis is simply a
supposition as to the way in which the present solar system may have
been developed from a nebula endowed with a motion of rotation and
with certain tendencies to condensation. Of course nothing could
have been developed out of the nebula, the germs of which had not
been originally implanted in it by the Creator.