Popular Science Monthly/Volume 3/June 1873/The Constitution of Nebulae
By Dr. H. SCHELLEN.
WHEN the starry heavens are viewed through a telescope of moderate power, a great number of stellar clusters and faint nebulous forms are revealed against the dark background of the sky which might be taken at first sight for passing clouds, but which, by their unchanging forms and persistent appearance, are proved to belong to the heavenly bodies, though possessing a character widely differing from the point-like images of ordinary stars. Sir William Herschel was able, with his gigantic forty-foot telescope, to resolve many of these nebula? into clusters of stars, and found them to consist of vast groups of individual suns, in which thousands of fixed stars may be clearly separated and counted, but which are so far removed from us that we are unable to perceive their distance one from the other, though that may really amount to many millions of miles, and their light, with a low magnifying power, seems to come from a large, faintly-luminous mass. But all nebulæ were not resolvable with this telescope, and, in proportion as such nebulæ were resolved into clusters of stars, new nebulæ appeared which resisted a power of 6,000, and suggested to this astute investigator the theory that, besides the many thousand apparent nebulas which reveal themselves to us as a complete and separate system of worlds, there are also thousands of real nebulæ in the universe composed of primeval cosmical matter out of which future worlds were to be fashioned.
Lord Rosse, by means of a telescope of fifty-two feet focus, of his own construction, was able to resolve into clusters of stars many of the nebulæ not resolved by Herschel; but there were still revealed to the eye, thus carried farther into space, new nebulæ beyond the power even of this gigantic telescope to resolve. Telescopes failed, therefore, to settle the question whether the unresolved nebulae are portions of the primeval matter out of which the
The Great Nebula in Orion.
existing stars have been formed; they leave us in uncertainty as to whether these nebulae were masses of luminous gas, which in the lapse of ages would pass through the various stages of incandescent liquid
That which was beyond the power of the most gigantic telescopes has been accomplished by that apparently insignificant, but really delicate, and almost infinitely sensitive instrument—the spectroscope; we are indebted to it for being able to say with certainty that luminous nebulae actually exist as isolated bodies in space, and that these bodies are luminous masses of gas.
The splendid edifice already planned by Kant in his "Theory of the Heavens" (1755), and erected by Laplace forty-one years later, in his "System of the Universe," has received its topmost stone through the discoveries of the spectroscope. The spectroscope, in combination with the telescope, affords means for ascertaining even now some of the phases through which the sun and planets have passed in their process of development or transition from masses of luminous nebulae to their present condition.
The Large Magellanic Cloud.
Great variety is observed in the forms of the nebulæ: while some are chaotic and irregular, and sometimes highly fantastic, others exhibit the pure and beautiful forms of a curve, a crescent, a globe, or a circle. A number of the most characteristic of these forms have been photographed on glass at the suggestion of Mr. Huggins; to these have been added a few others, taken from accurate drawings by Lord Rosse; and they may all be projected on to a screen by means of the electric or lime-light lantern, and made visible to a large audience.
The largest and most irregular of all the nebulae is that in the constellation of Orion (Figs. 1, 2). It is situated rather below the three stars of second magnitude composing the central part of that magnificent constellation, and is visible to the naked eye. It is extremely difficult to execute even a tolerably correct drawing of this nebula; but it appears, from the various drawings made at different times, that a change is taking place in the form and position of the brightest portions. Fig. 2 represents the central and brightest part of the nebula. Four bright stars, forming a trapezium, are situated in it, one of which only is visible to the naked eye. The nebula surrounding these stars has a flaky appearance, and is of a greenish-white color; single portions form long curved streaks stretching out in a radiating manner from the middle and bright parts.
Nebula of the Form of a Sickle.
Much less irregularity is apparent in the great Magellanic or Cape clouds (Fig. 3), which are two nebulae in the Southern Hemisphere, one of them exceeding by five times the apparent size of the moon. They are distinctly visible to the naked eye, and are so bright that they serve as marks for reconnoitring the heavens, and for reckoning the hour of the night.
The interest aroused by these irregular and chaotic nebulous forms is still further increased by the phenomena of the spiral or convoluted nebulæ with which the giant telescopes of Lord Kosse and Mr. Bond have made us further acquainted. As a rule, there stream out
Spiral Nebula in Canes Venatici.
from one or more centres of luminous matter innumerable curved nebulous streaks, which recede from the centre in a spiral form, and finally lose themselves in space. Fig. 4 represents a nebula in the form of a sickle or comet-tail, and Fig. 5 shows the most remarkable of all the spiral nebulæ, situated in the constellation Canes Venatici.
Transition from the Spiral to the Annular Form.
It is hardly conceivable that a system of such a nebulous form could exist without internal motion. The bright nucleus, as well as the streaks curving round it in the same direction, seems to indicate an accumulation of matter toward the centre, with a gradual increase of density, and a rotatory movement. But, if we combine with this motion the supposition of an opposing medium, it is difficult to harmonize such a system with the known laws of statics. Accurate measures are, therefore, of the highest interest for the purpose of showing
Annular Nebula in Lyra.
whether actual rotation or other changes are taking place in these nebulæ; but, unfortunately, they are rendered extremely difficult and uncertain by the want of outline, and by the remarkable faintness of these nebulous objects.
Nebula with Several Rings.
The transition state from the spiral to the annular form is shown in such nebulae as the one represented in Fig. 6; and they then pass into the simple or compound annular nebula of a type which is given in Fig. 7.
The space within most of these elliptie rings is not perfectly dark, but is occupied either by a diffused nebulous light, as in Fig. 7, or, as
Elliptical Annular Nebula.
in most cases, by a bright nucleus, round which sometimes one ring, sometimes several, are disposed in various forms. In Fig. 8 a representation is given of a compound annular nebula, with very elliptic rings and bright nucleus.
According as the ring has its surface or its edge turned toward us, or according as our line of sight is perpendicular or more or less obliquely inclined to the surface of the ring, its form approaches that of a circle, a ring, an ellipse, or even a straight line. Nebulæ of this latter kind are represented in Fig. 9 and in Fig. 10. When an elliptical ring is extremely elongated, and the minor axis is much smaller than the major one, the density and brightness of the ring diminish as its distance from the central nucleus increases; and this takes place to such a degree sometimes, that at the farthest points of the ring, the ends of the major axis, it ceases to be visible, and the continuity seems to be broken. The nebula has then the appearance of a double nebula, with a central spot as represented in Figs. 11, 12.
Those nebulæ, which appear with tolerably sharply-defined edges in the form of a circle or slight ellipse, seem to belong to a much higher stage of development. From their resemblance to those planets which shine with a pale or bluish light, they have been called planetary nebulæ; in form, however, they vary considerably, some of them being
Planetary Nebula with Two Stars.
spiral and some annular. Some of these planetary nebulæ are represented in Figs. 12, 14, 15. The first has two central stars or nuclei, each surrounded by a dark space, beyond which the spiral streaks are disposed; the second has also two nuclei, but without clearly separable dark spaces; the third is without any nucleus, but shows a well-defined ring of light.
The highest type of nebulae are certainly the stellar nebulæ, in which a tolerably well-defined bright star is surrounded by a completely rounded disk or faint atmosphere of light, which sometimes fades away gradually into space, at other times terminates abruptly with
a sharp edge. Figs. 16 and 17 exhibit the most striking of these very remarkable stellar nebulæ: the first is surrounded by a system of rings like Saturn, with the thin edge turned toward us; the second is a veritable star of the eighth magnitude, and is not nebulous, but is surrounded by a bright luminous atmosphere perfectly concentric. To the right of the star is a small dark space, such as often occurs in these nebulæ, indicating, perhaps, an opening in the surrounding atmosphere.
We have now passed in review all that is at present known of the nebulæ, so far as their appearance and form have been revealed by the largest telescopes. The information as yet furnished by the spectroscope on this subject is certainly much less extensive, but is nevertheless
of the greatest importance, since the spectroscope has power to reveal the nature and constitution of these remote heavenly bodies. It must here again be remembered that the character of the spectrum not only indicates what the substance is that emits the light, but also its physical condition. If the spectrum be a continuous one, consisting of rays of every color or degree of refrangibility, then the source of light is either a solid or liquid incandescent body; if, on the contrary, the spectrum be composed of bright lines only, then it is certain that the light comes from luminous gas; finally, if the spectrum be continuous, but crossed by dark lines interrupting the colors, it is an indication that the source of light is a solid or liquid incandescent body, but that the light has passed through an atmosphere of vapors at a lower temperature, which by their selective absorptive power have abstracted those colored rays which they would have emitted had they been self-luminous.
Spectrum of Nebula.
When Huggins first directed his telescope in August, 1864, to one of these objects, a small but very bright nebula, he found, to his great surprise, that the spectrum, instead of being a continuous colored band, such as that given by a star, consisted only of three bright lines.
This one observation was sufficient to solve the long-vexed question, at least for this particular nebula, and to prove that it is not a cluster of individual, separable stars, but is actually a gaseous nebula, a body of luminous gas. In fact, such a spectrum could only be produced
Spectrum of Nebula compared with the Sun and some Terrestrial Elements.
by a substance in a state of gas; the light of this nebula, therefore, was emitted neither by solid nor liquid incandescent matter, nor by gases in a state of extreme density, as may be the case in the sun and stars, but by luminous gas in a highly-rarefied condition.
In order to discover the chemical nature of this gas, Huggins followed the usual methods of comparison, and tested the spectrum with the Fraunhofer lines of the solar spectrum, and the bright lines of terrestrial elements. A glance at Fig. 19 will show at once the result of this investigation. The brightest line (1) of the nebula coincides exactly with the brightest line (N) of the spectrum of nitrogen, which is a double line. The faintest of the nebular lines (3) also coincides with the bluish-green hydrogen Hue Hß, or, which is the same thing, with the Fraunhofer line F in the solar spectrum. The middle line (2) of the nebula was not found to coincide with any of the bright lines of the thirty terrestrial elements with which it has been compared; it lies not far from the barium line Ba, but is not coincident with it.