CHAP. XII.

OF NEBULE.

403

most powerful can barely render resolvable, would be completely resolved by a further increase of instrumental force.

stars.

(619.) Of nebulæ, properly so called, the variety is again very great. By far the most remarkable are those represented in figs. 2. and 3. Plate N, the . former of which represents the nebulæ surrounding the quadruple (or rather sextuple) star 6, in the constellation Orion; the latter, that about, in the southern constellation Robur Caroli: the one discovered by Huygens, in 1656, and figured as seen in the twenty feet reflector at Slough; the other by Lacaille, from a figure by Mr. Dunlop, Phil. Trans. 1827. The nebulous character of these objects, at least of the former, is very different from what might be supposed to arise from the congregation of an immense collection of small It is formed of little flocky masses, like wisps of cloud; and such wisps seem to adhere to many small stars at its outskirts, and especially to one considerable star (represented, in the figure, below the nebula), which it envelopes with a nebulous atmosphere of considerable extent and singular figure. Several astronomers, on comparing this nebula with the figures of it handed down to us by its discoverer, Huygens, have concluded that its form has undergone a perceptible change. But when it is considered how difficult it is to represent such an object duly, and how entirely its appearance will differ, even in the same telescope, according to the clearness of the air, or other temporary causes, we shall readily admit that we have no evidence of change that can be relied on.

(620.) Plate II. fig. 3. represents a nebula of a quite different character. The original of this figure is in the constellation Andromeda near the star v. It is visible to the naked eye, and is continually mistaken for a comet, by those unacquainted with the heavens. Simon Marius, who noticed it in 1612, describes its appearance as that of a candle shining through horn, and the resemblance is not inapt. Its form is a pretty

long oval, increasing by insensible gradations of brightness, at first very gradually, but at last more rapidly, up to a central point, which, though very much brighter than the rest, is yet evidently not stellar, but only nebula in a high state of condensation. It has in it a few small stars; but they are obviously casual, and the nebula itself offers not the slightest appearance to give ground for a suspicion of its consisting of stars. It is very

large, being nearly half a degree long, and 15 or 20 minutes broad.

(621.) This may be considered as a type, on a large scale, of a very numerous class of nebulæ, of a round or oval figure, increasing more or less in density towards the central point: they differ extremely, however, in this respect. In some, the condensation is slight and gradual; in others great and sudden: so sudden, indeed, that they present the appearance of a dull and blotted star, or of a star with a slight burr round it, in which case they are called stellar nebulæ ; while others, again, offer the singularly beautiful and striking phænomenon of a sharp and brilliant star surrounded by a perfectly circular disc, or atmosphere, of faint light in some cases, dying away on all sides by insensible gradations; in others, almost suddenly terminated. These are nebulous stars. A very fine example of such a star is 55 Andromedæ R. A. 1h 43m, N. P. D. 50° 7′. ε Orionis and of the same constellation are also nebulous; but the nebula is not to be seen without a very powerful telescope. In the extent of deviation, too, from the spherical form, which oval nebulæ affect, a great diversity is observed: some are only slightly elliptic; others much extended in length; and in some, the extension so great, as to give the nebula the character of a long narrow, spindle-shaped ray, tapering away at both ends to points. One of the most remarkable specimens of this kind is in R. A. 12h 28m; N. P. D. 63° 4′.

(622.) Annular nebulæ also exist, but are among the rarest objects in the heavens. The most conspicuous of this class is to be found exactly half way between the

CHAP. XII.

PLANETARY NEBULE.

405

stars B and y Lyræ, and may be seen with a telescope of moderate power. It is small, and particularly well defined, so as in fact to have much more the appearance of a flat oval solid ring than of a nebula. The axes of the ellipse are to each other in the proportion of about 4 to 5, and the opening occupies about half its diameter: its light is not quite uniform, but has something of a curdled appearance, particularly at the exterior edge; the central opening is not entirely dark, but is filled up with a faint hazy light, uniformly spread over it, like a fine gauze stretched over a hoop.

(623.) Planetary nebulæ are very extraordinary objects. They have, as their name imports, exactly the appearance of planets: round or slightly oval discs, in some instances quite sharply terminated, in others a little hazy at the borders, and of a light exactly equable or only a very little mottled, which, in some of them, approaches in vividness to that of actual planets. Whatever be their nature, they must be of enormous magnitude. One of them is to be found in the parallel of Aquarii, and about 5m preceding that star. Its apparent diameter is about 20". Another, in the constellation Andromeda, presents a visible disc of 12", perfectly defined and round. Granting these objects to be equally distant from us with the stars, their real dimensions must be such as would fill, on the lowest computation, the whole orbit of Uranus. It is no less evident that, if they be solid bodies of a solar nature, the intrinsic splendour of their surfaces must be almost infinitely inferior to that of the sun's. A circular portion of the sun's disc, subtending an angle of 20′′, would give a light equal to 100 full moons; while the objects in question are hardly, if at all, discernible with the naked eye. The uniformity of their discs, and their want of apparent central condensation, would certainly augur their light to be merely superficial, and in the nature of a hollow spherical shell; but whether filled with solid or gaseous matter, or altogether empty, it would be a waste of time to conjecture.

(624.) Among the nebula which possess an evident symmetry of form, and seem clearly entitled to be regarded as systems of a definite nature, however mysterious their structure and destination, the most remarkable are the 51st and 27th of Messier's catalogue. The former consists of a large and bright globular nebula surrounded by a double ring, at considerable distance from the globe, or rather a single ring divided through about two fifths of its circumference into two lamine, and having one portion, as it were, turned up out of the plane of the rest. The latter consists of two bright and highly condensed round or slightly oval nebulæ, united by a short neck of nearly the same density. A faint nebu lous atmosphere completes the figure, enveloping them both, and filling up the outline of a circumscribed ellipse, whose shorter axis is the axis of symmetry of the system about which it may be supposed to revolve, or the line passing through the centers of both the nebulous masses. These objects have never been properly described, the instruments with which they were originally discovered having been quite inadequate to showing the peculiarities above mentioned, which seem to place them in a class apart from all others. The one offers obvious analogies either with the structure of Saturn or with that of our own sidereal firmament and milky way. The other has little or no resemblance to any other known object.

(625.) The nebulæ furnish, in every point of view, an inexhaustible field of speculation and conjecture. That by far the larger share of them consist of stars there can be little doubt; and in the interminable range of system upon system, and firmament upon firmament, which we thus catch a glimpse of, the imagination is bewildered and lost. On the other hand, if it be true, as, to say the least, it seems extremely probable, that a phosphorescent or self-luminous matter also exists, disseminated through extensive regions of space, in the manner of a cloud or fog-now assuming capricious shapes, like actual clouds drifted by the wind, and now con

CHAP. XII.

OF THE ZODIACAL LIGHT.

407

centrating itself like a cometic atmosphere around particular stars ;—what, we naturally ask, is the nature and destination of this nebulous matter? Is it absorbed by the stars in whose neighbourhood it is found, to furnish, by its condensation, their supply of light and heat? or is it progressively concentrating itself by the effect of its own gravity into masses, and so laying the foundation of new sidereal systems or of insulated stars? It is easier to propound such questions than to offer any probable reply to them. Meanwhile, appeal to fact, by the method of constant and diligent observation, is open to us; and, as the double stars have yielded to this style of questioning, and disclosed a series of relations of the most intelligible and interesting description, we may reasonably hope that the assiduous study of the nebulæ will, ere long, lead to some clearer understanding of their intimate nature.

(626.) We shall conclude this chapter by the mention of a phænomenon, which seems to indicate the existence of some slight degree of nebulosity about the sun itself, and even to place it in the list of nebulous stars. It is called the zodiacal light, and may be seen any very clear evening soon after sunset, about the months of April and May, or at the opposite season before sunrise, as a cone or lenticular-shaped light, extending from the horizon obliquely upwards, and following, generally, the course of the ecliptic, or rather that of the sun's equator. The apparent angular distance of its vertex from the sun varies, according to circumstances, from 40° to 90°, and the breadth of its base perpendicular to its axis from 8° to 30°. It is extremely faint and ill defined, at least in this climate, though better seen in tropical regions, but cannot be mistaken for any atmospheric meteor or aurora borealis. It is manifestly in the nature of a thin lenticularly-formed atmosphere, surrounding the sun, and extending at least beyond the orbit of Mercury and even of Venus, and may be conjectured to be no other than the denser part of that medium, which, as we have reason to belie