44

46

"to find a very red star, much brighter than the rest, occupying a conspicuous situation in them." Sir W. Herschel regards these as globular clusters in a less advanced state of condensation, conceiving all such groups as approaching, by their mutual attraction, to the globular figure, and assembling themselves together from all the surrounding region, under laws of which we have no other proof than the observance of a gradation by which their characters shade into one another, so that it is impossible to say where one species ends and the other begins. Fig. 41, formerly referred to, represents one of the globular clusters in the constellation Hercules. Fig. 42 is a view of a curious but somewhat irregular group, seen in the southern hemisphere, as sketched by Mr. Dunlop, at Paramatta, New South Wales. It is the 30 Doradûs, or Xipheas, and is rather a singular object, but evidently a large cluster of stars, presenting two or three very condensed strata, as if they were crowded to excess by an immense confluence of stars.

2. Another class is that termed resolvable nebula, or those which lead us to suppose that they consist of stars which would be separately distinguishable by an increase of light and magnifying power in the telescope. These may be considered as clusters too remote to be distinctly seen, the stars composing which are either too faint in their light or too small in size to make a definite impression upon the organs of vision. They are almost universally round or oval, which is supposed to be owing to their loose appendages and irregularities of form being extinguished by their distance, the general figure of the central or more condensed parts being only discernible. "It is under the appearance of objects of this character," says Sir J. Herschel," that all the greater globular clusters exhibit themselves in telescopes of insufficient optical power to show them well; and the conclusion is obvious, that those which the most powerful can barely render resolvable, would be completely resolved by a farther increase of instrumental force."

3. Besides the above, there is an immense variety of nebulæ, properly so called, which no telescopes have hitherto been able to resolve into stars, and which is supposed to be a species of matter diffused throughout infinite space, in various portions and degrees of condensation, and which may, in the course of ages, be condensed into stars or starry systems. The follow

ing is a description of some of the more remarkable varieties of this class of nebulæ.

Fig. 43 represents a nebula of an elliptical or spindle-like form. It is visible to the naked eye in a clear night, when the moon is absent, and has sometimes been mistaken for a small comet. It appears like a dull, cloudy, undefined spot upon the concave of the firmament, and has sometimes been compared to the light of a small candle shining through horn. Its central parts appear brightest, but its light gradually fades towards each extremity. A few small stars appear adjacent to it, and even within its boundaries, but it appears pretty evident that they have no immediate connexion with the nebula. Its form, as here delineated, may be seen with a telescope of moderate power, but no telescope hitherto constructed, even with the highest powers that could be applied, has yet been sufficient to resolve it into stars. In size, it is nearly half a degree long, and 12 or 15 minutes broad. Though the figure of this object appears oval or elliptical, it is not unlikely that it is in reality nearly of a globular figure, and that its oval appearance is owing to its position with regard to our eye. This nebula is situated in the girdle of Andromeda, within a degree or two of the star v of that constellation. It is about 150 nearly west from Almaach, and 8° north by west of Merach, with which stars it forms nearly a right-angled triangle. It may be seen in a northwesterly direction in the evenings of the months of January, February, and March, at a considerable elevation. It comes to the meridian about the middle of November, at nine o'clock in the evening. Its right ascension is Oh 33', and north declination 40° 20′. nebula may be considered as a representative, on a large scale, of a numerous class of nebulæ, which increase more or less in density towards the central point. The representation of it in the plate is somewhat longer and narrower than it appears through a telescope magnifying 140 times.

This

Fig. 44 represents a kind of elliptical nebula, with a vacancy of a lenticular form in the centre. It is pretty evident that such nebulæ are in reality large rings, which appear of an oval or lenticular form in consequence of their lying obliquely to our line of vision. This is undoubtedly a large starry system, comprising perhaps millions of stars, at such a distance that their combined light appears only like a faint nebula. It probably is not much unlike the form of our Milky Way in which

the sun is situated. Its right ascension is 2h 12', and north decl. 41° 35'. It lies near y Andromeda, or Almaach, about 4° to the eastward of that star, nearly in a line between it and Algol, in the head of Medusa, and about 19° east from the nebula represented in Fig. 43.

Fig. 45 is a representation of an annular nebula, which may be seen with a telescope of moderate power. It does not occupy so much space in the heavens as the preceding nebulæ, but it is well defined, and has the appearance of a flat, solid ring. It is not perfectly circular, but somewhat elliptical, the conjugate axis of the ellipse being to the transverse nearly in the proportion of 4 to 5. The opening occupies about half its diameter, and is not entirely dark, but filled up with a very faint, hazy light uniformly spread over it. Its light is not of a pure milky white, but is somewhat mottled in its appearance near the exterior edge. This curious phenomenon, like the preceding, is doubtless an immense stellar system, situated at an immeasurable distance in the profundity of space. It is situated in the constellation of Lyra, exactly half way between the stars ẞ and y, so that its position may be found by common observers without any difficulty. Its right ascension is 18h 47', and north declination 32° 49'. The following cut (fig. 46*) represents some of the principal stars in the constellation of the Lyre. The largest star near the upper part is Vega, a bright star of the first magnitude; the next larger star, south by east of which is ẞ; and the other star of the same magnitude to the southeast is Y; between which is the annular nebula, about 70 from Vega.

Fig. 46 represents an object somewhat similar to the above. It is situated between the constellations Anser and Cygnus, about 910 south from the star y Cygni, and 17° east from the phenomenon described above.* Its right ascension is 20h 9',

* It may not be improper here to remark, once for all, that the bearings or directions of the stars from one another, given here and in other parts of this volume, are strictly applicable only when the principal star, from which the bearings are stated, is on or near the meridian. When in other positions, they will appear, to a common observer, to have different bearings; for example, the star Vega, or Lyra, in the above figure, when about 500 or 60° above the western horizon, will appear at an equal altitude as the star ẞ, southwest by south of it; and when about 30° or 40° above the eastern horizon, the two stars will appear, the one directly above or below the other. This difference in the apparent directions of the stars from each other is most observable in those which are near the pole; for example, the stars of the Great Bear appear in one

and north declination 30° 3'. It comes to the meridian about the 10th of September, at nine o'clock in the evening.

The opposite page contains representations of several other kinds of nebulæ, some of which are extremely curious and singular. Fig. 47 is a very singular and wonderful object. It has the shape of a dumb-bell or hourglass of bright matter, surrounded by a thin, hazy atmosphere; the two connected hemispheres, and the space which connects them, are beautiful and pretty bright. The oval is completed by a space on each side, which is more dim and hazy than the two hemispheres. The whole has an oval form, like that of an oblate spheriod. The southern hemisphere is somewhat denser than the northern, and there are one or two stars in it. It appears evidently to be a dense collection of stars at an immeasurable distance from the region in which we reside, and leads us to form an idea of the endless diversities of shape and form among those countless assemblages of stars with which the

part of their revolution west from the pole, and in another part of their course east of it. These and other circumstances require to be attended to, in order to find particular stars by their bearings from one or more principal stars.