The so-called planetary nebulæ derive their name from their likeness to the planets, which they resemble in a more or less equable distribution of light and in their round or slightly oval form. While some of them have edges comparatively sharp and well defined, the outlines of others are more hazy and diffused. These nebulæ, which are frequently of a bluish tint, are comparatively rare objects, and most of those known belong to the southern hemisphere. When seen through large telescopes, however, they present a different aspect, and their apparent uniformity changes. The largest of these objects, No. 2,343 of the General Catalogue, is situated in the Great Bear, close to the star β. Its apparent diameter is, according to Sir J. Herschel, 2', 40", and "its light is equable, except at the edge, where it is a little hazy." In a study which I made of this object in 1876, with a refractor 6⅓ inches in aperture, I found it decidedly brighter on the preceding side, where the brightest part is crescent shaped. In Lord Rosse's telescope its disk is transformed into a luminous ring with a fringed border, and two small star-like condensations are found within. Another planetary nebula, near χ Andromedæ, has also shown an annular structure in Rosse's telescope.
The elliptical nebulæ, as their name implies, are elongated, elliptical objects; but while some of them are only slightly elongated ovals, others form ellipses whose eccentricity is so great that they appear almost linear. In all these objects the light is more or less condensed towards the centre; but while in some of them the condensation is gradual and slight, in others it is so great and sudden that the centre of the nebula appears as a large diffused star, somewhat resembling the nucleus of a comet. From the general appearance of these objects, it is not unlikely that some of them are either flattish, nebulous disks, like the planetary nebulæ, or nebulous rings, seen more or less sidewise. The condensation of light at their centres does not appear to be stellar, but nebulous like the rest, and it is a remarkable fact that very few, if any, of these objects are resolvable into stars.
Several elliptical nebulæ are remarkable for having a star at or near each of their foci, or at each of their extremities. Such are the elliptical nebulæ in Draco, Centaurus, and Sagittarius, Nos. 4,419, 3,706 and 4,395 of the General Catalogue, the last of which is in the vicinity of the triple star μ Sagitarii. Each of these nebulæ has a star at each of its foci, while No. 1, in Cetus, has a star at each of its extremities.
Among the most remarkable elliptic nebulæ may be mentioned Nos. 1,861 and 2,373 of Sir J. Herschel's catalogue, both situated in the constellation Leo. The first is one of Lord Rosse's spiral nebulæ, and the last, which is a very elongated object, is formed of concentric oval rings, which are especially visible towards its central part. The constellation Draco is particularly remarkable for the number of elliptical nebulæ found within its boundaries. Among them are Nos. 3,939, 4,058, 4,064, 4,087, 4,415, etc., which are quite remarkable objects of their class. No. 4,058, of which I have made a study, is bright, and has a decided lenticular form with a condensation in the centre. Its following edge is better defined than the preceding. In Lord Rosse's telescope this object exhibits a narrow, dark, longitudinal, gap in its interior.
By far the largest and the finest object of this class is the great nebula in Andromeda. Although this object belongs rather to the class of irregular nebulæ, yet it is generally considered as an elliptic nebula, since its complicated structure, being less prominent, was not recognized until 1848, when it was perceived by George P. Bond, Director of the Harvard College Observatory. This, the first nebula discovered, was found in 1612 by Simon Marius. It is situated in the constellation Andromeda, in the vicinity of the star ν, and almost in a line with the stars μ and β of the same constellation. It is visible to the naked eye, and appears as a faint comet-like object. It is represented at the upper left hand corner of Plate XIII., on the border of the Milky-way, as it appears to the naked eye.
The nebula in Andromeda is one of the brightest in the heavens, and is closely attended by two smaller nebulæ. Perhaps it would be rather more correct to say that it has three centres of condensation, as the two small nebulæ referred to are entirely involved in the same faint and extensive nebulosity. Its general form is that of an irregular oval, upwards of one degree in breadth and two and a half degrees in length. Its brightest and most prominent part, which alone was seen by the earlier observers, consists in a very elongated lenticular mass, which gradually condenses towards its centre into a blazing, star-like nucleus, surrounded by a brilliant nebulous mass. At a little distance to the south of this central condensation is found one of the lesser centres of condensation noted above, which is globular in appearance, with a bright, star-like nucleus like the former. The other centre of condensation is found to the north-west of the centre of the principal mass, and is quite elongated, with a centre of condensation towards its southern extremity, but it is not so bright as the others. Close to the western edge of the bright lenticular mass first described, and making a very slight angle with its longer axis, are found two narrow and nearly rectilinear dark rifts, running almost parallel to each other, and both terminating in a slender point in the south. These dark rifts, which are almost totally devoid of nebulous matter, are quite rare in nebulæ, and afford a good opportunity to watch the changes which this part of the nebulæ may undergo.
This nebula has never been positively resolved into stars, although Prof. Geo. Bond and others have strongly suspected its resolvability. In a study which I have made of it, with the same instrument employed by Bond, and also with the great Washington telescope, I detected a decided mottled appearance in several places, which might be attributable to a beginning of resolvability; but I do not consider this a conclusive indication that the nebula is resolvable. The continuous spectrum given by this nebula, showing that it is not in the gaseous state which its appearance seems to indicate, warrants the conclusion, however, that it will ultimately be found to be resolvable. This object, being situated on the edge of the Galaxy and involved in its diffused light, has a great number of small stars belonging to this belt projected upon it. During my observations I have mapped out 1,323 of these stars, none of which seems to be in physical connection with the nebula.
Among the circular and elliptical nebulæ a few exhibit a very remarkable structure, being apparently perforated, and forming either round, slightly oval, or elongated rings of great beauty. These Annular nebulæ are among the rarest objects in the heavens. In Scorpio, two such nebulæ are found involved in the Milky-way, and also one in Cygnus. One of those in Scorpio has two stars involved within its ring, at the extremities of its smallest interior diameter. A very elongated nebula in the vicinity of the fine triple star γ Andromedæ is also annular, and has two stars symmetrically placed at the extremities of its greatest interior axis. Another elongated annular nebula is also found north of η Pegasi.
The grandest and most remarkable of the annular nebulæ is found in the constellation Lyra, about midway between the two stars β and γ. It is slightly elliptical in form, and according to Prof. E. S. Holden, its major axis is 77".3 and the minor 58". From a study and several drawings which I have made of this object, with instruments of various apertures, I have found it decidedly brighter towards its outer border, at the extremities of its minor axis, than at the ends of the major axis. On very favorable occasions, some of its brightest parts have appeared decidedly, but very faintly mottled, and I have recognized three small centres of condensation. Its interior, in which Professor Holden has detected a very faint star, is quite strongly nebulous. In Lord Rosse's telescope, this nebula is completely surrounded by wisps and appendages of all sorts of forms, which I have failed to trace, however, both with the refractor of the Harvard College Observatory and with that of the Naval Observatory at Washington; Rosse, Secchi and Chacornac, have seen this nebula glittering as if it were a "heap of star dust," although its spectrum indicates that it is gaseous.
The nebula No. 1,541, in Camelopardus, of which I have also made a study and a drawing, is closely allied to the class of annular nebulæ. This object, which is quite bright, has a remarkable appearance. It consists principally of somewhat more than half of an oval ring, surrounding a bright, nebulous mass which condenses around a star; this mass being separated from the imperfect ring by a dark interval. Upon the bright portion of the ring, and on opposite points, are found two bright stars, between which lies the star occupying the central mass. The central mass extends at some distance outside of the ring on its open side. Several stars are involved in this object.