The other class of nebulæ, vastly inferior in size, of definite forms and great variety of character, are scattered through the remote heavens, or congregated in a great nebulous district far from the Milky Way. Many cling to stars like wisps of clouds, others are exactly like comets with comæ and tails; but the most definite forms are annular and lenticular nebulæ, nebulous stars, planetary and elliptical nebulæ, and starry clusters. However, there are two in the northern hemisphere differing from all of these, which are described by Sir John Herschel as amazing objects. One in Vulpecula is like an hourglass or dumb bell of bright matter, surrounded by a thin hazy atmosphere so as to give the whole an oval form, or the appearance of an oblate spheroid; with a higher optical power its form is much the same, but the brighter part is resolved into stars, and the hazy part, though still nebulous, assumes that mottled appearance which shows that the whole is a stellar system of the most peculiar structure: it is a phenomenon that bears no resemblance to any known object. (Fig. 3, [plate 8], and fig. 3, [plate 9]). The other is indeed most wonderful, and its history shows the gradual increase in the space-penetrating power of telescopes. To Messier it appeared merely to be a double nebula with stars; with Sir William Herschel’s telescope it presented the appearance of a bright round nebula encompassed at a little distance by a halo or glory, and accompanied by a companion; while in Sir John Herschel’s 20 feet reflector it appeared to “consist of a bright round nucleus, surrounded at a distance by a nebulous ring split through half its circumference, and having the split portions separated at an angle of 45 degrees each to the plane of the other.” (Fig. 1, [plate 10].) This nebula appeared to Sir John to “bear a strong similitude to the Milky Way, suggesting the idea of a brother system bearing a real physical resemblance and strong analogy of structure to our own.”
This object, which disclosed to Lord Rosse the astonishing phenomenon of spiral nebulæ seen in his telescope, presents the appearance of the fig. 1 in [plate 10], in which the partial division of the limb of the ring into two branches is at once recognised in the bright convolutions of the spiral. The outlying nebula is connected by a narrow curved band of light with the ring; the whole is either resolved into stars, or evidently might be with a still higher optical power. With regard to the marvellous nebula in question Lord Rosse observes, that “with each increase of optical power the structure has become more complicated, and more unlike anything that could result from any form of a dynamical law of which we find a counterpart in our system. The connection of the companion with this great nebula, of which there is not the least doubt, adds to the difficulty of forming any hypothesis. It is impossible that such a system could exist without internal movement, to which may be added a resisting medium; but it cannot be regarded as a case of mere static equilibrium.” This is by no means the only instance of a spiral nebula; Lord Rosse has discovered several others: some are easily seen—others require the highest powers of his telescope. From the numerous offsets that branch from the Milky Way and run far into space, it may possibly partake also of the spiral form.
There are seven annular nebulæ in the northern hemisphere, since Lord Rosse has discovered that five of the planetary nebulæ belong to this class. One of the finest examples of an annular nebula is to be seen midway between β and γ Lyræ (fig. 2, [plate 9]). According to Sir John Herschel, it is elliptical in the ratio of 4 to 5, and is sharply defined—the internal opening occupying about half the diameter. This opening is not entirely dark, but filled with a faint hazy light like fine gauze stretched over a hoop. Its diameter, if it is as far from us as 61 Cygni, must be 1300 times greater than the diameter of the earth’s orbit—dimensions that are most astounding. Lord Rosse’s telescope resolves this object into stars of extreme minuteness, with filaments of stars adhering to its edges and a pretty bright star in its interior. These rings are like hollow shells whose borders seem brighter because the nebulous substance, whatever it may be, is more condensed to appearance than the central part. The other annular nebula in the northern hemisphere described by Sir John Herschel is a small faint object, and more easily resolvable into stars. One of the annular nebulæ seen by Lord Rosse is surrounded by a faint external flat ring; another has ansæ, as if an annular nebulous ring encompassed it and was foreshortened. Two annular nebulæ have perforations as if the black sky was seen through openings in the interior haze, for in no instance is the central opening quite dark.
Some nebulæ are like very elliptical annular systems seen obliquely. If they be elliptical flat rings, the dark centre may be a real opening; but should the systems be a series of very long elliptical concentric shells surrounding a hollow, the dark axis may be merely a line of comparative darkness.
The connection of the elliptical nebulæ with double stars is mentioned as very remarkable. In one elliptical nebula whose longer axis is 50ʺ there are two individuals of a double star each of the 10th magnitude symmetrically placed rather nearer the vertex of the ellipse than the foci; in another the stars are unequal, but placed exactly at the extremities of the major axis, as in [plate 8]: besides these there are several other instances.
Double nebulæ are not unfrequent in both hemispheres, exhibiting all the varieties of distance, position, and relative brightness, with their counterparts the double stars. The rarity of single nebulæ as large, faint, and as little condensed in the centre as these, makes it extremely improbable that two such bodies should be accidentally so near as to touch, and often in part to overlap each other, as these do. It is much more likely that they constitute systems; and, if so, it will form an interesting object of future inquiry to discover whether they possess orbital motion.
Nebulous stars are beautiful objects, quite different from all the preceding. They are round or oval, increasing in density towards the centre. Sometimes the central matter is so vividly and sharply condensed and defined that the nebula might be taken for a bright star surrounded by a thin atmosphere. One is a star of the 8th magnitude exactly in the centre of a round bright atmosphere 25ʺ in diameter; the star is quite stellar, and not a nucleus: it has not the smallest appearance of being resolvable. Another nebulous star is ι Orionis, which has a broad atmosphere in which is a dark cavity not symmetrical with the star, and a small double star with a similar opening on the edge of the atmosphere. Lord Rosse observes that these openings appear to be of the same nature with that within the bright stars in the trapezium of Orion, the stars being at its edge; and he is convinced that the stars are not only connected with the nebula, but that they are equidistant with it; hence, if their parallax can be found, the distance of this nebula would be determined. The zodiacal light or lenticular shaped luminous haze surrounding the sun which may be seen extending beyond the orbits of Mercury and Venus soon after sunset in the months of April and May, or before dawn in November and December, seems to place our luminary in the class of nebulous stars. The extensive and delicate atmosphere of these nebulous stars assumes all degrees of ellipticity, from the circular to the spindle-shaped ray, or almost the right line.
Planetary nebulæ have exactly the appearance of planets with round or oval discs, sometimes sharply terminated, at other times hazy and ill-defined. Their surface, which is blue or blueish white, is equable or slightly mottled, and their light occasionally rivals that of the planets in vividness. They are generally attended by minute stars, which give the idea of accompanying satellites. These nebulæ are of enormous dimensions. One near γ Aquarii has a sensible diameter of about twenty seconds, and another presents a diameter of twelve. Sir John Herschel has computed that, if these objects be as far from us as the stars, their real magnitude, on the lowest estimation, must be such as would fill the orbit of Uranus. He concludes that, if they be solid bodies of a solar nature, their intrinsic splendour must be far inferior to that of the sun, because a circular portion of the sun’s disc subtending an angle of twenty seconds would give a light equal to that of a hundred full moons; while, on the contrary, the objects in question are hardly, if at all, visible to the naked eye. From the uniformity of the discs of these planetary nebulæ, and their apparent want of condensation, he presumes that they may be hollow shells emitting light from their surface only. The southern hemisphere is very rich in them, where twenty-eight or twenty-nine have been discovered, some in the midst of a cluster of stars, with which they form a beautiful contrast. Three are of a decided blue colour, one Prussian blue, or verditer green, the other two of a bright sky blue, of great beauty and delicacy. One seems to belong to the class of double nebulæ or double stellar nebulæ of the utmost remoteness. Since Lord Rosse’s telescope has shown that five of the planetary nebulæ are annular, some of those in the southern hemisphere may ultimately be found to belong to the same class.
Probably nine tenths at least of the nebulous contents of the heavens consist of spherical or elliptical forms presenting every variety of elongation and central condensation. Of these a great number have been resolved into stars, and a great many present that mottled appearance which renders it certain that an increase of optical power would decompose them. Those which resist do so on account of the smallness and closeness of the stars of which they consist.
Elliptical nebulæ are very common; by much the finest may be seen near the star υ in the girdle of Andromeda. It is visible to the naked eye, and has frequently been taken for a comet. With a low optical power it has the spindle-shaped form of fig. 6, [plate 5], the brightness being at first gradually and then rapidly condensed towards the centre, so that it has been compared to a star shining through horn, but had never appeared resolvable even with high optical powers till Mr. Bond examined it at the observatory of Cambridge in the United States. He found that its brightness extends over 21⁄2 degrees in length, and more than a degree in breadth, including two small adjacent nebulæ, so that it is oval. It is strongly and rapidly condensed into a nucleus on its northern side; and although it was not all resolved, it was seen to be strewed over with star dust, or extremely minute visible stars, which leaves not a doubt of its being a starry system. The most remarkable part of Mr. Bond’s discovery are two very narrow dark lines which extend along one side of the oval parallel to its major axis. These black streaks, difficult to distinguish, indicate a stratified structure, and are not the only instance of that arrangement in nebulæ. Fig. 1, in [plate 9], is from Mr. Bond’s drawing of this nebula.