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along the edge. Though apparently so small, its dimensions most be enormous. If no farther from the earth than 61 Cygni, the diameter would be 2,000,000,000 miles. It is probably immensely further distant.
The spiral or " whirlpool nebulse" are exceedingly curious in their appearance. The most remarkable one is that in Canes Venatici. It consists of brilliant spirals sweeping outward from a central nucleus, and all overspread with a multitude of stars. One is lost in attempting to imagine the distance of such a mass, and the forces which produce such a "tremendous hurricane of matter—perhaps of suns."
Planetary nebtdce, by their circular form and pale uniform light, resemble the disks of the most distant planets of our system. Their edges are generally well denned, though sometimes slightly furred. Threefourths of them are in the southern hemisphere. Several have a blue tinge. There is one in Ursa Major, which if located at the distance named before—that of 61 Cygni— would fill a space equal to
... PLANETARY NBBULA.
three times the entire orbit of Neptune. About twenty-five of these "island universes" have been found scattered through the ocean of space. Columbus discovered a new continent, and so immortalized his name; what shall we say of the astronomer who discovers a universe of worlds?
Irregular nebula, are those which have no definite form. Many of them present all the irregularities of clouds torn and rent by the tempest. Some of the likenesses which may be traced by the fancy are strangely fantastic: for example, the "dumb-bell nebula" in the constellation Vulpecula, and the "crab nebula" near the southern horn of Taurus.
There is also one known as " the great nebula in the sword-handle of Orion," in which may be seen a faint resemblance to the wings of a bird.
Nebulous stars are so called because they are enveloped by a faint nebula, usually of a circular form. The star is generally seen at the centre, although some which are elliptical surround two stars, one in each focus. It is thought that these may be suns possessing immense atmospheres, which are rendered visible somewhat as that of our sun is in the zodiacal light; and that in like manner our sun
itself to those in space presents the appearance of a nebulous star. The luminous atmosphere of the star in Cygnus, if located at the distance of <* Centauri, is of an extent equal to "fifteen times the distance of Neptune from the sun."
Variable nebuke.—Certain changes take place among the nebnlre which can be accounted for only under the supposition that they, like some of the stars, are variable. Mr. Hind tells us of one in Taurus which • was distinctly visible with a good telescope in 1852, but in 1862 it had vanished entirely out of the reach of a much more powerful instrument. It seems to have disappeared altogether. The great nebula in Axgo, when observed by Herschel in 1838, had in the centre a vacant space containing a star of the first magnitude completely enshrouded by nebulous matter. In 1863, the nebulous matter had disappeared, and the star was only of the sixth magnitude. These facts as yet defy explanation. They only illustrate the vast and wonderful changes constantly taking place in the heavens.
Double nebuke.—There seems to be a physical connection existing between some of the nebulse, similar to that already noticed in respect to certain stars. In the case of the latter, this inter-relation has been proved, since their movements even at their distances can yet be traced in the lapse of years. "But owing to the almost infinite depths in the abyss of the heavens at which these nebulsB exist, thousands of years, perhaps thousands of centuries, would be necessary to reveal any movement." (Guillemin.)
Magellanic Clouds.—Not far from the southern pole of the heavens there are two cloud-like masses, distinctly visible to the naked eye, known to navigators as " Cape Clouds." Sir John Herschel describes them as consisting of swarms of stars, clusters, and nebulse, seemingly grouped together in the wildest