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Explanation.—These phenomena are as yet little understood. A revolution about the axis would fail to explain the changes in color, besides being in itself a very unaccountable supposition. Some think that these stars revolve in enormous orbits of such eccentricity that at their most distant points théý fade out of sight. Arago has shown, in reply to this, that for a star to decrease in brightness from the first magnitude to the second by simply moving directly from us, would require six years, even if it should speed away with the velocity of light. As we have just seen, the star of 1866 underwent this change in brilliancy in a week. · The mind cannot help wondering if they are not instances of enormous conflagrations in which a world is overwhelmed in ruin! The investigations, of spectrum analysis indicate that the star of 1866 consisted of burning hydrogen gas. We can suppose that this was evolved by some convulsion, and taking fire, wrapped in flames the entire globe. This need not involve the idea of destruction, but only a change of form. In this manner a dark star may become luminous, or a bright one may be extinguished. :
Thus do we see that the process of apparent creation and destruction is going on in the heavens immodiately before the eye of the astronomer. Now stars flash into light, old stars are lost, worlds burst into flame, and their glowing embers fade into darkness. Are they re-created into new worlds? We know not. Wo only perceive that the same Al.
mighty power which fitted up this earth for our home is yet at work among the worlds about us, and We are thus witnesses of His eternal presence.
STAR CLUSTERS.—These are groups of stars so massed together as to present a hazy, cloud-like ap. pearance. Several of them have been already named—the Pleiades, the Beehive in Cancer, Berenice's Hair, the Hyades, and the group in the swordhandle of Perseus. The stars of which they are composed can generally be easily distinguished by
. Fig. 82
the naked eye, although by the use of a small opera or spy glass the number is largely increased. In the southern sky there are clusters still more remarkable. In the Cross is a group of 110 stars of
various colors, red, blue, and green, so that looking on it, says Herschel, is “like gazing into a casket of precious gems.” A cluster in Toucan is compact at the centre, where it is of an orange-red color; the exterior is composed of pure white stars, making a border of exquisite contrast. It is generally conceded that there is some close physical relation existing between the stars composing such an “ archipelago of worlds,” but its nature is a mystery. They seem generally crowded together toward the centre, blending into a continuous blaze of light. Yet, although they appear so densely compacted, it is probable that if we could change our stand-point, penetrating one of these groups of suns we should find it opening up and spreading out before us on our approach, until, in the midst, the suns would shine down upon us from the heavens as the stars do in our own sky.
NEBULÆ.—These are faint misty objects like specks of luminous clouds. They are generally either round or oval, and brightest at the .centre. They differ from "clusters” in not being resolvable into stars when viewed through the largest telescopes. With the constant improvement made in these instruinents, however, many nebulæ have been resolved, and thus the number of clusters has been increased, while new nebulæ are discovered to take their places. Until of late, it was thought that all nebulæ were simply groups of stars, which would be ultimately discerned in the more powerful telescopes yet to be made. Spectrum analysis shows, however, that
many of these luminons clouds are gaseous, and not solid. They cannot, therefore, be suns. Since they maintain the same position with respect to the stars, their distance must be inconceivably great, and in order to be visible to us, their magnitude must be proportionately vast. They are most abundant at the two poles of the Milky Way, but are more uniformly distributed over the heavens lying near the south pole. Those portions of the sky which are poorest in stars, are richest in nebulæ. Herschel was accustomed to say to his secretary, whenever for a brief time he saw no star passing the field of his telescope, as in the diurnal revolution the heavens swept by it, “Prepare to write ; nebulæ are about to arrive."
Nebulæ are divided, according to their form, into six classes—elliptic, annular, spiral, planetary, irregular nebulce, and nebulous stars.
The elliptic, or merely oval nebulæ, are the most abundant. Under this head is commonly classed the “great nebula in Andromeda," which was discovered over a thousand years
Fig. 83. ago. It is visible to the naked eye. Prof. Bond, of the Cambridge Observatory, has partly resolved it into stars. He has distinctly counted 1500, although its nebulous appearance is still retained. Under the telescope it is one of the
NBBULA IN ANDROMEDA
most glorious objects in the heavens. “If we suppose this nebula to be one continuous bed of stars of different sizes for its entire extent, it must comprise the enormous number of 30,000,000.” The distance of such nebulæ from the earth entirely passes our comprehension. Some astronomers have estimated that a ray of light would require 800,000 years to span the gulf that intervenes. Imagination wearies itself in the attempt to understand these figures. They only teach us something of the limitless expanses of that space in which God is working the mysterious problem of creation.
The annular nebulo have the form of a ring. There are but four of these “ring universes." In
the cut is a representation of one in Lyra—first as seen by Herschel, and having in the centre a nebulous film like a “bit of gauze stretched over a hoop;" second, as shown in Lord Rosse's great telescope, which resolves the filmy parts of the nebula into excessively minute stars, and reveals a fringe of stars