Distribution.—With regard to distribution these objects exhibit the utmost irregularity, for in certain regions of the heavens they are found to be very plentiful, while in others they are singularly rare. Thus, in Virgo, Coma Berenices, Leo, and Ursa Major large numbers of nebulæ abound, while in Hercules, Draco, Cepheus, Perseus, Taurus, Auriga, &c., very few are encountered. Taking the 7840 objects in the New General Catalogue of 1888 it will be found that their distribution in hours of Right Ascension is as follows:—
| R.A. | Nebulæ. | R.A. | Nebulæ. | |
|---|---|---|---|---|
| 0 H. | 387 | XII H | 858 | |
| I | 428 | XIII | 504 | |
| II | 398 | XIV | 375 | |
| III | 300 | XV | 212 | |
| IV | 276 | XVI | 230 | |
| V | 375 | XVII | 259 | |
| VI | 171 | XVIII | 203 | |
| VII | 196 | XIX | 117 | |
| VIII | 230 | XX | 153 | |
| IX | 362 | XXI | 188 | |
| X | 404 | XXII | 275 | |
| XI | 585 | XXIII | 354 |
The maximum is therefore reached at XII hours, while the minimum is shown at XIX h. There is a secondary max. at I h., and a secondary min. at VI h.
Early Observations.—The nebula in Andromeda appears to have been the one first discovered, for the distinguished Persian astronomer Al-Sûfi (who died in 986 A.D.) was undoubtedly acquainted with it. The nebula is figured upon a Dutch map of the stars nearly 400 years old. In 1612 Simon Marius redetected this object, and appropriately likened its appearance to that of a “candle shining through a piece of horn.” In 1618 the nebula in Orion was certainly known, for Cysatus of Lucerne compared it with the head of the fine comet visible in December of that year. Huygens alighted upon the same object in 1656, and appears to have been unconscious of its prior discovery. Only six “nebulæ or lucid spots” were known in 1716, and enumerated by Halley in the ‘Phil. Trans.’ vol. xxix. These included those of Andromeda and Orion. A third was situated in the space between the bow and head of Sagittarius. This is M. 22, and consists of a bright globular cluster of Stars. The fourth was the fine star-group involving ω Centauri, which Halley himself found in 1677. The fifth was another fine group in the right foot of Antinous. This is M. 11, and was discovered by Kirch in 1681. The sixth was the magnificent globular cluster (M. 13) in Hercules, discovered by Halley in 1714.
In 1735 the Rev. W. Derham published a list of 16 of these objects, and in 1761 Lacaille summarized 42 nebulæ and star-clusters which he had observed in the southern sky. This was followed by Messier’s tables of 45 nebulæ &c. in 1771, and of 103 in 1781[64]. But these contributions, important though they severally were, sunk into insignificance beside the splendid results obtained by Sir W. Herschel, who during his prolonged and systematic sweeps of the heavens picked up no less than 2500 new nebulæ and clusters which he formed into three catalogues printed in the ‘Phil. Trans.’ as follows:—1786, 1000 objects, 1789, 1000 ditto, 1802, 500 ditto.
Variable Nebulæ.—It is in the highest degree probable that changes occur in the physical appearances of certain nebulæ, though the opinion is not perhaps supported by a sufficient number of instances. Until Sir W. Herschel began his review of the heavens very few nebulæ were known, and the information possessed about them was very incomplete. The early records, obtained with small and inferior telescopes, scarcely admit of comparison with recent observations, for in matters of detail little agreement will be found; and this proceeds certainly not so much from real changes in the objects as from differences due to the variety of instruments employed, to atmospheric vagaries, and to “personal equation.” Bullialdus and Kirch in 1667 and 1676 and Le Gentil in 1759 supposed that remarkable changes were operating in the great elliptical nebula of Andromeda. But G. P. Bond fully investigated the evidence, and concluded that the variability of the object was by no means proved. Some observers have represented the nucleus as stellar, while others have drawn it as a gradual condensation, and Dr. Copeland has shown that different magnifying powers alter the aspect of the nucleus, “the lower powers making it more star-like, the higher ones more soft-looking and extensive.”
Mairan and others entertained the view that the large irregular nebula in Orion was subject to change. This object received much attention from Sir W. Herschel, and he concluded that it underwent great alteration between 1774 and 1811. D’Arrest, from his own researches and a discussion of other results, expressed himself in 1872 that “the observed changes in this vast mass of gas seem exclusively to turn out to be temporary fluctuations of brightness.” Prof. Holden has arrived at a similar conclusion, and says:—“The figure of the nebula has remained the same from 1758 till now (if we except a change in its apex about 1770, which seems quite possible); but in the brightness of its parts undoubted variations have taken place, and such changes are still going on”[65] (‘Monograph of the Nebula in Orion,’ p. 225).
Hind discovered a faint nebula, with a diameter of about 1′, on Oct. 11, 1852. It was situated in Taurus, the position being R.A. 4h 15m 33s, Dec. +19° 15′·6 (for 1890), or about 2° W. of the star ε Tauri (mag. 3·7). D’Arrest, on Oct. 3, 1861, searched for this object, but found it had quite disappeared! A small round nebula was seen in 1868, about 4′ preceding Hind’s, but this resisted some later attempts at observation. In Oct. 1890, Burnham and Barnard, with the 36-inch refractor of the Lick Observatory, saw two nebulæ here, one a very small, condensed nebula, with a stellar nucleus, and the other an exceedingly faint nebulosity about 45″ in diameter (see ‘Monthly Notices,’ vol. li. pp. 94, 95).
The nebula surrounding the star ζ Argûs has been suspected of variation, particularly by Abbott, of Hobart Town, Tasmania. Vols. xxv., xxx., and xxxi. of the ‘Monthly Notices’ contain many references to, and figures of, this interesting object. But the alleged changes have not been substantiated, and there seems no reason to doubt that they were purely imaginary.
The trifid nebula in Sagittarius (M. 20) is supposed by Prof. Holden to have altered its position with reference to a triple star now situated in the S. following part of the nebula. Sir J. Herschel, more than half a century ago, had described this star as placed in the middle of the vacuity by which the nebula is divided. Dreyer, however, points out that the drawings of this object differ in many details, and that, though changes of brightness may have taken place, it is difficult to understand that the nebula should move so as to envelop the star in about 1835, “after which no sensible change occurred again, so far as published observations go.”