The nebula (M. 17) just N. of the bow of Sagittarius was also inferred by Holden to have shifted its place relatively to the small stars figured by Lassell in this object; but Dreyer adduces facts which controvert this assumption. (See ‘Monthly Notices,’ vol. xlvii. pp. 412-420, where much valuable information will be found as to supposed variable nebulæ.)
On Oct. 19, 1859, Tempel discovered a faint, large nebulosity attached to the star Merope, one of the Pleiades, and at first mistook it for a diffused comet. Its position is R.A. 3h 39m·6, Dec. +23° 26′ (1890). An impression soon gained ground that this object was variable; for while Schmidt, Chacornac, Peters, and others saw it with small instruments, it could not be discerned by D’Arrest and Schjellerup with the large refractor at Copenhagen. Swift saw the nebula easily in 1874 with a 4½-inch refractor, and has observed it with the aperture contracted to 2 inches. Backhouse re-observed it in 1882 with a 4-1/4-inch refractor. Yet in March 1881 Hough and Burnham sent a paper to the Royal Astronomical Society with an endeavour to prove that the nebula did not exist! They had frequently searched for it during the preceding winter, but not a vestige of the object could be seen in the 18½-inch refractor at Chicago, and they regarded the supposed nebula as due to the glow proceeding from Merope and neighbouring stars. But photography has entirely refuted this negative evidence, and has shown, not only Tempel’s nebula, but others involving the stars Maia, Alcyone, and Electra belonging to this cluster. As to the alleged variations in the Merope nebula, there is every reason to suppose these were not real.
Proper motion has been suggested in regard to a very small, faint nebula (N.G.C. 3236) a few degrees following α Leonis. But Dreyer has disproved this by showing that there was no proper motion between 1865 and 1887, whence “it may be safely inferred that there has been none since 1830, unless we are to believe, in this and similar cases, that nebulæ in the good old days moved about as they liked, but have been on their good behaviour since 1861.”
Nebulous Stars.—This name was applied by Hipparchus and other ancient observers to the clusters of stars which, to the naked eye, appear as patches of nebulous light. Sir W. Herschel, in 1791, showed this designation to be incorrect, and used it in connection with stars actually involved in nebulosity. In sweeping the heavens he met with several instances of this kind. Thus, 3° E.S.E. of ζ Persei he found a star of the 9th mag. surrounded by a nebula 3′ in diameter. He picked up another close to the star 63 Geminorum. This is a remarkable object—a star of the 9th mag. surrounded by two dark and two bright rings. On Feb. 3, 1864, Lord Rosse’s telescope showed an opening in the outer bright ring, and the latter seemed connected with the inner bright ring; so that the object presented the aspect of a spiral nebula with a star in the centre. The diameter of the whole nebulosity is 45″. Key observed this object with an 18-inch reflector in 1868, and described it as symmetrical—a central star, with intervening dark and bright rings. He found a power of 510 the best, for, “like the annular nebula in Lyra, it bears magnifying wonderfully well.” Since Herschel’s time many nebulous stars have been discovered. There is one of about 6th mag. in R.A. 8h 6m·1, Dec.-12° 36′. The nebulosity round the star fades away gradually, and its extreme diameter is 157″. There is a 7th mag. star at R.A. 21h 0m 14s, Dec. +67° 44′ involved in a very large, faint nebulosity. This is a striking object, and I have frequently picked it up while comet-seeking. The star has such a foggy, veiled appearance that on first remarking it the observer thinks his lenses are dewed, but on viewing neighbouring stars he sees them sharp and clear on the dark sky, and the contrast is very pronounced. The nebulous star is much isolated, though in a part of the sky where small stars abound. This is one of Herschel’s discoveries and No. 7023 of the N. G. C.; Dreyer says he has seen the nebulosity particularly distinct north and south of the star. In some cases a double star is involved in nebulosity, and there are instances in which two double stars are placed within an elliptical nebula.
The Magellanic Clouds[66].—These are marked as Nubecula Major and Nubecula Minor on celestial globes and charts. They form two extensive aggregations of nebulæ and star-clusters, and are readily visible to the naked eye in or near Hydrus, and not far from the south pole of the heavens. They may be likened to detached patches of the Milky Way. Sir J. Herschel says the Nubecula Major is situated between the meridians of 4h 40m and 6h and the parallels of 66° and 72° of S. declination, and extends over a space of some 42 square degrees. The Nubecula Minor lies between 0h 28m and 1h 15m and 72° and 75° of S. declination, and spreads over about 10 square degrees. The composition of these objects is very complex and diversified, and affords very rich ground for exploration with a large telescope. Nebulæ exist in profusion and in every variety, and are intermingled with star-clusters varying in condensation from the compact globular form to groups more loosely scattered, and such as we often find in the Milky Way. Nearly three hundred nebulæ and clusters are included in the major “cloud,” while more than fifty others closely outlie its borders. In the minor about forty such objects have been discovered. It is very strange to find them collected together in this manner; for in other regions of the firmament they are usually found separated, and certain classes appear to have their own special zones or localities of distribution. Sir J. Herschel pointed out that “globular clusters (except in one region of small extent) and nebulæ of regular elliptic forms are comparatively rare in the Milky Way, and are found congregated in the greatest abundance in a part of the heavens most remote possible from that circle, whereas in the Nubeculæ they are indiscriminately mixed with the general starry ground and with irregular though small nebulæ.”
Double Nebulæ.—Instances are not wanting of conspicuous double nebulæ. M. 51 and 76, near ζ Ursæ and θ Andromedæ, may be classed in this category. There is a very interesting, though a smaller object just W. of α and β Geminorum, or in R.A. 7h 18m·6, Dec. +29° 43′. Two bright, round nebulæ are separated by an interval of 28″. These double nebulæ are usually round, and are sometimes resolvable into stars. Whether they are physical or mere optical pairs has yet to be ascertained. So many examples exist that it seems highly probable they have a real connection, though no motion has yet been certainly detected to prove they are binary systems. Such motion may, however, be very slow, and require observations extending over a much longer interval before it is revealed.
Real Dimensions of Nebulæ and Clusters.—It may be readily imagined that these objects are of immense size; for though placed at distances of the utmost remoteness, they spread over perceptible and comparatively large areas. Gore remarks that, on the assumption that the globular cluster in Hercules (M. 13) is 5′ in diameter, and its parallax one tenth of a second, its real diameter must be 3000 times the Sun’s mean distance from the Earth, or nearly 280 billions of miles! He further points out that, though this group contains as many as 14,000 stars, according to Sir W. Herschel, yet each component may be separated many millions of miles from the others, owing to the vast dimensions of the group. Details like these are of course only approximate, as the distance of a nebula or star-cluster has not yet been definitely ascertained. The great nebulæ of Orion and Andromeda must extend over prodigious regions in distance-space; but to quote figures seems useless, in consequence of our inability to form just conceptions of such immensity.
Round Nebulæ and Clusters.—Resolvable nebulæ and clusters are frequently circular in outline. The central condensation is an indication of their globular form, though not always so, for many of these objects become suddenly much brighter in the middle, and show an apparently stellar nucleus. The material or stars forming the object cannot therefore be equally distributed. Where it suddenly brightens there is a great condensation, and in some cases several of these are evident in the form of bright rings, intensifying as the nucleus is approached. This irregular aggregation denotes the operation of “a force of condensation directed from all parts towards the centre of such systems.” In regard to planetary nebulæ, they cannot be globular or they would exhibit a brightness increasing from the margin to the centre. Their even luminosity throughout affords the evidence of a special structure. Sir J. Herschel thought the planetary nebula (M. 97) near β Ursæ Majoris must either be in the form of a hollow globe or a flat circular disk lying perpendicular to the line of vision.
Description of Nebulæ and Clusters of Stars.—The latter objects are included in this chapter for several reasons. In a small telescope nearly all such clusters exhibit the aspect of nebulæ, and they have been catalogued with them, though, as already explained, some great distinctions are to be drawn. To the naked eye the cluster Præsepe, in Cancer, is usually visible as a patch of nebulosity, though on a very clear, dark night stars may be glimpsed sparkling about the spot, and a very small glass will suffice to show it as a nest of stars. This object, and some others of a more difficult character (their component stars being smaller and more compressed), are tabulated (I.) at the end of this chapter. A summary (II.) of globular clusters is also given, together with a list (III.) of nebulæ, a few of which are resolvable into stars[67]. It must be understood that these selections, though comprising many notable objects, are by no means exhaustive, the intention being merely to indicate some typical examples of fine nebulæ and clusters and of peculiarities of form or appearance, such as planetary, annular, elliptical, and centrally condensed nebulæ and loose, compressed, and globular clusters. Some of these objects deserve individual references, as they present interesting details to the telescopic observer and come within the reach of moderate appliances.
Great Nebula in Andromeda (M. 31). This object has often been mistaken for a comet, for it is readily perceptible to the eye on a moonless night. It is very large—4° by 2½°, according to Bond, with a 15-inch refractor. He discovered a pair of dark streaks in the brightest region of the nebula, and these may be well seen in a 10-inch reflector. It is really triple; for about 25′ S. of the nucleus there is a very bright, round, resolvable nebula, discovered by Le Gentil, and a third, observed by Caroline Herschel, lies rather further to the N.W. Photographs by Roberts show dark rings dividing the bright interior parts of the nebula from the outer, and imparting to it a decided spiral tendency. This nebula has hitherto resisted attempts to resolve it into stars, though many hundreds have been seen in the foreground. But its spectrum is continuous, so that its stellar character is to be inferred.