1911 Encyclopedia Britannica
NEBULA (Lat. for " cloud," connected with the Gr. vecEXri, mist or cloud), in astronomy, the name given to certain luminous cloudy patches in the heavens. They resemble the stars in that they retain the same relative positions, and thus may be distinguished from the comets which appear to wander across the stars. When examined with sufficient telescopic power, a great many of these luminous patches are perceived to be composed of clusters of little stars, which in a smaller telescope are invisible separately, but whose rays of light blend together so as to produce a confused luminous appearance. Others, however, cannot be resolved into individual stars even with the bes t telescopes, and in many cases the spectroscope gives direct evidence that the nebula has a constitution altogether different from that of a star-cluster. We thus distinguish between the nebulae proper and the star-clusters; but owing to the difficulty of deciding the nature in any particular case, and especially owing to the fact that some of the earlier observers.believed it probable that all nebulae would with sufficient telescopic power become resolvable into stars, the term nebula is often used to cover both star-clusters and the true nebulae.
An enumeration of nebulae was made by Charles Messier in Paris in 1771, who recorded 103; Sir William Herschel increased the number known to over 250o; whilst Sir John Herschel between 1825 and 1847 catalogued and described 3926 nebulae (including 1700 observed at the Cape of Good Hope). About 1848 the earl of Rosse with his famous six-foot reflector at Parsonstown began his examination of the nebulae, which added greatly to our knowledge of their forms and structure. In more modern times the development of photography has enabled the features of the nebulae to be ascertained and recorded with a certainty, which, unfortunately, the older visual observations and drawings cannot claim to possess. In this connexion the photographic work of Isaac Roberts, A. A. Common, E. E. Barnard and J. E. Keeler in particular must be mentioned. The total number of known nebulae has, too, been enormously increased; Perrine estimates that the number within the power of the Crossley reflector at Lick is not less than half a million.
Nebulae may be conveniently classified according to their telescopic appearance; we enumerate below some of the principal forms that have been recognized, but it must be observed that this classification is rather superficial, and that the differentiation is often one of appearance only and not of real structure. The types are: (r) Irregular nebulae, examples: the great nebula of Orion (M. 42), 1 the " key-hole " nebula near n Argus, 1 i.e. No. 42 in Messier's catalogue. Nebulae not contained in that catalogue are generally known by their number in Dreyer's New General Catalogue (N.G.C.). the " Omega " nebula (M. 17); (2) Annular nebulae, example: M. 37 in Lyra; ( 3) Double nebulae, example: the dumb-bell nebula (M. 27) in Vulpecula; (4) Planetary nebulae, examples: the " owl " nebula (M. 97) in Ursa Major, M. i in Taurus; (5) Elliptical nebulae, example: the great nebula of Andromeda (M. 31); (6) Spiral nebulae, example: M. 51 in Canes Venatici; ( 7) Nebulous stars; (8) Diffused nebulosities. Most of these names require little explanation. The first class have ill-defined irregular boundaries; their forms often suggest the appearance of curdled liquid or wreaths of smoke. The annular nebulae have a ringed appearance, the centre being much darker than the outer parts, though it is filled with faintly luminous matter. Double nebulae have two principal centres of condensation. The planetary nebulae are nearly uniformly illuminated compact patches of light generally circular or elliptical in shape; they were so called because they appeared to possess disks like planets. Elliptical nebulae are usually nebulae of some flat type (such as annular or spiral) seen rather edgeways, so that the structure is not readily recognizable. The typical spiral nebulae are in the form of a double spiral, the two branches of which proceed from diametrically opposite points of a bright nucleus and wind round it in the same sense; the whole is generally studded with points of condensation. The great majority of the nebulae, including the abundant small nebulae which shine with a white light (in contrast with the blue-green light of the planetary and irregular nebulae - see below Spectra of nebulae ), are generally classed as spiral nebulae. The spiral structure has been shown to exist in a few of them, but for the remainder it is only inferred. Nebulous stars are true stars surrounded by an atmosphere or aureole of nebulous light. Diffused nebulosities are very faint nebulae of enormous extent, sometimes forming the background of a whole constellation. We proceed to describe some of the more famous nebulae.
One of the most remarkable nebulae is that which is situated in the sword-handle of Orion and about the multiple star 0 Orionis; it is faintly visible to the naked eye. It seems to have been first noticed by Huygens in 1656, who described and figured it in his System Saturnium. It has now been found that nebulous streamers connected with the bright nucleus wind through the whole constellation of Orion. It is well known that all the brighter stars of the constellation except Betelgeuse appear to be related to one another by their similarity both of spectra and of proper motion; it seems probable that they are actually situated in the nebula and in some way connected with it.
The only other nebula which can be seen with the naked eye is the elliptical nebula in Andromeda. Modern photographs show very clearly that its structure is spiral. The nucleus is large and appears circular, but the spirals proceeding from it lie in a plane inclined at a rather sharp angle to the line of sight, and this gives to the nebula its elliptical appearance. Two small dense nebulae accompany it, and appear to belong to the system.
The finest example of a ring nebula is M. 57 between f3 and y Lyrae. The ring is slightly elliptical, its dimensions being 87" by 64". At the ends of the major axis the ring becomes very faint, so that the form of the bright part may justly be compared to a pair of marks of parenthesis (). The centre is marked by a star which appears to be intimately associated with the ring, for the whole space within the ring is filled with a very faint nebulosity. According to Schaeberle, there is evidence of a spiral structure in this nebula also. It must, however, clearly be of an essentially different character from the structure of an ordinary spiral nebula, and the spectroscope reveals a fundamental difference between the annular and spiral nebulae.
The " dumb-bell " nebula in Vulpecula consists of two almost separated fan-shaped patches of light. It exhibits a close resemblance to the annular nebula; for we have only to assume a continuation of the thinning out along the longest diameter and a slight filling in of the centre of the Lyra nebula to obtain the dumb-bell form. .
(I) GREAT NEBULA IN ORION, 1901, OCTOBER 19.
By permission of Yerkes Observatory
(2) NEBULA IN ANDROMEDA, 1901, SEPTEMBER 18.
By permission of Yerkes Observatory.