NEBULA (Lat. for “cloud,” connected with the Gr. νεφέλη, 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 best 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 2500; 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: (1) Irregular nebulae, examples: the great nebula of Orion (M. 42),[1] the “key-hole” nebula near η Argus, 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. 1 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) Defused 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 θ 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 β and γ 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.
| Plate I. | ||
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| (1) GREAT NEBULA IN ORION, 1901, OCTOBER 19. | (2) NEBULA IN ANDROMEDA, 1901, SEPTEMBER 18. | |
| By permission of Yerkes Observatory. | By permission of Yerkes Observatory. | |
| Plate II. | ||
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| (1) ANNULAR NEBULA, LYRA, 1899, JULY 14. | (2) SPIRAL NEBULA, CANES VENATICI, 1899, MAY 10. | |
| By permission of Lick Observatory. | By permission of Lick Observatory. | |
Of planetary nebulae one of the best known is the “owl nebula” in the Great Bear about midway between “the pointers.” As seen with Lord Rosse’s reflector, it presented a startling appearance, resembling the face of a goblin; two faint stars shone in the centres of the two dark circles which represented the saucer-eyes of the creature. Some change has certainly taken place since then, for the two stars no longer could be supposed to represent the pupils of the eyes; the cause may, however, be merely the proper motion of the stars or of the nebula.
The discovery of great regions having a faint nebulous background is one of the most remarkable results of modern work. Particularly interesting is the fact that, whilst the large telescopes are unable to render them perceptible to the eye or to photograph them, they are revealed by what at first sight seems an absurdly simple apparatus. For the study of the ordinary nebulae large reflecting telescopes (preferably of short focal length) are used, the great light-gathering power being all important; but for photographing these diffused nebulosities portrait lenses of very small aperture and focal length are most successful. Thus the great extension of the Orion nebula was photographed by W. H. Pickering in 1890 with a lens 2·6 in. in aperture and of 8·6 in. focal length; the exposure was rather more than six hours. Other extensive nebulous regions of a similar character have been found by Barnard in the constellations Ophiuchus, Scorpio and Taurus.
Spectra of Nebulae.—Owing to the feebleness of their light the study of the spectra of nebulae is one of particular difficulty. Two varieties of spectra are recognized; the one consists of a few narrow bright lines with sometimes a faint continuous spectrum for a background; the other consists of a continuous spectrum crossed by dark lines and is indistinguishable from that of ordinary stars. The former variety unmistakably shows that the light proceeds from diffuse incandescent vapour; nebulae showing this spectrum are accordingly called “gaseous.” Irregular, annular and planetary nebulae are of this nature. The visual spectrum is marked by three bright lines in the blue and green of wave-lengths 5007, 4959 and 4861. Of these the last is the line H β of the hydrogen series; the other two are of unknown origin, and as they are always found together and have always the same relative intensity, they have both been attributed to the same unknown element, which has been named “nebulium.” Usually there are no other conspicuous lines in the visual spectrum, but in the ultra-violet region numerous lines can be photographed, including most of the hydrogen series. The yellow line (D3) of helium can be detected in many nebulae. The great majority of the nebulae, however, show the second variety of spectrum, and are thus indistinguishable spectroscopically from irresolvable star-clusters. The great nebula of Andromeda and the spiral nebulae are of this kind. It is not necessary to conclude that they, therefore, are star-clusters whose components are, owing to their remoteness from us, too faint and close together to be separately distinguishable. A gaseous mass only gives a bright line spectrum when it is so rarefied as to be transparent through and through. If the density and thickness are such that a ray of light cannot pass through it the spectrum will, in general, be continuous like that of a solid body.
The inquiry into the physical state and constitution of the nebulae raises problems of great difficulty. In the case of “gaseous” nebulae it is very hard to understand how such extremely tenuous masses are maintained in a state of incandescence. Only one theory has been put forward which at all accounts for this fact, and unfortunately, it is not altogether satisfactory in other respects. This is Sir Norman Lockyer’s “Meteoritic Hypothesis,” which attributes the light to collisions between numbers of small discrete solid particles, these being vaporized and made luminous owing to the heat developed by their impacts. Formidable difficulties, however, prevent the entire acceptance of this suggestion. The spiral nebulae are not distributed at random over the sky, nor are they condensed along the galactic plane like the clusters which they spectroscopically resemble. There is a well-marked centre of aggregation of the northern nebulae near the north galactic pole. In the southern hemisphere they are more evenly distributed, but the avoidance of the galactic plane is marked. The remarkable Nubeculae or Magellanic Clouds in the southern hemisphere, which look like detached portions of the Milky Way, are found on telescopic examination to consist, not of stars alone, like the Milky Way, but of stars and nebulae clustering together. In the greater cloud Sir John Herschel counted 286 nebulae; in the lesser cloud they are rather less numerous.
References.—The characters of nebulae receive treatment in all text-books on descriptive astronomy; mention may be made of Miss A. M. Clerke, The System of the Stars (2nd ed., 1905), which contains a full account of these objects, illustrated by many photographs; the same work is replete with references to original papers. Of recent catalogues of nebula, we notice J. L. E. Dreyer, “A new general catalogue of nebulae and clusters of stars,” Memoirs R.A.S. (1888), published separately in 1890; and “Index Catalogue of Nebulae (1888–1894),” Mem. R.A.S. (1895). Excellent photographs of the more famous nebulae are given in Sir R. Ball’s Popular Guide to the Heavens (1905); a more comprehensive collection is given in Isaac Roberts, Photographs of Stars, Star Clusters and Nebulae (2 vols., 1873–1899). (A. S. E.)
- ↑ 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.).