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NEBUCHADREZZAR, or NEBUCHADNEZZAR
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See also:NEBUCHADREZZAR, or NEBUCHADNEZZAR , See also: To this See also:period probably belong an inscription of Nebuchadrezzar on the See also:north See also:bank of the Nahr el-Kelb near See also:Beirut, and another in the See also:Wadi Brissa in the See also:Lebanon. From his See also:inscriptions we gather that Nebuchadrezzar was a See also:man of peculiarly religious See also:character. A younger See also:brother of his is called Nabo-sum-lisir. See See also:Josephus, Cont. See also:Apion, i. 19; See also:Eusebius, Praep. Evangel. x. NEBULA (See also:Lat. for " See also:cloud," connected with the Gr. veckXn, mist or cloud), in See also: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 See also:power, a great many of these luminous patches are perceived to be composed of clusters of little stars, which in a smaller See also:telescope are invisible separately, but whose rays of See also:light blend together so as to produce a confused luminous See also:appearance. Others, however, cannot be resolved into individual stars even with the best telescopes, and in many cases the spectroscope gives See also:direct See also:evidence that the nebula has a constitution altogether different from that of a See also: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 See also: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 See also:term nebula is often used to See also:cover both star-clusters and the true nebulae. An enumeration of nebulae was made by See also: The types are: (I) Irregular nebulae, examples: the great nebula of See also:Orion (M. 42),1 the " See also: The first class have See also:ill-defined irregular boundaries; their forms often suggest the appearance of curdled liquid or wreaths of See also:smoke. The annular nebulae have a ringed appearance, the centre being much darker than the See also:outer parts, though it is filled with faintly luminous See also: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 See also:planets. Elliptical nebulae are usually nebulae of some See also: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 See also:form of a double spiral, the two branches of which proceed from diametrically opposite points of a See also:bright See also:nucleus and See also:wind See also:round it in the same sense; the whole is generally studded with points of condensation. The great See also:majority of the nebulae, including the abundant small nebulae which shine with a See also: 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 See also: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 See also:lie in a See also:plane inclined at a rather See also:sharp See also:angle to the See also: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 See also:ring nebula is M. 57 between O and y Lyrae. The ring is slightly elliptical, its dimensions being 87" by 64". At the ends of the major See also:axis the ring becomes very faint, so that the form of the bright See also:part may justly be compared to a pair of marks of See also: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 See also: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 See also:fan-shaped patches of light. It exhibits a See also:close resemblance to the annular nebula; for we have only to assume a continuation of the thinning out along the longest See also:diameter and a slight filling in of the centre of the Lyra nebula to obtain the dumb-bell form. Of planetary nebulae one of the best known is the "owl nebula " in the Great See also:Bear about midway between " the pointers." As seen with See also:Lord Rosse's reflector, it presented a startling appearance, resembling the See also: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 See also: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 See also:discovery of great regions having a faint nebulous back-ground 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 See also:simple apparatus. For the study of the ordinary nebulae large reflecting telescopes (preferably of See also:short See also:focal length) are used, the great light-gathering power being all important; but for photographing these diffused nebulosities portrait lenses of very small See also:aperture and focal length are most successful. Thus the great See also:extension of the Orion nebula was photographed by W. H. See also:Pickering in 1890 with a See also:lens 2.6 in. in aperture and of 8.6 in. focal length; the exposure was rather more than six See also:hours. Other extensive nebulous regions of a similar character have been found by Barnard in the constellations Ophiuchus, See also: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 See also:wave-lengths 5007, 4959 and 4861. Of these the last is the line H /3 of the See also:hydrogen See also: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 See also:element, which has been named " nebulium." Usually there are no other conspicuous lines in the visual spectrum, but in the ultra-See also:violet region numerous lines can be photographed, including most of the hydrogen series. The yellow line (D3) of See also: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 See also: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 distinguish-able. A gaseous See also:mass only gives a bright line spectrum when it is so rarefied as to be transparent through and through. If the See also:density and thickness are such that a See also:ray of light cannot pass through it the spectrum will, in general, be continuous like that of a solid See also:body. The inquiry into the See also:physical See also: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 See also:Norman See also:Lockyer's "Meteoritic See also:Hypothesis," which attributes the light to' collisions between See also:numbers of small discrete solid particles, these being vaporized and made luminous owing to the See also:heat See also:developed by their impacts. Formidable difficulties, however, prevent the entire See also:acceptance of this See also:suggestion. The spiral nebulae are not distributed at See also:random over the See also:sky, nor are they condensed along the galactic plane like the clusters which they spectroscopically resemble. There is awell-marked centre of See also:aggregation of the See also:northern nebulae near the north galactic See also:pole. In the See also: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. Additional information and CommentsThere are no comments yet for this article.
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