(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. |
Scale: Minutes of arc 01, 2 I 3 | Scale: Minutes of arc 12 3357 8 4 Io |
I 20 30 40 50 Scale: Minutes of arc 10 20 30 0 Scale: Minutes of arc 40 50 (I) [[Annular Nebula, Lyra, 1899, July 14. (2) Spiral Nebula, Canes Venatici, 1899, May]] To.
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 saucereyes 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.
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 /3 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 (D 3) 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 Iook 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.
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 Popula r 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.)
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