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THE RADIAL VELOCITY
GREATER MAGELLANIC CLOUD
By RALPH ELMER WILSON
SEMI-CENTENNIAL PUBLICATIONS, UNIVERSITY OF CALIFORNIA
THE RADIAL VELOCITY OF THE GREATER
By RALPH ELMER WILSON
The discovery,' in 1914, that certain of the gaseous nebulae in the Greater Magellanic Cloud have large and approximately equal radial velocities made it of special interest to determine the velocities of as many as possible of these nebulae, and likewise the velocities of stars apparently in the Cloud.
The Greater Magellanic Cloud presents many characteristics in common with the Milky Way. It contains some 278 nebulous objects, nineteen of which are known to have bright-line spectra; these form a large proportion of such objects of this class as are distant more than 10° from the galactic plane. The twenty-one stars of Class Oa within its borders are the only stars of this class, with a single exception in the Lesser Cloud, which lie more than 12° from the galactic plane. It contains, also, four stars of the type of P Cygni, and a number of stars whose spectra contain bright hydrogen lines. These objects are essentially all those of their respective spectral classes known to exist in this region of the sky, and it is therefore difficult to doubt that they lie within the Cloud and form integral parts of it.
The gaseous nebulae in this region differ greatly in size and shape, varying from the small, round planetary to the large, irregular nebula and the cluster composed of stars and nebulosity. None of them, so far as we have been able to determine, is stellar. They are all, including the brightest one, 30 Doradus, N.G.C. 2070, intrinsically faint, and their spectra may be recorded only by means of comparatively long exposures with rapid cameras and sensitive plates.
The first large radial velocities of these nebulae were discovered on weak plates taken with a one-prism spectrograph, camera of 18-inch focal length, designed for stellar observations. Most of the observations here presented, however, have been secured with a camera of S^-inch focal length and two-prism dispersion. The spectra of all of the nineteen gaseous nebulae have been examined visually, but two of them, N.G.C.2 2116 and N.G.C..., 2117, in whose spectra we have been able to see bright lines, were found to be so faint as to make guiding upon them very uncertain with the means at hand, and even with perfect guiding the exposures would be very long.2
The positions for 1900.0, the approximate magnitudes as estimated with low power, the mean radial velocities observed for the several objects to date, the number of spectrograms, the spectral classifications as assigned in Annals II. C. O., 76, No. 3, and the estimated relative intensities of the three green nebular lines, are given as completely as they are available in the following table:
i Proc. N. A. 8., 1, 183, 1915. '.! See p. 93.