The wave-lengths of the three lines from which the radial velocities were deduced, and the wave-lengths of the comparison lines, are as determined or adopted by Campbell and Moore in 1915 for similar work at Mount Hamilton.3 It is evident at a glance that the seventeen observed nebulae are traveling through space with approximately the same high radial velocity. As they are distributed over the area of the Cloud, and as no bright-line nebulae are known to exist in a large region of ordinary sky background surrounding the Cloud, it seems reasonable to assume that the mean of the seventeen observed velocities is the radial velocity of the nebulous structure of the Greater Magellanic Cloud; that is, 276 kilometers per second, recession. Whether or not this velocity is truly representative of the Cloud as a whole can perhaps only be determined when we shall have secured observations of stars as faint as our instruments can be made to observe and have shown that some of these stars are traveling with essentially the same high velocities. The seventeen stars, of various spectral types, visual magnitudes between 4.5 and 9.0, which we have already observed in this region, are all moving with relatively small radial velocities, the average for the seventeen being about 6 kilometers per second, recession; and this is more than accounted for by the recession of the solar system from that region of the stellar system. In all probability the seventeen stars lie between us and the Cloud, against which they are seen by projection. The close relationship of the bright-line nebulae in general and the stars of Class Oa (WolfRayet stars), and the remarkable facts that twenty-one Class Oa stars have been found in the area of the Greater Magellanic Cloud and no such stars in a vastly larger region of surrounding sky, lead us to inquire whether the Class Oa stars in the Cloud have radial velocities comparable with those of the nebulae in the Cloud. We have obtained low-dispersion spectrograms of the brightest three of the twenty-one Class Oa stars, to test the question. These are Cape P.D.M. -66° 343, mag. 9.1; 69° 361, mag. 9.4; and 67° 433, mag. 9.5. Unfortunately, the spectrograms secured to date are much under-exposed and otherwise unsatisfactory. There is a bright Hẞ line on the spectrograms of 69° 361, which yields a radial velocity of +279 km., but there is a good chance that the line really proceeds from a nebulous background, and not from the star itself. When opportunity comes this question will be tested by exposing with a long slit, keeping the star fixed upon the center of the slit. The dark lines in the spectra of -66° 343 and 69° 361 give some indications of high positive velocity, but the evidence is unsatisfactory, and a definite statement on the subject must await the securing of better plates. RADIAL VELOCITY OF THE GREATER MAGELLANIC CLOUD 189 It will be seen from the co-ordinates of the seventeen nebulae in the Greater Cloud that they fall naturally into four groups. The co-ordinates and radial velocities of the four groups are: The two southern groups have the smaller positive velocity, 27 km. per second smaller. Whether this difference has a significance for the motions of corresponding regions in the Cloud cannot now be determined. Professor Pickering has said, "30 Doradus appears to be the nucleus of the whole system. It seems to be the center of a great spiral, and to bear the same relation to the entire system that the nebula in Orion bears to the great spiral nebula which covers a large part of that constellation." If we assume that 30 Doradus marks the center of the Cloud, we may say that the northern regions seem to be receding with velocities systematically higher than those nearer the center; suggesting possibly a rotation of the Cloud as a whole about an axis approximately at right angles to the line of sight and parallel to the celestial equator. It should be said that the evidence for this is at present so meager as to assign the hypothesis to the domain of speculation. 3 2 As to the character of the nebulae in the Magellanic Clouds, Miss Cannon remarks," "the spectra of several nebulae in the Large Magellanic Cloud are intermediate between classes Pc and Pd, considering the relative intensities of Hy and 15007 (N,)." Some points of interest are brought out in the following tabulation of the relative intensities of the lines N1, N2, Hẞ, and N. (4686A) as estimated from the spectrograms of all the nebulae observed by the D. O. Mills Expedition. The intensity of N, in each case is set down as 10. For some of the fainter objects on the list only the N, line was recorded, and that very feebly, in which cases the estimates of relative intensities could, of course, not be made. For a few other faint objects only the N1 and N. lines were recorded, and the relative intensity of Hẞ cannot be estimated, except that in these cases it is certainly no brighter than N2. It should be said that the estimates printed must not be considered as correct in absolute values. In several cases the percentage of error in the numbers for N2, Hẞ, and N, may be considerable, but relatively they are fairly satisfactory. The use of two kinds of sensitive plates has complicated this phase of the subject. Seed Graflex plates are more sensitive in the N, region than are the Seed 30 plates, but the sensitivities of the two kinds of plates are practically equal in the Hẞ and N, regions; and for fully exposed images the Seed 30 plates give appreciably stronger contrasts. In effect the sensitivity curves for the two kinds of plates are different and lead to different estimates of relative intensities of the lines. The lines N, and Hẞ are invisible (inv.) on the spectrograms of four nebulae in the list. 2 a 3 RELATIVE INTENSITIES Remarks Nebula 17 nebulae Orion Nebula 5 30 Ni Ng Hẞ Ng In Lesser Cloud In Greater Cloud Trapezium region; extended Planetary 10 4 2 Planetary, "stellar'' 10 4 4 Annals H. C. O., 27, pt. 2, 206. 5 It should not be assumed that the great curves of faint nebulosity in the constellation of Orion are parts of a typical spiral nebula.-R. E. W. 6 Annals H. C. O., 76, pt. 3, 24. In Class Pc the hydrogen Hy line is stronger than the N, line. In Class Pd N, is stronger than Hy, generally at least five times as strong.-Miss CANNON. η It will be noticed, first, that in the nebulae of the Greater Cloud the Hẞ line is as bright as or brighter than N2, without exception; secondly, that a similar condition occurs in the two irregular and extended nebulae observed, the Orion nebula and the Carinae nebula; thirdly, that in all planetary nebulae observed outside of the Clouds, excepting the stellar nebula at 17h 53m3, Hẞ is decidedly fainter than N,; fourthly, that N, appears in certain planetary nebulae outside of the Clouds, but not in the nebulae of the Clouds. Our knowledge of the characteristics of the Smaller Magellanic Cloud is very limited, and our present consideration of it must rest for the most part upon analogies with the Greater Cloud. It would not be surprising, in view of the observed velocities within the Greater Cloud, if we should eventually find that the radial velocity, 168 kilometers per second, recession, of the one gaseous nebula within the Lesser Cloud is representative of the Cloud, but we have not enough evidence to establish this hypothesis. Hertzsprung has estimated the distance of this Cloud as 30,000 light years, assuming space to be perfectly transparent, and this would put the Cloud beyond the limits assigned to our stellar system by modern researches. The spiral appearance of the Greater Cloud, as described by Pickering, and the high velocities of the nebulae within it, when considered in connection with the high velocities derived by Slipher for the spiral nebulae, suggest that the Greater Cloud may be a typical spiral nebula, nearer to us than most and possibly all of the other spirals. If the spiral nebulae are isolated universes similar to our Milky Way system, as seems quite reasonable in view of accumulated evidence, it is possible that the Magellanic Clouds, especially the Greater Cloud, may afford opportunities for studying some of the detailed characteristics of spiral nebulae. Plate XL, figure 1, reproduces Schaberle's photograph of the Greater Magellanic Cloud, and the positions of the observed nebulae in the Cloud may be determined from Magister Thiele's copy of it in figure 2. I am indebted to my colleagues, Messrs. R. F. Sanford, A. A. Scott, and C. M. Huffer, for aid in securing and measuring the plates upon which the present paper is based. PART VI THE WAVE-LENGTHS OF THE NEBULAR LINES AND GENERAL OBSERVATIONS OF THE SPECTRA OF THE GASEOUS NEBULAE BY WILLIAM HAMMOND WRIGHT ASTRONOMER IN THE LICK OBSERVATORY |