N.G.C. 1976: GREAT NEBULA IN Orion. a= 5h 30m4; 8 = -5° 27′; λA 81° 36'; ẞ=—28° 41′ Thirteen observations of the Orion nebula made by Keeler with the Brashear grating spectroscope attached to the 36-inch refractor, in 1890–91, gave the observed radial velocity as +17.7 km. per second.15 With the original Mills 3-prism spectrograph attached to the 36-inch refractor Wright secured two observations each on the Hẞ and Hy hydrogen lines, in 1901, for the radial velocity of the Orion nebula. His result is +16.2 km.16 Vogel and Eberhard obtained ten observations with a 3-prism spectrograph attached to the 32.5 cm. refractor, in 1901-02, which yielded a radial velocity of +17.4 km.1 17 Frost and Adams secured eleven observations with a 1-prism spectrograph attached to the 40-inch refractor, in 1903-04. Their result is +18.5 km.18 It appears that the four groups of observations refer to approximately the same bright region of the nebula, that immediately preceding the Trapezium. The simple mean of the four results quoted is 17.4 km. per second. Our observations of the Orion nebula spectrum in the past three years have been very extensive: 86 spectrograms in number, with total exposure time amounting to nearly four hundred hours. As with the planetary nebulae, so with the Orion nebula, improved methods of observing and increased accuracy came with the progress of the work, and for that reason we repeated many of the early observations. It was further found that a long slit could be used to advantage: about 80 seconds of are in length seemed to be the practicable limit for quantitative results. The "long" slit was usually 100 seconds of are or more in length, but the outer-end sections of the comparison lines and nebular lines were found to give unreliable results, probably because the corresponding illuminations of the collimator lens were incomplete or unsymmetrical. The end sections were accordingly not used. Great care was exercised in directing the telescope upon the region which it was desired to observe. We beg to acknowledge our indebtedness to Bond's catalogue and charts19 of the stars in the Orion nebula, which we used constantly for this purpose. As a preliminary to combining the observations, each observed velocity was set down on a large chart at its proper right ascension and declination. A good many points in the nebula, especially in the vicinity of the Trapezium, had several observations each. Those observations which refer to the same point or to closely adjacent points were combined into one resulting observed velocity, whose weight is the sum of the weights of the individual observations. The positions of the observed points are given in the table of observations, under Remarks. Aa and As are the co-ordinates of these points with reference to the origin, at star Bond No. 628. Figure 16 contains the observed velocities arrived at in this manner for all regions of the nebula whose right ascensions and declinations do not differ from those of the principal star (the southern star, Bond No. 628) in the Trapezium by more than one minute of arc. The weight of each such velocity is written underneath it. To illustrate: the velocity of the nebula at the origin of co-ordinates, the south point of the Trapezium, is +21.0 km. per second, and its relative weight is 15. The positions of the four stars defining the Trapezium are connected by the right lines of the four-sided figure near the center of the chart. As Buisson, Fabry, and Bourget established in 1914, the radial velocities of different regions in the nebula are not the same. The highest velocity observed by us in the area covered by figure 16 is for a small region within the Trapezium, about 5" east of its center. The lowest velocity, at Aa=-1', A8+025, is especially well established, as shown by its relative weight = 15 Publ. Lick Obs., 3, 197, 1894. 16 Wright, Lick Obs. Bull., 1, 155, 1902. 17 Vogel and Eberhard, Astroph. Jour., 15, 303, 1902; and Sitzungsber. Kgl. Akad. Wiss. Berlin, p. 260, 1902. 18 Frost and Adams, Astroph. Jour., 19, 354, 1904. 19 Annals H. C. O., 5, 1867. and the good accord of the individual results for that point. The range of observed velocities within the bounds of figure 16 is 13.5 km. It is a question of the integrated radial velocity of the Orion nebula, for use in statistical studies of the bright-line nebulae. The brilliant parts of the nebula correspond to a comparatively small area near the center of figure 16, and it is here that the observed velocities are most numerous and on the whole most accurate. The velocities set down in the outer parts of figure 16, on the other hand, may be said to represent larger areas of the nebula. It therefore seems to us that we cannot do better than to accept the straight arithmetical mean of the 49 plotted velocity values as the apparent radial velocity of the Orion nebula. This mean value is +17.5 km. per second. -8' - 6' - 4' - 2' 0' Figure 17, on a much smaller scale, represents all the observations, with two exceptions, that we have made on the Orion nebula. The area of figure 16, 2' on each side, is embraced within the small square bounded by right lines at the center of figure 17, and in this small square the results on figure 16 appear necessarily in a highly condensed form, as there is not room for legible charting of the individual results. The weights of these condensed results have not been set down. The velocities charted outside of the small central square are much less condensed and their relative weights are quoted underneath them. The two observed velocities not represented in figure 17 are listed in the table of observations under dates 1915 October 8.0 and October 13.0. Their relative weights are in each case less than . To some extent the two observations must be considered as unreliable, and to include them on figure 17 would possibly create - 8' +2' 16,0 +4 17.4 3 17.0 + 8' 249 Fig. 17-Observed radial velocities of the Orion Nebula. ·E-· |