236 CARNEGIE INSTITUTION OF WASHINGTON. 



short-focus lens of ratio F 1.8. The resulting small image and large 

 dispersion give promising preliminary results in the study of such faint 

 extensive objects as the exterior nebulosity in Orion. 



A provisional classification of the spectra of 90 nebulous stars clearly 

 indicates a progression in type with decreasing brightness. Almost 

 without exception the objects brighter than magnitude 7.5 are of a 

 type earlier than B 5. Fainter stars are distributed uniformly over the 

 range BO to A 3. Two stars, B. D.+31°597, 7.5, and - 19°4357, 6.2, 

 are, however, of advanced type, being G 5 and K 0, respectively. 



The spectrographs at the Cassegrain focus of the 60-inch and 

 100-inch telescopes have been used by Mr. Hubble for a detailed 

 study of the spectra of some 25 nebulous stars. No characteristic 

 deviation from a normal distribution of hues has been detected. Two 

 of these stars have the enhanced hnes of a Cygni and two are of late 

 type — G5 and KO. In these four cases some idea of the absolute 

 magnitude, and hence of the parallax, can be obtained by the spectro- 

 scopic method. One star shows bright Ha and H^, the latter, at 

 least, reversed in the center. Radial velocities are being determined 

 and the plates on hand suggest a high percentage of binaries. 



COLOR OF NEBULOUS STARS. 



The importance of the continuous part of these spectra is emphasized 

 by the abnormally large color-index which nebulous stars are now 

 known to possess. It had been noticed by Mr. Hubble that certain 

 nebulous stars looked decidedly yellowish in the telescope, and when 

 their spectra were found to be of early type, the matter was investi- 

 gated photographically by Mr. Scares and Mr. Hubble, using the method 

 of exposure ratios with the 60-inch reflector. For certain of the stars, 

 color-indices were also determined by polar comparisons with the 

 10-inch telescope. Complete data — spectra as well as colors — are now 

 available for 42 objects, and it is found that as a class these stars are 

 appreciably redder tlian would be inferred from their spectral type. 

 The amount of the color excess varies between rather wide limits, but 

 in the average corresponds to about one spectral subdivision or 0.4 

 mag. in the color index. B. D. — 12?1771 is the only conspicuous 

 exception, although a few stars, formerly supposed to be nebulous, 

 show little or no color excess. Long-exposure photographs indicate, 

 however, that none of these objects is nebulous in the sense defined by 

 Herschel. At the other extreme is B. D. — 22?4510, the central star in 

 the northern mass of the Trifid Nebula. Its spectrum is A 3, while the 

 color class is g 4. The color excess is thus 2.1 spectral subdivisions, or 

 about 0.8 mag. B. D. -6?1415 (N. G. C. 2170) shows an even larger 

 excess of color, but the result is uncertain and requires confirmation. 

 A plausible expknation of the phenomenon is that the light of these 

 stars is scattered by the surrounding nebulosity, but the possibility of 

 some form of resonance excitation should also be borne in mind. 



