A Notation For Use In The Discussion Of Star Colors(en)(2s)

ASTRONOMY: F. H. SEARES 481 A NOTATION FOR USE IN THE DISCUSSION OF STAR COLORS By Frederick H. Scares MOUNT WILSON SOLAR OBSERVATORY. CARNEGIE INSTITUTION OF WASHINGTON Presented to the Academy, August 4. 1915 The extension of absolute scales of photographic and photovisual magnitudes to the fainter stars provides a method of determining the colors of objects at present beyond the reach of spectroscopic investigation. For the statistical discussion of such color results it is convenient to adopt, tentatively at least, in place of the conventional color index, a notation similar to that used for spectral classification. Where for the latter we employ the letters B, A, F, G, etc., to represent various spectral types, we choose b, a, f, g, etc., to indicate different classes of color. When a more exact specification is required, a decimal subdivision of the classes may be employed. Spectral type is determined by the number, character, and distribution of the lines in the spectrum. Color class depends upon the relative intensity of the continuous spectrum background in two different regions whose location varies somewhat with the instrumental equipment and the method of observing. Since both spectral type and color class stand in an intimate relation to the temperature, they are necessarily connected with each other, and when properly defined the color class indicates at once the general character of the spectrum. To secure this advantage, the classes b, a, f, etc., are assumed to represent the colors corresponding to the mean for typical spectra of the classes B, A, F, etc., respectively. We cannot assume that there is an invariable correspondence between b and B, a and A,f and F, etc., for the observed color C, which specifically is represented by some one of the letters b, a, f, etc., may consist of at least three elements, namely, C = Cs + CMS + , (1)