.318S SIX-COLOR PHOTOMETRY OF STARS .102. III. THE COLORS OF 238 STARS OF DIFFERENT SPECTRAL TYPES* Joel Stebbins1 and A. E. Whiteord2 1945ApJ. Mount Wilson Observatory and Washburn Observatory Received June 8,1945 ABSTRACT Colors have been obtained for 238 stars of all spectral types from O to M by measuring intensities i six spectral regions from X 3530 to X 10,300 A (Tables 2 and 3). The early-type stars from O to B3 sho small dispersion in intrinsic color, but many are strongly affected by space reddening. A dozen late-tyx giants in low latitudes are likewise affected. The most marked effect of absolute magnitude is near spe< trum K0, where the colors of dwarfs, ordinary giants, and supergiants are all different {Fig. 1). The observed colors of the stars agree closely with the colors of a black body at suitable temperatur« (Fig. 2). The derived relative color temperatures are based upon the mean of ten stars of spectrum dG with an assumed temperature of 5500°K. On this scale the values are 23,000° for O stars, 11,000° for A( and 5950° for dGO. An alternative scale, with 6700° and spectrum dG2 for the sun, gives 140,000° fc O stars, 16,000° for A0, and 6900° for dGO (Table 7). A definitive zero point for the temperature seal has not been determined. The bluest O and B stars agree very well with each other, but there is still the possibility that all ai slightly affected by space reddening. A dozen bright stars of the Pleiades seem normal for their type. Tl colors of P Cygni are anomalous, as is the space reddening of the Trapezium cluster of Orion (Fig: 3 and 4). Comparisons of the new colors with the International colors and our previous photoelectric colors C of the North Polar Sequence give the ratios of the scales of color index. These ratios depend upon whethí change of color is caused by change of spectral type or by change of space reddening (Table 11). Goo agreement is found with the Greenwich gradients and especially with the results of the spectrophotometr by John S. Hall (Fig. 5). I. THE OBSERVATIONS . The various kinds of stellar photometry may be classified as one-color, two-coloi multicolor, and spectrophotometry, depending upon the number of spectral regions s€ lected for measurement. Except in spectrophotometry the spectral regions are detei mined by the characteristics of the receiver, such as the eye, the photographic plate, th photoelectric cell, the thermocouple, etc., or by a combination of such a receiver wit filters suitable for the desired result. Even with a black-body receiver like the thermc couple, equally receptive to all wave lengths, the measured responses to radiation ar dependent upon the transmission of the atmosphere and of the optical system involvec When a dispersing agent like a prism or grating is used, it is possible to isolate a sma range of wave lengths; and since the smaller the spectral range the less will be the tots energy available, it follows that anything approaching monochromatic photometry c stars will need a large telescope. In fact, stellar spectroscopy with high dispersion ma be looked upon as the natural application of stellar photometry to narrow regions c wave length. The present six-color photometry of stars lies between one- and two-color photometr} on the one hand, and spectrophotometry, on the other. Obviously, measures in six spec tral regions should give more information than measures in only two, while they shoul be easier to obtain than measures for the entire spectrum. The photocell gives a linea scale of response over a wide range of intensities and of wave lengths; also, freedom fror many difficulties of photographic photometry. But, despite these advantages, there r< * Contributions from the Mount Wilson Observatory, Carnegie Institution of Washington, No. 712. 1 Research Associate of the Mount Wilson Observatory, Carnegie Institution of Washington. 2 On leave at Massachusetts Institute of Technology. 318 © American Astronomical Society • Provided by the NASA Astrophysics Data System SIX-COLOR PHOTOMETRY OF STARS 319 Lain the limitations of any method where spectral regions extending over a thousand or tore angstroms are integrated by the receiver into a single intensity. The effects of :rong absorption lines or bands are buried in the results. In this respect the present rork has the same drawback as ordinary photographic photometry; but even in spectro- hotometry, where measures are limited to the continuous spectrum between spectral nes, there is always the possibility of a veil over the surface of a star which will modify he ideal black-body radiation, even if such were emitted in the first place. It remains to •e seen what is the best method for deriving the color temperature of a star; up to the ♦resent there is much disagreement among the results from all methods. Our photometer with a photocell and six filters was devised primarily for measures of ixtragalactic nebulae; but for the observations to be significant, such nebulae must be eferred to standard stars of different spectral types. In the course of the work, so much TABLE 1 Observations of a Ursae Minoris, August 18,1944 Color U B X(A)..,. 3530 4220 4880 5700 7190 10,300 1A (m-1) 2.83 2.37 2.05 1.75 1.39 0.97 0^530 0^292 0^180 0^136 0^58 0^30 Deflection, d 27.8 50.1 74.4 65.8 58.4 60.8 log¿ 1.444 1.700 1.872 1.818 1.766 1.784 0.4a sec z 0.382 0.210 0.130 0.098 0.042 0.022 log do 1.826 1.910 2.002 1.916 1.808 1.806 log (B, G, R)-log d0.. ■ 0.083 0.001 0.093 0.007 + 0.101 -f- 0.103 A mag 0.21 0.00 0.23 0.02 + 0.25 + 0.26 Reduction to standard. .06 .14 + .17 .01 - .16 - .07 Final color ;. .15 .14 .06 .03 + .09 + .19 Another set .18 .14 .06 .00 + .06 + .17 Mean. + 0^165 - 0m14 — 0^6 - 0^15 + 0^75 -b 0“18 Av. deviation. ± 0.015 ± 0.00 ± 0.00 ± 0.015 ± 0.015 ± 0.010 of interest was found in the measures of various stars that this secondary program of calibration has perhaps grown to outweigh in emphasis the original program on the nebu- lae. We found that measures of normal and reddened B-type stars3 gave a new determi- nation of the law of space reddening by interstellar material, and other possibilities were opened up by the wide spectral range from 3530 A to 10,300 A. The observations used in the present paper were made at Mount Wilson during the five summers of 1940-1944. Some measures were made with the 100-inch reflector, but most of them with the 60-inch, which, besides having the advantage of reaching the pole, is, in general, more readily manipulated than the larger telescope. A description of the installation and the methods of observation and reduction is given in our paper on B stars,3 but a sample reduction is given here. In Table 1, giving data for a Ursae Minoris, the designations of the different quanti- ties under each color are mostly self-explanatory. The units used in Table 1 and through- out this paper are: angstroms for X, microns "1 for 1/X, and stellar magnitude for col- ors. In the next line below 1/X in Table 1, is given a, the extinction in magnitude at the zenith. The deflection d, in millimeters, is the mean of two deflections, taken forward 3 ML W. Contr., No. 680; Ap. /., 98, 20, 1943. © American Astronomical Society • Provided by the NASA Astrophysics Data System .318S 320 JOEL STEBBINS AND A. E. WHITFORD .102. and backward through the colors. For this bright star a 3-mag. absorbing screen ove the large mirror was used; also, a shunt on the galvanometer giving one-third the ful 1945ApJ. sensitivity. To log d is added 0.4 a sec z to obtain log do for outside the atmosphere The log do is then subtracted from 1.909, the corresponding mean of blue, green, and red to obtain A log; then A mag. = 2.5A log. The reduction to standard color, applied to thi A mag., gives the final color referred to the mean of ten stars of average spectrum dG6 The results of a second set of observations, made the same night, are in the next line, anc then the mean. The average deviation in all the colors from the mean of two sets i ± 0.009 mag., which is not particularly good for successive measures on the same night The use of a mean extinction coefficient for each color is justified by the accordance o the results for the same star at different zenith distances on different nights and in differ ent seasons. A control on the extinction is given by measures of a polar star, usualb NFS 4; this control also takes care of any variation in the color sensitivity of the photo cell. Usually, in a run of several good nights the colors of the control star remain nearb enough the same, but occasionally an increased extinction will appear in the ultraviolet- say several hundredths, or even a tenth, of a magnitude. But there are also variations h the infrared, caused presumably by the strong water-vapor bands p and 4> on either sid< of the effective wave length at 10,300 A.