. The Adolfo Stahl lectures in astronomy, delivered in San Francisco, California, in 1916-17 and 1917-18, under the auspices of the Astronomical Society of the Pacific. brighter stars. In the meantime, spectral types have beendetermined for large numbers of stars, but these are necessa-rily restricted to the brighter objects. The colors found fromspectra therefore relate to stars which are comparatively near,for in general, the brighter stars are much less distant than thefainter objects. Counts of about nine thousand stars from the Revised Har-vard Photometry give for the number of stars brig
. The Adolfo Stahl lectures in astronomy, delivered in San Francisco, California, in 1916-17 and 1917-18, under the auspices of the Astronomical Society of the Pacific. brighter stars. In the meantime, spectral types have beendetermined for large numbers of stars, but these are necessa-rily restricted to the brighter objects. The colors found fromspectra therefore relate to stars which are comparatively near,for in general, the brighter stars are much less distant than thefainter objects. Counts of about nine thousand stars from the Revised Har-vard Photometry give for the number of stars brighter than aspecified magnitude the totals shown in Table V®, which issimilar to Table I, but includes no stars fainter than magni-tude We note first that the ratios for adjacent totals given inthe right-hand part of the last column, for all the starstogether, agree sensibly with those of Table I. This in factmust be the case, for the six principal spectral classes hereconsidered include the great majority of all the stars. Theratios for the G, K, M stars are nearly the same as those for Derived from Harvard Annals, 64, 138, 140. r*r^ ? V .;;,*** •, I i. PLATE XLMII. Kaptevx Selected Area No. 20h 46 «; 6 + 45° (1910). The central star opposite the arrow points (at the right and at the bottom)is magnitude. The Brightness of the Stars 225 all classes together; but for the blue and white stars we finda very interesting result. The numbers for the B stars increasevery slowly, while for the A, F stars the totals accumulatewith unusual rapidity. The star-ratios show clearly thephenomenon in question ; those for the B stars decrease rapidly,and apparently would become equal to unity near ; below this limit the totals would be constant, and weshould conclude that there are no B stars fainter than aboutmagnitude These objects therefore must thin out veryrapidly with increasing distance. TABLE \ Numbers ok Stars Brighter than a Given Magnitude a
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