+1.50 5.3 1.8 8.89 2.8 1.2 8,600 +2.20 1.9 1.9 2.31 7,000 +2.5 1.6 1.8 5.48 }680 +2.9 1.2 1.4 3.37 6,080 +3.5 1.3 1.0 5.98 +4.2 4.28 }7600 5,460 +4.5 -1.5 4.78 4,820 +4.8 +0.6 8.98 4,240 +6.20 +1.05 19.65 }160 (giant) 6.85 3,600 +7.20 +0.50 4.80 }22 (giant) 3,380 + 10.20 +0.40 2.10 Excitation and ionization conditions differ so widely for different atoms that it would be expected that two factors, one of which encourages ionization, while the other discourages recombination, would not in every case balance exactly, even when their mean effect was constant, as it is for any one Draper class. The Henry Draper Catalogue, as we have emphasized, was made on the basis of the general resemblance of the spectra, an arrangement which corresponds to the greatest physical homogeneity that can be obtained. As regards features of their spectra, it is therefore to be expected that the members of any one class will correspond closely, and care [Pg 198] must be exercised in eliminating redundancies from discussions of the homogeneity of the individual classes. There are, however, other types of discussion, independent of spectroscopic data, and such investigations have shown that the Draper classes have indeed a significance far beyond the mere formation of homogeneous groups of spectra. In illustration of the profound statistical significance of the classification, the table on page 197 of the present chapter contains a brief synopsis of some of the most salient features that have been correlated with spectral class. Successive columns contain the class, the effective temperature,[513] the mean absolute magnitude,[514] the galactic concentration,[515] the percentage of the class in the Draper catalogue,[516] and the computed number per million cubic parsecs.[517] FOOTNOTES: [497] H. A., 91-99. [498] Rep. I. A. U., Rome, 1922. [499] H. A., 28, 131, 1901. [500] Observatory, 38, 381, 1915. [501] Mt. W. Contr. 199, 1918. [502] Harper and Young, J. R. A. S. Can., 18, 9, 1924. [503] Observatory, 38, 381, 1915. [504] Chapter XIII, p. 178. [505] Observatory, 38, 381, 1915. [506] Chapter III, p. 36. [507] Chapter III, p. 35. [508] See above, p. 193. [509] P. 141. [510] M. N. R. A. S., 83, 403, 1923. [511] Chapter II, p. 31. [512] Adams and Joy; Mt. W. Contr. 142, 1917. [513] A. N., 219, 361, 1923. [514] Lundmark, Pub. A. S. P., 34, 147, 1922. [515] Shapley, H. B. 796, 1924. [516] Shapley and Cannon, Proc. Am. Ac. Sci., 59, 217, 1924. [517] Shapley and Cannon, ibid., 59, 230,1924. [Pg 199] CHAPTER XV ON THE FUTURE OF THE PROBLEM