The Spectra of the Bright Stars of Types F5-K5.

Spectral types and luminosity classes have been obtained for 641 stars in the spectral range ES-KS brighter than 5.50 visual magnitude and north of declination - 20 . These types show the well-known absence of subgiants later than K1 and, in addition, show that the stars near zero absolute magnitude become plentiful eztremely abruptly between G5 and G8. In the range GOGS we have 4 giants; at G8 we have 67. Color-magnitude diagrams show that this change occurs in a very small range of temperature. New calibrations of the absolute magnitudes are made for all spectral types and luminosity classes which contain enough stars to yield significant results. For the most part these agree well with older determinations. A surprising result is that the K0 III-K2 III stars are about 0.8 mag. fainter, on the average, than the giants at G8 and in the classes K3-K5. That this is not merely a systematic effect in the classification is shown by the fact that the mean absolute magnitudes for all stars of luminosity classes II-IV show the same phenomenon. Space velocities corrected for the solar motion have been computed for each star fainter than luminosity class II. In the spectral range G8-K1 it is found that spectroscopic differences exist among stars of the same spectral type and luminosity class. If these stars are separated spectroscopically, it is found that the velocity distributions within the different groups are also different. The two groups which contain 85 per cent of the stars are about equally numerous. They are separated by the facf that the G band and X 4226 of Ca I are stronger relative to the remainder of the spectrum in one group than they are in the other. The velocity distributions indicate that the group with the stronger G band and X 4226 is dynamically related to the group of weak-line stars in the earlier types and that the other group is dynamically related to the strong-line stars. In addition, two other groups exist which are characterized by peculiarities in the cyanogen absorption. One of these has fairly strong cyanogen which is characterized by a peculiar appearance of the region near X 4150 and by a strengthening of the absorption in this region compared to that near X 4120. Dynamically, these stars cannot be distingnished from the weak-line group. The fourth group is characterized by unusually weak cyanogen absorption. As has been known for some time, these stars are the "high-velocity" stars. Mthough some members of this group have fairly low velocities and some members of the weak-line group have fairly high ones, the mean velocities of the stars with weak CN is 95 km/sec, as compared to a mean of 41 km/sec for the weak-line group.