The Color-Magnitude Diagram for the Galactic NGC 7789.

Colors and magnitudes for nearly seven hundred stars are given for the cluster NGC 7789 The color- magnitude diagram and the absence of large numbers of faint stars show this to be a galactic cluster Photoelectric calibrations in three colors for forty-three stars were obtained with the 60-inch telescope Photographic plates taken with the 100-inch telescope were used for the photometry The cluster has a strong yellow giant branch which slopes blueward toward faint magnitudes until the Herxsprung gap is reached at (B-V)0 = 092. The gap is A(B- V) = 045 mag wide The main sequence is reached at V 14 and has the characteristic steep slope due to evolution Blue stars exist brighter than the main-sequence break point, as in M67 and M3 This feature is unexplained by evolutionary theory The apparent distance modulus is m - M = 12 20 + 02, the reddening is E(B- V) = 028, the true modulus is 11 36 + 0 2, and the distance is 1870 + 170 parsecs The yellow giant stars reach (B - V)0 = 1 62 This color would correspond to M2 III stars if our photometry is correct and if color is a good indicator of spectral type in this range. However, spectra obtained with the 60-inch telescope indicate a type K4 III for the stars at the top of the giant branch NGC 7789 fits into the systematic scheme of C-M diagrams for other galactic clusters in the expected way It is older than Praesepe or Hyades but younger than M67 The giant sequence of NGC 7789 crosses the giant sequence of Ml 1 This again shows the non-unique massluminosity relation for luminosity class III giants