Photometric Data for the Old Galactic Cluster NGC 188.

Abstract

Photoelectric and photographic data for about 500 stars in and near the galactic cluster NGC 188 are reported. The color-magnitude diagram suggests that NGC 188 is the oldest galactic cluster isolated so far The main-sequence termination point is about 1 mag. fainter than that of M67. Confirmatory spectroscopic evidence was obtained which shows that the stars in NGC 188 near the main-sequence turning point are 0 3 of a spectral class later than similar stars in M67. The color-magnitude diagram for NGC 188 lies within 0.2 mag. of the lower envelope of the evolved part of the diagram for field stars near the sun Despite their great age, these old stars and the stars in NGC 188 appear to have nearly the same metal abundance as do the much younger Hyades stars. If this is so, then the enrichment of the heavy elements in the interstellar medium near the sun has been negligible in the disk of our Galaxy for most of the life of the disk. Analysis of the random photometric errors is given in Appendix A Comparison of the present data with the photometric results of Barkhatova is given in Appendix B.