The galactic cluster Collinder 121

Abstract

Photoelectric data have been obtained for 21 stars of the cluster Cr 121, which lie around the red supergiant o' CMa. Color-magnitude arrays and color-color arrays were made. They both show that there are 11 stars on the main sequence. The brightest of them is a Wolf-Rayet star, HD 50896, of type WN5. Its absolute magnitude derived from the C-M diagram is Mᵥ= -2.0. According to some authors this star displays variation in the radial velocity of some of the emission lines, which would suggest some binary motion, but this was not confirmed by our photometry. The red supergiant o' CMa of spectral type K3 Iab belongs to the cluster, and its absolute magnitude is Mᵥ= - 5.0, in very good agreement with the value of Mᵥ= -5.2 obtained by Wilson and Bappu (1957) from the K emission line. The cluster is younger than the Pleiades and one could suggest that perhaps the Wolf - Rayet is in some stage of rapid evolution. The complete results will be published somewhere else.