Extended Main-Sequence of Some Stellar Clusters

The hypothesis of this paper is that considerable mass-transfer can occur between the components of close binary stars, a possibility that has been widely discussed in the literature. The purpose here is to study the consequences for cluster HR diagrams. In a cluster that contains close binaries and that has evolved far enough to produce red giants, we expect cases to occur of mass-transfer from an incipient giant to a secondary companion. The latter thus becomes a star on or near a continuation of the main-sequence beyond its normal turn-off point for the cluster. The extent of this continuation is predicted to be about 2.5 magnitudes. At any one epoch, the number of stars constituting this continuation would be relatively small; the theory relates this number to the number of stars in a certain interval of the giant branch of the cluster diagram. Such predictions are in good qualitative accord with well known features particularly of the clusters M3 and M67. Under certain conditions, reversal of the mass-transfer may subsequently occur. In that case, or in some cases of the original mass-transfer, the material involved would be turned inside-out; the result would be a star whose outer part is poor in hydrogen and rich in helium. There are known cases of such stars. Further attention to these phenomena might provide novel ways for observational study of stellar evolution.