The Hertzsprung-Russell Diagram for F5-K2 Stars with the Most Accurate Absolute Magnitudes.

A Hertzsprung-Russell diagram has been constructed for 185 F5 -K2 field stars which have absolute magnitudes with probable errors of 0.2 or less. Many of these stars appear to have evolved from the main sequence, like the stars in the Hyades cluster, NGC 752, or M67. There are a group of 16 stars which appear to be older than the stars in M67. The main sequence below 1Iv = t4 has a breadth of over 1 mag. Part of this spread may be due to the effects of chemical composition on the star's structure. The remainder may be explained in at least three possible ways: (1) Most of the stars above the xero-age main sequence may be unresolved binaries. This is shown to be unlikely. (2) Stars above the xero-age main sequence may already have evolved off the main sequence, implying an age of more tban 10 X 10 years. (3) The B - V color may not be a true indication of the star's effective temperature.