  Modern instrumentation makes it possible to measure the mass to radius ratio
for main sequence stars in open clusters from gravitational redshifts. For
stars where independent information is available for either the mass or the
radius, this application of general relativity directly determines the other
quantity. Applicable examples are: 1) measuring the radii of solar metallicity
main sequence stars for which the mass - luminosity relation is well known, 2)
measuring the radii for stars where model atmospheres can be used to determine
the surface gravity (the mass to radius squared ratio), 3) refining the mass -
radius relation for main sequence stars, and 4) measuring the change in radius
as stars evolve off the main sequence and up the giant branch.
