The gravitational redshift of solar-type stars from Gaia DR3 wide binaries

Abstract

Light escaping from a gravitational potential suffers a redshift with magnitude proportional to the depth of the potential. This "gravitational redshift" is easily measurable in dense stars such as white dwarfs, but is much weaker and has evaded unambiguous detection in main-sequence stars. I show that the effect is directly measurable in the Gaia DR3 radial velocities (RVs) of the components of wide binary stars. In a sample of $\sim$500 wide binaries containing a solar-type main-sequence star and a red giant or red clump companion, the apparent RV of the giant is on average $0.49 \pm 0.02 \,\, \rm km\,s^{-1}$ lower than that of the main-sequence star. This owes primarily to the giants' weaker gravitational fields and is in reasonably good agreement with the value expected from general relativity.