  We report on the measurement of the mass and radius of the neutron star in
the low-mass X-ray binary 4U 1820-30. The analysis of the spectroscopic data on
multiple thermonuclear bursts yields well-constrained values for the apparent
emitting area and the Eddington flux, both of which depend in a distinct way on
the mass and radius of the neutron star. The distance to the source is that of
the globular cluster NGC 6624, where the source resides. Combining these
measurements, we uniquely determine the probability density over the stellar
mass and radius. We find the mass to be M = 1.58 +/- 0.06 M_sun and the radius
to be R = 9.11 +/- 0.40 km.
