Abstract
We present systematic calculations of the mass-radius (MR) relation of a neutron star (NS) using a large variety of modern equations of state (EOS) of dense matter. The role of strangeness on the EOS and on the MR relation is particularly emphasized. Theoretical results are then compared with the semiempirical MR relation recently extracted for the x-ray burst source 4U 1820-30. Based on this comparison, we propose that the compact object in 4U 1820-30 is a kaon-condensed nucleon star or a strange star.