IS HER-X-1 A STRANGE STAR

Though the existence of strange stars have been conjectured, it is ver y difficult to distinguish them from neutron stars, since both strange stars and neutron stars share similarities in many observable phenome na. But the mass - radius (M - R) relations of strange stars are diffe rent from those of neutron stars. The M - R relation of an X-ray pulsa r can be determined semi-empirically from the observations of its spin variation and cyclotron spectral line. Here we calculate the semi-emp irical M - R relation of the X-ray pulsar Her X-1. Comparing the resul t with the theoretical interior models of a neutron star and of a stra nge star respectively, we find that a strange star model is more consi stent with Her X-1 and suggest it is a strange star.