The Hubble Constant and HST

摘要

The Hubble diagram of distant type la supernovae, zero-pointed by nearby SNe la with Cepheid distances from HST observations yields H_0 = 62.3. The basis of this value has been weakened because the period-luminosity relation of Cepheids is not universal, but depends on their metallicity and possibly other parameters. An independent (Pop. II) distance scale is needed, as provided by RR Lyrae stars and the tip of the red-giant branch, to buttress the value of H_0. Present results show good agreement.