Distance to Virgo Kicks Off HST Assault on the Hubble Constant

One of the “Key Projects” intended for the Hubble Space Telescope from the start was to pin down the distance to the Virgo cluster by measuring Cepheid variable stars in that massive assemblage of galaxies some 50‐million light‐years away. Cepheids are the primary yardsticks for astronomical distances beyond the reach of parallax measurement. The inability to determine cosmologically relevant distances with confidence has been the principal source of confusion in our knowledge of $H_o,$ the Hubble constant. The HST’s measurement of Cepheids in the Virgo cluster was to be the opening salvo in an assault that would soon pin down this fundamental parameter of cosmology to within 10%, once and for all. Estimates of the Hubble constant in recent years have differed by as much as a factor of two, and the stakes are high. The reciprocal of $H_o$ is, after all, the first approximation to the age of the universe.