Measuring the Hubble Constant with the Hubble Space Telescope

The spectacular success of the Hubble Space Telescope (HST) repair mission now makes it possible to realize one of the astrophysical goals for which the HST was designed. Despite recent progress in ground-based measurements of the distance scale, the value of the Hubble constant remains uncertain at the +/- 20%-30% level. The incompatibility of high values of H_0_ with the stellar age scale in an Einstein-de Sitter universe underscores the need for a definitive calibration of the distance scale. This paper briefly reviews the current status of the distance scale calibration, and describes ongoing programs to measure H_0_ with HST, with special emphasis on the Extragalactic Distance Scale Key Project. The objective of the Key Project is to combine HST observations of Cepheid variables in ~25 galaxies with ground-based observations of five secondary distance indicators to measure H_0_ to an accuracy of +/- 10%. The sample selection, observing strategy, data analysis methods, and error budget are designed to minimize susceptibility to the kinds of systematic errors which have often plagued distance scale determinations. The successful measurement of Cepheids in the Virgo cluster spiral M 100 demonstrates the feasibility of this program, and already constrains H_0_ to the range 80+/-17 km s^-1^ Mpc^- 1^. The ultimate goal of measuring H_0_ to +/- 10% awaits successful completion of the Key Project over the next three years.