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A decade of observing with HST also coincides with the completion of the last of the initial three Key Projects for HST, the measurement of the Hubble constant, H0. Here we present the final results of the Hubble Space Telescope (HST) Key Project to measure the Hubble constant, summarizing our method, the results and the uncertainties. The Key Project results are based on a Cepheid calibration of several secondary distance methods applied over the range of about 60 to 400 Mpc. Based on the Key Project Cepheid calibration and its application to five secondary methods (type Ia supernovae, the Tully-Fisher relation, surface brightness fluctuations, type II supernovae, and the fundamental plane for elliptical galaxies), a combined value of H0 = 72 ± 8 km/sec/Mpc is obtained. An age conflict is avoided for current estimates of globular clusters and H0 if we live in A-dominated (or other form of dark energy) universe.
When planning HST, pinning down H0 was one of the scientific programs that drove the design and construction of the telescope. Although the original plans for a Large Space Telescope were scaled down during the mid-1970s, one of the primary arguments for an aperture of at least 2.4m was to enable the detection of Cepheid variables in the Virgo cluster (Smith 1989), a goal that was achieved within months of the corrective optics being installed in HST in December, 1993.