THE HUBBLE PARAMETER

Distance determinations to individual galaxies, groups, and clusters are critically reviewed. All modem distance determinations have been reduced to a common and internally consistent system. These distances are then used to discuss the present status of the extragalactic distance scale and of the Hubble Parameter, H0. It is shown that the classical distance-scale "problem" starts quite abruptly at D greater than or similar to 3 Mpc. It is emphasized that the reasons for this discrepancy are entirely independent of difficulties associated with large-scale deviations from a smooth Hubble flow. From eight different techniques the mean distance modulus of the Virgo cluster is found to be (m - M)0 = 31.0 +/- 0.15 (D = 15.8 +/- 1.1 Mpc). Twelve determinations of the difference between the distance moduli of the Coma and Virgo clusters yield DELTA(m - M)0 = 3.71 +/- 0.05 mag and hence a Coma distance modulus (m - M)0 = 34.71 +/- 0.16 (D = 87 +/- 6 Mpc). Combining this distance with the Coma cluster's cosmological velocity of 7210 km s-1 yields a value H0(local) = 83 +/- 6 km s-1 Mpc-1. From the velocity-distance relation for the brightest galaxies in clusters of Bautz-Morgan types I and I-II it is found that H0(global)/H0(local) = 0.92 +/- 0.08, in which "local" refers to northern clusters with V0 < 10,000 km s-1. In other words, the cluster data do not show a statistically significant difference between the local and global values of the Hubble parameter. If one nevertheless adopts H0(global)/H0(local) = 0.92 +/- 0.08 then one obtains H0(global) = 76 +/- 9 km s-1 Mpc-1. This observed value of H0 differs at the approximately 3sigma level from that predicted by theories of stellar evolution in conjunction with canonical models of the Universe with OMEGA = 1 and LAMBDA = 0.