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Published online by Cambridge University Press: 07 August 2017
It is generally agreed that the Hubble parameter lies in the range 50 < Ho (km s−1 Mpc−1) < 100. Recent observations, which are discussed in the present paper, favor Ho ≳ 75 km s−1 Mpc−1. Hubble Space Telescope observations of Cepheids in Virgo cluster galaxies will probably reduce the uncertainty in Ho to ∼ 20%. It should be possible to lower this remaining uncertainty in the Hubble parameter by strengthening the calibration of the Cepheid period-luminosity relation, the MV(RR) versus [Fe/H] relation of RR Lyrae stars, and the luminosity calibration of the subdwarf main sequence, by using parallaxes obtained with Hipparcos.
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