The Hubble Constant from Observations of the Brightest Red Giant Stars in a Virgo-Cluster Galaxy

The nearest large groups of elliptical galaxies, in the Virgo and Fornax clusters, play central roles in determining the Hubble constant H 0 and thus the cosmological rate of expansion. the between these two clusters and more remote clusters are well known, robust distance determinations to Virgo and Fornax will establish the Hubble constant for the local universe. In addition, elliptical galaxies reside predominantly in the cores of clusters, so that distance calibrations for ellipticals will minimize the uncertainties due to the possible large extent of clusters along the line of sight. A powerful and direct way of establishing such distances is to use the brightest red-giant stars, which have nearly uniform luminosities 1 , 2 . Here we report the direct observation of the old red giant stars in a dwarf elliptical galaxy in the Virgo cluster. We determine a distance to this galaxy, and thus to the core of the Virgo cluster, of ∼ 15 . 7 ± 1 . 5 Megaparsecs, from which we estimate a Hubble constant of H 0 = 77 ± 8 km s − 1 Mpc − 1 . Under the assumption of a low-density Universe with the simplest cosmology, the age of the Universe is no more than 12-13 billion years.