HST has so far provided Cepheid distances to nine galaxies .
Although not sufficient yet to determine the distance of the extended Virgo cluster , they are decisive for the distance scale in two ways .
( 1 ) Seven of the galaxies contribute to a much improved calibration of the Tully-Fisher relation .
Applying this to a complete sample of Virgo spirals one obtains a cluster distance of ( m - M ) = 31.79 \pm 0.09 .
Other distance indicators support this value .
The adopted linear distance of 22.0 \pm 0.8 Mpc combined with the cluster velocity of 1178 \pm 32 ~ { } { km s ^ { -1 } } ( in the CMB frame ) gives H _ { 0 } = 54 \pm 2 ( internal error ) .
( 2 ) Six of the galaxies have been the site of seven SNe Ia with well observed maxima .
Their resulting calibration in absolute magnitudes gives M _ { B } ( { max } ) = -19.53 \pm 0.07 and M _ { V } ( { max } ) = -19.49 \pm 0.07 with negligible intrinsic scatter .
If this calibration is used to determine the distances of all distant SNe Ia with known maxima and with 1100 < v < 30 000 { km s ^ { -1 } } , H _ { 0 } becomes 56 \pm 3 ( internal error ) .
Systematic errors tend to make this an upper limit ; in particular the case H _ { 0 } \geq 70 can be excluded .
– The conclusion is that the large-scale value of the Hubble constant is H _ { 0 } = 55 \pm 10 ( external error ) .