We present here the final results of the Hubble Space Telescope ( HST ) Key Project to measure the Hubble constant .
We summarize our method , the results and the uncertainties , tabulate our revised distances , and give the implications of these results for cosmology .
Our results are based on a Cepheid calibration of several secondary distance methods applied over the range of about 60 to 400 Mpc .
The analysis presented here benefits from a number of recent improvements and refinements , including ( 1 ) a larger LMC Cepheid sample to define the fiducial period–luminosity ( PL ) relations , ( 2 ) a more recent HST Wide Field and Planetary Camera 2 ( WFPC2 ) photometric calibration , ( 3 ) a correction for Cepheid metallicity , and ( 4 ) a correction for incompleteness bias in the observed Cepheid PL samples .
We adopt a distance modulus to the LMC ( relative to which the more distant galaxies are measured ) of \mu _ { 0 } ( LMC ) = 18.50 \pm 0.10 mag , or 50 kpc .
New , revised distances are given for the 18 spiral galaxies for which Cepheids have been discovered as part of the Key Project , as well as for 13 additional galaxies with published Cepheid data .
The new calibration results in a Cepheid distance to NGC 4258 in better agreement with the maser distance to this galaxy .
Based on these revised Cepheid distances , we find values ( in km/sec/Mpc ) of H _ { 0 } = 71 \pm 2 _ { r } ( random ) \pm 6 _ { s } ( systematic ) ( type Ia supernovae ) , H _ { 0 } = 71 \pm 3 _ { r } \pm 7 _ { s } ( Tully–Fisher relation ) , H _ { 0 } = 70 \pm 5 _ { r } \pm 6 _ { s } ( surface brightness fluctuations ) , H _ { 0 } = 72 \pm 9 _ { r } \pm 7 _ { s } ( type II supernovae ) , and 82 \pm 6 _ { r } \pm 9 _ { s } ( fundamental plane ) .
We combine these results for the different methods with 3 different weighting schemes , and find good agreement and consistency with H _ { 0 } = 72 \pm 8 km s ^ { -1 } Mpc ^ { -1 } .
Finally , we compare these results with other , global methods for measuring H _ { 0 } .