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Several groups of astronomers conducted observational projects to determine Hubble's constant, the most important cosmological parameter, during the late 1990s. Notably, the Hubble Key Project, carried out by American astronomers Wendy Freedman, Robert Kennicutt, and Barry Madore, used the Hubble Space Telescope to observe Cepheid variable stars in distant galaxies, following the Leavitt-Shapley method.

The Hubble Space Telescope can distinguish and follow such stars in galaxies much farther away from Earth than ground-based telescopes can. Their final value was a Hubble constant of 72 kilometers per second per megaparsec (45 miles per second per megaparsec). A parsec is about 3.26 light-years (a light-year is the distance that light could travel in a year—9.5 × 1012 km, or 5.9 × 1012 mi). The units of the Hubble constant mean that for each million parsecs (megaparsec) of distance between two objects, the space between them expands by 72 kilometers every second. Their result was accurate to within about 10 percent. It corresponds to an age of the universe of 12 billion to 14 billion years, depending on the rate of deceleration.


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