NGC 4603 is 108 million light years away, in the cluster of galaxies of Centaurus, one of the most massive. It is the farthest galaxy in which the periodic variations of brightness of cepheid stars have been studied. Cepheids of greater size and brightness have longer periods than small ones. This relationship between period and mass allows you to accurately calculate its distance.
Although they have a powerful shine, the Cepheids are faint and difficult to find at great distances (the stars in the image above, with their "pointed" brightness, are background objects). Thanks to the sharp vision of the Hubble Space Telescope, more than 36 cepheid signals have been identified in NGC 4603, from now on the most distant galaxy in which these stars have been identified. In fact, using this telescope to select cepheids in galaxies closer than NGC 4603, the Hubble Fundamental Program Team has recently announced the end of 8 years of efforts to accurately measure galactic distances and the average speed of expansion of the universe, Hubble's constant.
Comparing galactic distances and recession rates, the team reports that the Hubble constant is equal to 70 kilometers per second per megaparsec, with an uncertainty of 10 percent. This means that a galaxy increases its apparent recession rate by 257,000 km / h for every 3.3 million light-years of travel. When it was launched in 1990, one of the main goals of the Hubble Space Telescope was the exact measurement of the Hubble constant.
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