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Hubble Space Telescope image of spiral galaxy NGC 4603. It is the most distant galaxy with Cepheids used to measure the Hubble Constant.
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NASA

Scientists Find the True Hubble Constant!
News story originally written on May 26, 1999

After eight long years, the Hubble Space Telescope Key Project Team finally revealed what they believe is the real Hubble Constant. The Hubble Constant is the speed which the Universe is expanding at. For over 70 years, scientists have argued over the exact number.

Astronomers are able to use the constant to "work backwards", in order to find the age of the Universe. Now that the Hubble team has found Hubble's Constant to be 70 kilometers per second per megaparsec, they have determined the age of the Universe to be 12 billion years old. At one time, scientists differed on the value of the constant by a factor of 2. Now, the constant has been measured with an error of only 10 percent.

The Hubble team used a certain type of star called a Cepheid variable star. A Cepheid is a special pulsating star that can be used to make precise distance measurements. Over 800 Cepheids were found during the 10 year project.

Finding the Hubble Constant was one of three original goals set by astronomers when the telescope was first launched in 1990. The telescope is named after Edwin Hubble, the first to believe the Universe is expanding.

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Windows to the Universe, a project of the National Earth Science Teachers Association, is sponsored in part is sponsored in part through grants from federal agencies (NASA and NOAA), and partnerships with affiliated organizations, including the American Geophysical Union, the Howard Hughes Medical Institute, the Earth System Information Partnership, the American Meteorological Society, the National Center for Science Education, and TERC. The American Geophysical Union and the American Geosciences Institute are Windows to the Universe Founding Partners. NESTA welcomes new Institutional Affiliates in support of our ongoing programs, as well as collaborations on new projects. Contact NESTA for more information. NASA ESIP NCSE HHMI AGU AGI AMS NOAA