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The newspaper of The Johns Hopkins University August 4, 2003 | Vol. 32 No. 41
70,000,000,000,000,000,000,000 Stars in the Sky

By Michael Purdy

In 2002, astronomers from Johns Hopkins University made headlines around the world with the announcement that they'd determined the color of the universe. The observation was a by-product of efforts to pursue more serious scientific goals, but it captured the fancy of non-astronomers worldwide.

Last month, a Hopkins astronomer and four of his colleagues presented another scientific study with a tangential finding that once again proved to be the prolific newsmaker. In looking at the distribution and luminosity of galaxies, the group was also able to put together an estimate of the total number of stars in the visible universe: about 70 sextillion, or a seven followed by 22 zeros.

"Actually, a lot of our data came from the same survey that produced the color of the universe finding--the 2dF Galaxy Redshift Survey," said Nicholas Cross, a Physics and Astronomy associate research scientist in the Krieger School of Arts and Sciences.

Cross and astronomers from various institutions in England, Scotland and Australia used data from the 2dF survey and observations taken from the wide field camera on the Isaac Newton Telescope in the Canary Islands to put together their survey, known as the Millennium Galaxy Catalogue.

"We were much more interested in getting a very accurate count of the number of galaxies in the area of space we surveyed, and a very careful assessment of their brightness," said Cross, who noted that earlier surveys of the same factors in local galaxies had produced unresolved discrepancies. To remove those discrepancies and advance efforts to understand the evolution of galaxies and the universe, Cross and his colleagues took highly accurate measurements of galaxies in a patch of sky on the celestial equator.

Brightness measurements of a galaxy can be used to estimate the number of stars in a galaxy, so astronomers totaled the estimated number of stars in the 10,000 galaxies they found in the patch of sky they surveyed. They combined that star count with a few other estimated factors, such as the size of the universe, to arrive at the 70 sextillion figure. Simon Driver, who presented the findings at an astronomy meeting in Australia, emphasized that the estimate was for visible stars within the range of modern telescopes.


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