70,000,000,000,000,000,000,000 Stars in the
Sky
By Michael Purdy Homewood
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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