[meteorite-list] Scientists Refines Cosmic Clock to Determine Age of Milky Way

[Ron Baalke]

http://www.eurekalert.org/pub_releases/2005-06/uoc-src062905.php

Public release date: 29-Jun-2005

Contact: Steve Koppes
skoppes at uchicago.edu 
773-702-8366
University of Chicago 

Scientist refines cosmic clock to determine age of Milky Way

The University of Chicago's Nicolas Dauphas has developed a new way to
calculate the age of the Milky Way that is free of the unvalidated
assumptions that have plagued previous methods. Dauphas' method, which
he reports in the June 29 issue of the journal Nature, can now be used
to tackle other mysteries of the cosmos that have remained unsolved for
decades.

"Age determinations are crucial to a fundamental understanding of the
universe," said Thomas Rauscher, an assistant professor of physics and
astronomy at the University of Basel in Switzerland. "The wide range of
implications is what makes Nicolas' work so exciting and important."

Dauphas, an Assistant Professor in Geophysical Sciences, operates the
Origins Laboratory at the University of Chicago. His wide-ranging
interests include the origins of Earth's atmosphere, the oldest rocks
that may contain evidence for life on Earth and what meteorites reveal
about the formation of the solar system.

In his latest work, Dauphas has honed the accuracy of the cosmic clock
by comparing the decay of two long-lived radioactive elements,
uranium-238 and thorium-232. According to Dauphas' new method, the age
of the Milky Way is approximately 14.5 billion years, plus or minus more
than 2 billion years.

That age generally agrees with the estimate of 12.2 billion years-nearly
as old as the universe itself-as determined by previously existing
methods. Dauphas' finding verifies what was already suspected, despite
the drawbacks of existing methods: "After the big bang, it did not take
much time for large structures to form, including our Milky Way galaxy,"
he said.

The age of 12 billion years for the galaxy relied on the characteristics
of two different sets of stars, globular clusters and white dwarfs. But
this estimate depends on assumptions about stellar evolution and nuclear
physics that scientists have yet to substantiate to their complete
satisfaction.

Globular clusters are clusters of stars that exist on the outskirts of a
galaxy. The processes of stellar evolution suggested that most of the
stars in globular clusters are nearly as old as the galaxy itself. When
the big bang occurred 13.7 billion years ago, the only elements in the
universe were hydrogen, helium and a small quantity of lithium. The
Milky Way's globular clusters have to be nearly that old because they
contain mostly hydrogen and helium. Younger stars contain heavier
elements that were recycled from the remains of older stars, which
initially forged these heavier elements in their cores via nuclear fusion.

White dwarf stars, meanwhile, are stars that have used up their fuel and
have advanced to the last stage of their lives. "The white dwarf has no
source of energy, so it just cools down. If you look at its temperature
and you know how fast it cools, then you can approximate the age of the
galaxy, because some of these white dwarfs are about as old as the
galaxy," Dauphas said.

A more direct way to calculate the age of stars and the Milky Way
depends on the accuracy of the uranium/thorium clock. Scientists can
telescopically detect the optical "fingerprints" of the chemical
elements. Using this capability, they have measured the uranium/thorium
ratio in a single old star that resides in the halo of the Milky Way.

They already knew how fast uranium and thorium decay with time. If they
also know the ratio of uranium and thorium when the star was formed-the
production ratio-then calculating the star's age becomes a problem with
a straightforward solution. Unfortunately, "this production ratio is
very poorly known," Dauphas said.

Dauphas solved the problem by combining the data from the
uranium/thorium observations in the halo star with measurements of the
uranium/thorium ratio that other scientists had made in meteorites. "If
you measure a meteorite, you ultimately have the composition of the
material that formed the sun 4.5 billion years ago," he said. And this
material included debris from many generations of other stars, now long
dead, that still contains information about their own uranium/thorium
composition.

"We have very good instruments in the laboratory that allow us to
measure this ratio with very, very good precision," Dauphas said.

Following the change in amount of two sufficiently long-lived
radioactive elements is a sensitive way of measuring the time since they
were formed, Rauscher said. "The problem is to set the timer correctly,
to know the initial amounts of uranium and thorium. By clever
combination of abundances in stars and meteorites, Nicolas provides the
important starting value for the uranium/thorium clock," he said.

Scientists can now use that clock to determine the age of a variety of
interstellar objects and particles, including cosmic rays, Rauscher
said. These subatomic scraps of matter continually bombard the Earth
from all directions. Where they come from has baffled scientists for
almost a century.

Dauphas' work may also lead to a better understanding of how stars
produce gold, uranium and other heavy elements that play an important
role in everyday life, Rauscher said.

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Photo illustration is available upon request.