Radioactive decay of elements gives age of starsUNIVERSITY OF TEXAS AT AUSTIN NEWS RELEASEPosted: January 13, 2002

Gold, silver, platinum and other exotic heavy elements
forged in the explosions of massive stars are leading the way to
understanding the birth of elements in our Milky Way galaxy. According
to Christopher Sneden, professor of astronomy at the University of Texas
at Austin, supernova explosions were the main influence on the earliest
formation of elements in the Galaxy. Sneden reviewed his work in an invited lecture at the 199th meeting of the American Astronomical
Society in Washington, D.C.

Entitled "Early Galactic Nucleosynthesis of the Heaviest Elements," the
talk highlighted recent high-resolution spectroscopic studies of the
oldest Milky Way stars. The observations were done with the 2.7-meter
Harlan J. Smith Telescope at the UT-Austin McDonald Observatory, the
Hubble Space Telescope, and the Keck I Telescope in Hawaii.

To work out the Galaxy's element-formation history, Sneden studies the
oldest stars in the Milky Way. To find the ages of his target stars,
he uses a sleuthing method usually known for its archeological
applications: the radioactive decay of elements. Sneden focuses on
extremely heavy elements like precious metals, lead, europium, barium,
and thorium.

"For example, we can detect thorium in the earliest stars," he said.
"Thorium has a half-life of 14 billion years. So we observe how much
thorium the star has now, and compare that to how much we think it was
born with. Thus, we have a clock," Sneden said. "This method gives us
the ages of these stars directly: 12 to 16 billion years. These numbers
are very similar to what other scientists are saying is the age of the
Galaxy."

Sneden then compares the amounts of different heavy elements in these
old stars. These heavy elements are made by two distinct processes, so
such comparisons offer a unique way to gain insight into exactly how
elements formed early in our galaxy. "All of the elements of the
Periodic Table heavier than iron are mostly made in what are called
neutron bombardment reactions," Sneden said. "That means adding
neutrons to the nucleus of an atom to make a different, heavier
isotope." There are two ways this can happen. In each case, there
must be a source of free neutrons.

The slow process (s-process) occurs inside highly-evolved, late-type
stars. These stars have exhausted all of their hydrogen fuel, and have
begun to burn helium. The helium burning creates free neutrons, which
hit seed nuclei of ordinary metals. The neutrons have no electrical
charge, so they aren't repelled. They enter the nucleus of the atom,
turning it into an isotope. This neutron-capture continues until there
are too many neutrons inside the nucleus for the isotope to remain
stable. Then beta decay occurs: The isotope emits an electron, and is
now a stable atom of the next element on the Periodic Table.

The rapid process (r-process) is quite different. When a massive
star dies in a supernova explosion, it creates an enormous blast of
neutrons that pulverize atomic nuclei. These nuclei have no chance for
beta decay. This creates incredibly neutron-rich nuclei, which then
rapidly decay.

"The slow process can create some isotopes, the rapid process creates
others, and some are formed both ways," Sneden said. "For example, the
s-process builds almost no europium, but lots of barium," he said. "We
find that the most metal-poor stars -- these are the oldest stars in
the Milky Way -- contain more europium than barium." Thus we know that
the early formation of elements in our galaxy was more influenced by
supernova explosions than anything else.

"Contributions from the s-process came later," Sneden said. "This is
shown by the generally higher metallicity levels of stars that have
neutron-capture element abundance ratios that are more nearly like
those of the Sun."

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