Radioactive decay of elements gives age of stars
Posted: 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."