Astronomy research team with UT Austin professor develops ancient stellar timeline by studying the formation of heavy elements in the Milky Way

November 15, 2000

AUSTIN, Texas—A group of astronomers including the chairman of The University of Texas at Austin astronomy department has identified a distinct series of epochs of galaxy-wide chemical formation. The group developing this evolutionary timeline had been studying how elements heavier than iron were produced in the early Milky Way.

This timeline, stretching from the Big Bang onward for several billion years, has the potential to serve as a cosmic "fingerprint manual" that could help astronomers categorize some of the quirky high-redshift galaxies seen in recent samples of the ancient Universe, such as the Hubble Deep Field, and in deeper surveys to come.

Debra Burris of Oklahoma City Community College, is lead author of a paper outlining the developments that is scheduled for the Nov. 20 issue of The Astrophysical Journal,with co-author Catherine Pilachowski of the National Optical Astronomy Observatory (NOAO) in Tucson.

The research team co-authoring the paper includes Dr. Christopher Sneden, chair of the astronomy department at UT Austin. Sneden is the Rex G. Baker, Jr. And McDonald Observatory Centennial Research Professor in Astronomy. Taft Armandroff of NOAO in Tucson, John Cowan from the University of Oklahoma and Henry Roe from the University of California at Berkeley also are co-authors.

The research team looked at nearly 100 stars in the halo around the Milky Way, carefully selecting them to be relatively nearby, old and at least 10-to-100 times depleted of metals as compared to the Sun. Subsequent plots of elemental ratios in the stars revealed obvious trends over time that allow a larger chronology to be developed.

"This is one of the largest studies yet of the abundances of heavy elements in galactic halo stars," said Burris. "The behavior of the trends in abundances of these elements give us major clues about the conditions and populations of stars that existed early in the Milky Way's history."

"Our results tell us that the history of the galaxy is tied very closely to the ways that stars change from generation to generation," explained Pilachowski. "Certain chemical elements don't form until the stars that make them have had time to evolve. Therefore, we can read the history of star formation in the compositions of the oldest stars."

The research team developed this timeline to explain their observations:

• The Pre-Stellar Epoch —- The Big Bang jumpstarts the initial large-scale production of hydrogen, deuterium, helium and lithium.

• The Epoch of Very Massive Stars — The earliest stages of heavy element formation in the galaxy were dominated by stars with masses 10 times that of the Sun or more, and lifetimes of a few million years or less, producing small amounts of all the elements. Their presence can be identified most clearly by excesses of elements like strontium, yttrium and zirconium.

• The Europium Epoch — For the next 30-to-100 million years, element formation was dominated by supernovae from stars with about eight to-10 times the mass of the Sun, enriching the Milky Way in heavier elements like barium, europium, and other lanthanide elements in the Periodic Table, such as cerium.

• The Double Shell Epoch — A major shift from previous epochs, lasting from about 100 million to a billion years after the Galaxy formed, it featured stars with perhaps three to seven times the mass of the Sun, producing more strontium, barium and some particular lanthanides from nuclear-burning interior shells.

• The Iron Epoch — From one billion to three billion years after the Galaxy formed, supernovae from white dwarf stars a bit larger than the Sun produced large amounts of iron.

The measurements used to develop these epochs were obtained with spectrographic instruments on the National Science Foundation's Mayall four-meter telescope and Coude-Feed telescope operated by Kit Peak National Observatory near Tucson.

For more information, contact Douglas Isabel of the National Optical Astronomy Observatory

(520) 318-8214, e-mail disbell@noao.edu or Kathi Etherton of Oklahoma City Community College (405) 682-1611, ext. 7675 or Sandra Preston, director of the public information office at UT Austin's McDonald Observatory (512) 475-6765.