June 28, 2002

Contact: Ranjit Arab, University Relations, (785) 864-8855.

KU astronomers help chart evolution of Milky Way galaxy

LAWRENCE -- The research that Bruce Twarog and his wife, Barbara Anthony-Twarog, conduct can best be described as taking a cosmic census.

The couple, both professors of physics and astronomy at the University of Kansas, methodically comb the Milky Way galaxy as they try to classify the age, location and chemical content of stars, gas, and the large collection of hundreds or even thousands of stars known as star clusters.

"When we put all of that information together, hopefully we can figure out the past history of the entire system," Bruce Twarog said.

The couple recently made a breakthrough in their research by identifying the star cluster NGC 6253 as the most metal-rich star cluster in our galaxy. By "metals" the scientists are referring collectively to all of the elements -- other than hydrogen and helium -- that are present in the atmosphere of a star.

The metals are quite rare, since hydrogen and helium usually make up more than 98 percent of the mass of the stars. The stars in star cluster NGC 6253, a moderately old cluster that was formed some 3 billion years ago, had a metal content of about 5 percent, they concluded.

Tracking the abundance of metal-rich stars not only gives the astronomers a chance to see what the Milky Way currently looks like, but it also gives them insights into the galaxy's appearance at the time the stars formed.

For the most part, the oldest stars tend to have a very low metal content. They were formed out of gas at a time when the Milky Way galaxy was purely hydrogen and helium. However, as the stars evolved, they created metals in their core through nuclear fusion. When these stars died and exploded, they ejected the metals back into the galactic gas. The stars that formed in the aftermath from this existing gas had a higher metal content.

"These elements are made by stars' lives and deaths, so tracking the amount of metal abundance as a function of time and space in the galaxy is one of our techniques for studying the evolution of the galaxy," said Anthony-Twarog.

To conduct this research, the astronomers brought back dozens of images of the star cluster taken at the National Science Foundation's Cerro Tololo Interamerican Observatory in Chile. The General Research Fund from KU and the Department of Physics and Astronomy supported these visits and purchased the special filters the astronomers required.

Back in Lawrence, they ran the images through elaborate software designed by the National Observatories. Enlisting the help of their undergraduate research student, Nathan De Lee, a May 2002 KU graduate from Pittsburg, the researchers endured the time-intensive task of determining the metal content of the hundreds of stars in that cluster. Bruce Twarog delivered the results of the study earlier this month at the meeting of the American Astronomical Society in Albuquerque, N.M.

The entire process has taken about three years, but Anthony-Twarog said they purposely took their time to make sure they were as accurate as possible. Their work is far from over. They will continue sampling other clusters, in all corners of the galaxy, for metal content.

An interesting offshoot of this research is that it may lead to the discovery of additional planets.

"It is connected, although that wasn't our motivation," Anthony-Twarog said. "It has become apparent in the last few years -- for reasons we don't really understand -- that the stars that people are finding planets around tend to have more metals than the sun. So one of the strategies people are using to find more planets is to look at stars that are more enhanced in metal content -- that's been pretty successful, though we don't know why."

Anthony-Twarog said this connection between metal-rich stars and the possibility of additional planets has provided several other research opportunities. Still, she said, gaining a better understanding of the Milky Way was thrilling in and of itself.

"The Milky Way is a big place; our picture of it has been painted with a broad brush," she said. "There are big gaps in our understanding of how the galaxy formed, and they will only be filled in when we understand more of the details about its appearance."