October 2002

From Michigan State University

MSU astronomer part of international team that identifies ancient star

Ancient star identified

EAST LANSING, Mich. -- A Michigan State University astronomer is part of an international team that has identified an ancient star, one that may be the oldest ever found and which provides clues to what the universe was made of shortly after the Big Bang.

The finding of the star, which is known as "HE 0107-5240," is chronicled in the Oct. 31 issue of the British journal Nature.

The star is anywhere from 14 billion to 15 billion years old, said Timothy C. Beers, MSU professor of physics and astronomy. That's more than three times the age of our sun, which is a mere 4.5 billion years old.

"This is the closest astronomers have come to having direct knowledge of the elemental composition of the universe shortly after the Big Bang," Beers said. "Chemically, this is the most primitive object that's ever been found."

The clue to its age is the fact that the star has an extremely low metal content. Beers said this particular star has 1/200,000th of the solar content of heavy elements, which is about 20 times more metal-poor than the previous record for this kind of star.

When stars began to form some time after the Big Bang, the two main elements in the universe were hydrogen and helium. The first stars to form are thought to have been hundreds or even thousands of times more massive than the sun. After extremely short lives, these stars exploded as supernovae and began to pollute the universe with heavier metals such as iron, carbon, nitrogen and oxygen. Consequently, older stars have very low levels of these metals.

This is what's known as a Population II star, said Beers, a classification of stars that formed shortly after the first, post-Big Bang stars began to die off.

"This is perhaps the first identification of a true second-generation star," he said. "In spite of its deficiency of heavy metals, the small abundance of metals we do see in this star is evidence of the previous generation of massive stars that exploded as supernovae.

"This also tells us that stars were able to form right from the beginning. There wasn't a significant delay. It tells us where we started from and sets the chemical recipe of the early universe."

This star was found in the halo of our Milky Way Galaxy, a low-density region of the galaxy where the stars are considerably older than those found in the disk, or the plane, of the galaxy, where the sun is located.

This star is approximately 36,000 light years from Earth. Not visible with the naked eye, it is located in the constellation Phoenix in the southern hemisphere.

For Beers, the discovery is the result of nearly 25 years of searching the sky for a star such as this. Quite a task, considering there are as many as 100 billion stars in our galaxy.

"The first task was to separate the interesting ones from the uninteresting ones," he said. "We began with around 4 million stars in our survey, and narrowed that down to around 8,000 interesting candidates. This was one of those 8,000."

Other members of the team are astronomers from Germany, Sweden, Australia and Brazil. The discovery spectrum of this star was obtained on the Siding Springs Observatory 2.3m telescope in Australia. A high-resolution spectrum was then obtained using the European Southern Observatory's 8m Very Large Telescope in Chile.

The research was funded by the National Science Foundation and the National Aeronautics and Space Administration.











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