Jul 11, 2008

Star Cluster with 3 Birthdays

Astronomers have found three different age groups in the ancient open star cluster NGC 6791. It is like having three different clocks in your house, each chiming at a different time. These new findings may fundamentally challenge the way astronomers estimate ages of star clusters.

How this mystery was uncovered? Using NASA's Hubble Space Telescope, astronomers were busy in studying the dimmest stars in the NGC 6791, which is one of the oldest and largest open clusters known, about 10 times larger than most open clusters and containing roughly 10,000 stars. The cluster is located in the constellation Lyra. The astronomers found that three different populations of stars exist in an object where all the stars should have formed at the same time out of an interstellar cloud of gas and dust. How this could happen? Now, astronomers are in great puzzle.

Two of the star populations are burned-out stars called white dwarfs. One group of these low-wattage stellar remnants (red circles) appears to be 6 billion years old, another (blue circles) appears to be 4 billion years old. The ages are out of sync with those of the cluster's normal stars, which are 8 billion years old.

"The age discrepancy is a problem because stars in an open cluster should be the same age. They form at the same time within a large cloud of interstellar dust and gas. So we were really puzzled about what was going on," explained astronomer Luigi Bedin, who works at the Space Telescope Science Institute in Baltimore, Md. Link


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