July 10, 2008: Imagine having three clocks in your house, each chiming at a different time. Astronomers have found the equivalent of three out-of-sync "clocks" in the ancient open star cluster NGC 6791. The dilemma may fundamentally challenge the way astronomers estimate cluster ages, researchers said.
Using NASA's Hubble Space Telescope to study the dimmest stars in the cluster, astronomers uncovered three different age groups. Two of the 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.See the rest: