Washington, July 11 : Astronomers have used NASA's Hubble Space Telescope to determine that the ancient open star cluster NGC 6791 has three different birthdays.
NGC 6791 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.
Using the Hubble 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 appears to be 6 billion years old, another 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," said astronomer Luigi Bedin, who works at the Space Telescope Science Institute in Baltimore, Maryland.
According to Ivan King of the University of Washington and leader of the Hubble study, "This finding means that there is something about white dwarf evolution that we don't understand."
After extensive analysis, members of the research team realized how the two groups of white dwarfs can look different and yet have the same age.
It is possible that the younger-looking group consists of the same type of stars, but the stars are paired off in binary-star systems, where two stars orbit each other.
Because of the cluster's great distance, astronomers see the paired stars as a brighter single star.
"It is their brightness that makes them look younger," said team member Maurizio Salaris of Liverpool John Moores University in the United Kingdom.
Binary systems are also a significant fraction of the normal stellar population in NGC 6791, and are also observed in many other clusters. This would be the first time they have been found in a white-dwarf population.
"Our demonstration that binaries are the cause of the anomaly is an elegant resolution of a seemingly inexplicable enigma," said team member Giampaolo Piotto the University of Padova in Italy.