What's My Age? Mystery Star Cluster Has 3 Different Birthdays
Imagine having three clocks in your house, each chiming at a different
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 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," explained astronomer Luigi Bedin, who works at the Space
Telescope Science Institute in Baltimore, Md.
Ivan King of the University of Washington and leader of the Hubble study
said: "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
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
"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.
Bedin and his colleagues are relieved that they now have only two ages
to reconcile: an 8-billion-year age of the normal stellar population and
a 6-billion-year age for the white dwarfs. All that is needed is a
process that slows down white-dwarf evolution, the researchers said.
Hubble's Advanced Camera for Surveys analyzed the cooling rate of the
entire population of white dwarfs in NGC 6791, from brightest to
dimmest. Most star clusters are too far away and the white dwarfs are
too faint to be seen by ground-based telescopes, but Hubble's powerful
vision sees many of them.
White dwarfs are the smoldering embers of Sun-like stars that no longer
generate nuclear energy and have burned out. Their hot remaining cores
radiate heat for billions of years as they slowly fade into darkness.
Astronomers have used white dwarfs as a reliable measure of the ages of
star clusters, because they are the relics of the first cluster stars
that exhausted their nuclear fuel.
White dwarfs have long been considered dependable because they cool down
at a predictable rate-the older the dwarf, the cooler it is, making it a
seemingly perfect clock that has been ticking for almost as long as the
cluster has existed.
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