What's My Age?
Mystery Star Cluster Has 3 Different Birthdays
(10 July 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 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.
In studying the dimmest burned-out stars in globular star cluster NGC 6791, NASA's Hubble Space Telescope has uncovered a paradox: 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. [Left] This is a ground-based telescopic view of NGC 6791, located 13,300 light-years away in the constellation Lyra. The green inset box shows the view with Hubble's Advanced Camera for Surveys. [Top right] The full Hubble Advanced Camera for Surveys field is full of stars estimated to be 8 billion years old. Two background galaxies can be seen at upper left. [Bottom right] A blow up of view of a small region of the Advanced Camera for Surveys field reveals very faint white dwarfs. The blue circles identify hotter dwarfs that are 4 billion years old. The red circles identify cooler dwarfs that are 6 billion years old. (courtesy: NASA, ESA, and L. Bedin (STScI))
"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
realised 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.
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 analysed 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 smouldering 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.
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.
The first
results appeared in the May 10 issue of The Astrophysical Journal, and the
clarification about binaries was in the May 20 issue of The Astrophysical
Journal Letters.
Other members of the research team are Santi Cassisi of
the Collurania Astronomical Observatory in Italy, and Jay Anderson, of the
Space Telescope Science Institute.
(source: Space Telescope Science
Institute)
