A Galactic Fossil: Star is Found to be 13.2 Billion Years Old(Forwarded)

ESO Education and Public Relations Dept.

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Contact

Anna Frebel
McDonald Observatory, Texas
Phone: +1 512-461-7907

Norbert Christlieb
Department of Astronomy and Space Physics, Uppsala University, Sweden
Phone: +46-18-471-5982

For Immediate Release: 10 May 2007

ESO Science Release 23/07

A Galactic Fossil

Star is Found to be 13.2 Billion Years Old

How old are the oldest stars? Using ESO's VLT, astronomers recently
measured the age of a star located in our Galaxy. The star, a real fossil,
is found to be 13.2 billion years old, not very far from the 13.7 billion
years age of the Universe. The star, HE 1523-0901, was clearly born at the
dawn of time.

"Surprisingly, it is very hard to pin down the age of a star", the lead
author of the paper reporting the results, Anna Frebel, explains. "This
requires measuring very precisely the abundance of the radioactive
elements thorium or uranium, a feat only the largest telescopes such as
ESO's VLT can achieve."

This technique is analogous to the carbon-14 dating method that has been
so successful in archaeology over time spans of up to a few tens of
thousands of years. In astronomy, however, this technique must obviously
be applied to vastly longer timescales.

For the method to work well, the right choice of radioactive isotope is
critical. Unlike other, stable elements that formed at the same time, the
abundance of a radioactive (unstable) isotope decreases all the time. The
faster the decay, the less there will be left of the radioactive isotope
after a certain time, so the greater will be the abundance difference when
compared to a stable isotope, and the more accurate is the resulting age.

Yet, for the clock to remain useful, the radioactive element must not
decay too fast -- there must still be enough left of it to allow an
accurate measurement, even after several billion years.

"Actual age measurements are restricted to the very rare objects that
display huge amounts of the radioactive elements thorium or uranium," says
Norbert Christlieb, co-author of the report.

Large amounts of these elements have been found in the star HE 1523-0901,
an old, relatively bright star that was discovered within the Hamburg/ESO
survey [1]. The star was then observed with UVES on the Very Large
Telescope (VLT) for a total of 7.5 hours.

A high quality spectrum was obtained that could never have been achieved
without the combination of the large collecting power Kueyen, one of the
individual 8.2-m Unit Telescopes of the VLT, and the extremely good
sensitivity of UVES in the ultraviolet spectral region, where the lines
from the elements are observed.

For the first time, the age dating involved both radioactive elements in
combination with the three other neutron-capture elements europium,
osmium, and iridium.

"Until now, it has not been possible to measure more than a single cosmic
clock for a star. Now, however, we have managed to make six measurements
in this one star"," says Frebel.

Ever since the star was born, these "clocks" have ticked away over the
eons, unaffected by the turbulent history of the Milky Way. They now read
13.2 billion years.

The Universe being 13.7 billion years old, this star clearly formed very
early in the life of our own Galaxy, which must also formed very soon
after the Big Bang.

More Information

This research is reported in a paper published in the 10 May issue of the
Astrophysical Journal ("Discovery of HE 1523-0901, a Strongly r-Process
Enhanced Metal-Poor Star with Detected Uranium", by A. Frebel et al.).

The team includes Anna Frebel (McDonald Observatory, Texas) and John E.
Norris (The Australian National University), Norbert Christlieb (Uppsala
University, Sweden, and Hamburg Observatory, Germany), Christopher Thom
(University of Chicago, USA, and Swinburne University of Technlogy,
Australia), Timothy C. Beers (Michigan State University, USA), Jaehyon
Rhee (Center for Space Astrophysics, Yonsei University, Korea, and
Caltech, USA).

Note

[1]: The Hamburg/ESO sky survey is a collaborative project of the
Hamburger Sternwarte and ESO to provide spectral information for half of
the southern sky using photographic plates taken with the now retired
ESO-Schmidt telescope. These plates were digitized at Hamburger
Sternwarte.

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The Netherlands: Ms. Marieke Baan, +31-20-525 74 80
Portugal: Prof. Teresa Lago, +351-22-089 833
Spain: Dr. Miguel Mas-Hesse, +34918131196
Sweden: Dr. Jesper Sollerman, +46-8-55 37 85 54x
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United Kingdom: Mr. Peter Barratt, +44-1793-44 20 25

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*****

McDonald Observatory
University of Texas at Austin

Contact:
Rebecca Johnson
ph: 512-475-6763 fax: 512-471-5060

Additional Contacts:
Dr. Anna Frebel, McDonald Observatory
The University of Texas at Austin
512-461-7907

Dr. Henri Boffin, Deputy Head of Public Affairs
European Southern Observatory
+49-89-3200-6222

10 May 2007

Texas Astronomer Finds Six 'Cosmic Clocks' in Star Born Soon After Big
Bang

AUSTIN -- How old are the oldest stars? An international team of
astronomers led by Dr. Anna Frebel of The University of Texas at Austin
McDonald Observatory recently measured the age of an ancient star in our
Milky Way galaxy at an extraordinary 13.2 billion years. This measurement
provides a lower limit to the age of the universe and will help to
disentangle the chemical history of our galaxy. Frebel's results are
published in today's edition of The Astrophysical Journal Letters.

The team used radioactive decay dating techniques to date the star, called
HE 1523-0901. This is close to the age of the universe of 13.7 billion
years. "This guy was born very shortly after the Big Bang," Frebel said.

"Surprisingly, it is very hard to pin down the age of a star," she said,
"although we can generally infer that chemically primitive stars have to
be very old." Such stars must have been born before many generations of
stars had chemically enriched our galaxy.

Astronomers can only accurately measure the ages of very rare old stars
that contain huge amounts of certain types of chemical elements, including
radioactive elements like thorium and uranium.

Similar to the way archaeologists use carbon-14 and other elements to date
Earth relics thousands of years old, astronomers use radioactive elements
found in stars to deduce these stars' ages, which may be millions or
billions of years.

"Very few stars display radioactive elements," Frebel said. "I'm looking
at a very rare subgroup of these already rare stars. I'm looking for a
needle in a haystack, really."

Frebel made the extremely difficult measurement of the amount of uranium
in the star HE 1523-0901 using the UVES spectrograph on the Kueyen
Telescope, one of four 8.2-meter telescopes that comprise The Very Large
Telescope at the European Southern Observatory in Chile.

"This star is the best uranium detection so far," she said, explaining
that while uranium has been discovered in two other stars previously, only
one could be used to get a good age for the star. HE 1523-0901 also
contains thorium, another radioactive element that is useful in age-dating
of stars. Uranium, with a half-life of 4.5 billion years, is a better
clock than thorium, Frebel says. Thorium's half-life of 14 billion years
is actually longer than the age of the universe.

But astronomers need more than just radioactive elements like uranium and
thorium to age-date a star. For each radioactive element, "you have to
anchor it to another element within the star," Frebel said. Because she
detected so many of these anchor elements in HE 1523-0901, she can come up
with an extremely accurate age. In this case, the anchor elements are
europium, osmium, and iridium.

The combination of two radioactive elements with three anchor elements
discovered in this one star provided Frebel six so-called "cosmic clocks."

"So far, for no other star was it possible to employ more than one cosmic
clock," she said. "Now we are suddenly provided with six measurements in
just one star!"

How did she find this amazing star? Frebel says it was a case of "informed
serendipity." She was researching a sample of old stars for her PhD thesis
while a graduate student at The Australian National University, and
recognized the consequences of this star's extraordinary spectrum after
she measured it with ESO's Very Large Telescope.

"When you do discovery work, you never know what you're going to find,"
Frebel said. "You hope to find interesting objects. Depending on what you
find, you then move in that direction."

The new result will be used by Frebel and her team to gain important clues
to the creation and evolution of the chemical elements shortly after the
Big Bang. It will also provide theorists with new, important experimental
data. "Stars such as HE 1523-0901 are ideal cosmic laboratories to study
nucleosynthesis," she said.

Frebel is now working with her colleagues Chris Sneden, Volker Bromm,
Carlos Allende Prieto, Matthew Shetrone, and graduate student Ian Roederer
at The University of Texas at Austin to further research extremely old
stars with the 9.2-meter Hobby-Eberly Telescope at McDonald Observatory.

The Hobby-Eberly Telescope is a joint project of The University of Texas
at Austin, The Pennsylvania State University, Stanford University,
Ludwig-Maximilians-Universität München and Georg-August-Unversität
Göttingen.