The European Southern Observatory (ESO) press release 23-07 is reproduced below:

ESO 23/07 - Science Release

10 May 2007
For Immediate Release



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."


From left: Recent cosmological studies show that the Big Bang occurred 13.7 billion years ago. The metal-poor star HE 1523 formed in our Milky Way galaxy soon afterward, cosmologically speaking: 13.2 billion years ago. The primitive star contained the radioactive heavy elements uranium and thorium, and the amounts of those elements decay over time, each according to its own half-life. Today, astronomer Anna Frebel of the The University of Texas atAustin McDonald Observatory and her colleagues have deduced the star's age based on the amounts of radioactive elements it contains compared to certain other "anchor" elements, specifically europium, osmium, and iridium. The study of the star's chemical make-up was made using the UVES spectrograph on the Kueyen Telescope, part of ESO's Very Large Telescope, at Paranal, in Chile. © ESO

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.


The observed spectrum (dots) of the old star HE 1523-0901in the region of the uranium (U II) line at a wavelength of 385.96 nm. The origin of some of the other spectral lines in the region is also indicated (e.g. iron, neodymium, samarium, magnesium). The synthetic spectrum (thin and dotted lines) was computed for the adopted abundances of the stable elements and for four different values of the abundance of uranium atoms in the atmosphere of the star. The uppermost line (corresponding to no uranium at all) clearly does not fit the observed spectrum at all. The best fit is provided by the middle (red) line. © ESO

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.

Contact

Anna Frebel
McDonald Observatory, Texas

Phone: +1 512-461-7907
Email: anna (at) astro.as.utexas.edu

Norbert Christlieb
Department of Astronomy and Space Physics, Uppsala University, Sweden
Phone: +46-18-471-5982
Mobile: +49-176-67 67 14 08
E-mail: norbert (at) astro.uu.se

Source: ESO Press Release pr-23-07

The McDonald Observatory, University of Texas at Austin press release is reproduced below:

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.

— END —

Additional Contacts:

Dr. Anna Frebel, McDonald Observatory
The University of Texas at Austin
512-461-7907; anna@astro.as.utexas.edu

Dr. Henri Boffin, Deputy Head of Public Affairs
European Southern Observatory
+49-89-3200-6222; hboffin@eso.org

Source: McDonald Observatory Press Release