Mit Hilfe des radioaktiven Zerfalls von Uran konnte das Alter eines Sterne zu 13,2 Mrd. Jahren bestimmt werden. Er ist somit schon als Teil unserer Galaxis 500 Mill. Jahre nach dem Urknall entstanden.



ESO 23/07 - Science Release

10 May 2007 

For Immediate Release


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). 



[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.




Forscher um Anna Frebel vom McDonald-Observatorium konnten den Stern HE 1523-0901 anhand radioaktiver Elemente auf ein Alter von 13,2 Milliarden Jahre datieren. Er stammt demnach aus der Frühzeit des Universums, dessen Alter auf 13,7 Milliarden Jahre geschätzt wird. Es handelt sich um eine der bislang genauesten Bestimmungen eines Sternalters überhaupt. 


Die Wissenschaftler maßen den Uran-Gehalt des Sterns mit dem UVES-Spektrometer des Kueyen-Teleskops, das zur Gruppe des Very Large Telescope der Europäischen Südsternwarte gehört. Mit einer Halbwertszeit von 4,5 Milliarden Jahren eignet sich das radioaktive Element gut zur Altersbestimmung bei Sternen. Außerdem enthält HE 1523-0901 Thorium, das mit einer Halbwertszeit von 14 Milliarden Jahren ebenfalls zur Sterndatierung eingesetzt wird. Da der Stern zudem Europium, Osmium und Iridium aufweist, konnten die Forscher ihre Messungen gleich mehrfach absichern. 


Frebel war in den Daten des Hamburg/Eso-Surveys auf HE 1523-0901 gestoßen. Nur wenige Objekte enthalten Uran und Thorium in ausreichend hohen Konzentrationen, dass sie für eine genaue Messung geeignet sind. Dass der Stern zudem drei weitere geeignete Elemente für Kontrollmessungen enthielt und damit den Vergleich von sechs "kosmischen Uhren" ermöglichte, ist ein besonderer Glücksfall. (af)




Astrophysical Journal Letters 660: L117-L120 (2007), Abstract


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