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ESO 23/07 - Associated Image

10 May 2007
For Immediate Release

A Galactic Fossil

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. (c) ESO

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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. (c) ESO

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