Physics – Optics
Scientific paper
Feb 2001
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2001eso..pres....2.&link_type=abstract
ESO Press Release, 02/2001
Physics
Optics
Scientific paper
First Reading of a Basic Cosmic Chronometer with UVES and the VLT
Summary
Most astronomers would agree that the age of the Universe - the time elapsed since the "Big Bang" - is one of the " holy grails of cosmology ".
Despite great efforts during recent years, the various estimates of this basic number have resulted in rather diverse values. When derived from current cosmological models, it depends on a number of theoretical assumptions that are not very well constrained by the incomplete available observational data. At present, a value in the range of 10-16 billion years [1] is considered most likely.
But now, an international team of astronomers [2] has used the powerful ESO Very Large Telescope (VLT) and its very efficient spectrograph UVES to perform a unique measurement that paves the way for a new and more accurate determination of the age of the Universe. They measured for the first time the amount of the radioactive isotope Uranium-238 in a star that was born when the Milky Way, the galaxy in which we live, was still forming. It is the first measurement ever of uranium outside the Solar System .
This method works in a way similar to the well-known Carbon-14 dating in archaeology, but over much longer times. Ever since the star was born, the Uranium "clock" has ticked away over the eons, unaffected by the turbulent history of the Milky Way. It now reads 12.5 billion years . Since the star obviously cannot be older than the Universe, it means that the Universe must be older than that .
Although the stated uncertainty is still about 25% or about ±3 billion years, this is only to a minor extent due to the astronomical observation. The main problem is the current absence of accurate knowledge of some of the basic atomic and nuclear properties of the elements involved. However, further laboratory work will greatly improve this situation and a more accurate value for the age of the star and implicitly, for the Universe, should therefore be forthcoming before long .
This important result is reported in the international research journal Nature in the issue of February 8, 2001.
PR Photo 05a/01 : The 12.5-billion-year old star CS 31082-001. PR Photo 05b/01 : The telltale spectral line in CS 31082-001 - the first detection of uranium outside the Solar System . Heavy elements in stars
While hydrogen, helium and lithium were produced during the Big Bang, all heavier elements result from nuclear reactions in the interiors of stars. When stars die, heavy-element enriched matter is dispersed into surrounding space and will later be incorporated in the next generations of stars. In fact, the gold in the ring on your finger was produced in an exploding star and deposited in the interstellar cloud from which the Sun and its planets were later formed.
Thus, the older a star is, the lower is generally its content of heavy elements like iron and other metals. Measurements have shown that old stars that are members of large agglomerations known as globular clusters are normally quite "metal-poor"- their metal-content ranges down to about 1/200 of that of the Sun, in which these metals constitute only 2% of the total mass, the rest being still in the form of hydrogen and helium. Very old stars in the Milky Way galaxy
After decades of mostly fruitless efforts, a large spectral survey by American astronomer Timothy C. Beers and his collaborators has recently uncovered hundreds of stars with much lower metal content than even the globular clusters, in some cases only 1/10,000 of the solar value. It is evident that these extremely metal-poor stars must have formed during the very infancy of the Milky Way, an important, but still poorly understood phase in the life of our galaxy.
These particular stars exhibit a great variety of element abundances that may ultimately throw more light on the processes at work during this early period. As a step in this direction, an international team of astronomers [2] decided to study these stars in much more detail. They were awarded observing time for a Large Programme in 2000-2001 with the powerful combination of the ESO VLT and its very efficient high-dispersion spectrograph UVES. The first observations have been carried out and, not unexpectedly, have already proven to be a true gold mine of new information. Cosmochronology with radioactive isotopes
It is possible to make a fundamental determination of the age of a star that is quite independent of stellar evolution models, provided it contains a suitable long-lived radioactive isotope [3]. The use of a "radioactive chronometer" depends on the measurement of the abundance of the radioactive isotope, as compared to a stable one.
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 time scales.
For the method to work well, the right choice of radioactive isotope is very critical. Contrary to 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, 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. Thorium and Uranium clocks
This leaves only two possible isotopes for astronomical measurements, thorium ( 232 Th or Thorium-232, with a half-life of 14.05 billion years [4]) and uranium ( 238 U or Uranium-238, half-life 4.47 billion years).
Several age determinations have been made by means of the Thorium-232 isotope. Its strongest spectral line is measurable with current telescopes in a handful of comparatively bright stars, including the Sun. However, the decay is really too slow to provide sufficiently accurate time measurements. It takes around 47 billion years for this isotope to decay by a factor of 10, and with a typical measuring uncertainty of 25%, the resulting age uncertainty is about 4-5 billion years, or approx. one third of the age of the Universe. This slow-moving clock runs forever, but is hard to read accurately!
The faster decay of Uranium-238 would make it a much more precise cosmic clock. However, because uranium is the rarest of all normal elements, its spectral lines in stars are always very weak; if visible at all, they normally drown entirely in a vast ocean of stronger spectral lines from more abundant elements.
Nevertheless, this is exactly where the low abundance of heavier elements in very old stars comes to the rescue. In the stars that were studied by the present team at the VLT, with typically 1000 times less of the common elements than in the Sun, the predominance of the maze of atomic and molecular lines in the spectrum is greatly reduced. The lines of rare elements like uranium therefore stand a real chance of being measurable. This is particularly so, if for some reason uranium atoms were preferentially retained in the debris of those early supernova explosions that also created the iron-group elements we see in the stars today. The uranium line in CS 31082-001
ESO PR Photo 05a/01
ESO PR Photo 05a/01
[Preview - JPEG: 337 x 400 pix - 32k] [Normal - JPEG: 674 x 800 pix - 120k]
Caption : PR Photo 05a/01 displays the Milky Way star field around CS 31082-001 , the 12th-magnitude star at the centre. The "cross" is caused by reflections in the telescope optics, a typical effect for relatively bright stars. Technical information about this photo is available below.
ESO PR Photo 05b/01
ESO PR Photo 05b/01
[Preview - JPEG: 501 x 400 pix - 42k] [Normal - JPEG: 1001 x 800 pix - 128k] [Full-Res - JPEG: 1502 x 1200 pix - 200k]
Caption : PR Photo 05b/01 The observed spectrum (dots) of the old star CS 31082-001 in the region of the uranium (U II) line at 385.96 nm. The origi
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