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10\30 ESO - A Glimpse of the Young Milky Way
Part 1 of 2

        Information from the European Southern Observatory

 ESO Press Release 19/02
 30 October 2002                                   [ESO Logo]

 Embargoed until Wednesday, October 30, 2002,
 19:00 hrs CET (18 hrs UT)
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A Glimpse of the Young Milky Way
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VLT UVES Observes Most Metal-Deficient Star Known [1]

Summary

A faint star in the southern Milky Way, designated HE 0107-5240, has 
been found to consist virtually only of hydrogen and helium. It has 
the lowest abundance of heavier elements ever observed, only 1/200,000 
of that of the Sun - 20 times less than the previous record-holding 
star.

This is the result of a major ongoing research project by an 
international team of astronomers [2]. It is based on a decade-long 
survey of the southern sky, with detailed follow-up observations by 
means of the powerful UV-Visual Echelle Spectrograph (UVES) on the 
8.2-m VLT KUEYEN telescope at the ESO Paranal Observatory in Chile.

This significant discovery now opens a new window towards the early 
times when the Milky Way galaxy was young, possibly still in the stage 
of formation. It proves that, contrary to most current theories,
comparatively light stars like HE 0107-5240 (with 80% of the mass of 
the Sun) may form in environments (nearly) devoid of heavier elements.

Since some years, astronomers have been desperately searching for 
stars of the very first stellar generation in the Milky Way, 
consisting only of hydrogen and helium from the Big Bang. None have 
been detected so far and doubts have arisen that they exist at all.

The present discovery provides new hope that it will ultimately be
possible to find such stellar relics from the young Universe and 
thereby to study "unpolluted" Big Bang material.

PR Photo 25a/02: The sky region around the very metal-deficient
 star HE 0107-5240.
PR Photo 25b/02: Comparison of UVES spectra of stars with different
 metal abundances.
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Stellar generations in the Milky Way galaxy
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The Milky Way galaxy in which we live formed from a gigantic cloud of 
gas, when the Universe was still young, soon after the initial Big 
Bang. At the beginning, this gas was presumably composed almost 
exclusively of hydrogen and helium atoms produced during the Big Bang.

However, once the first stars formed by contraction in that gas, many
heavier elements were built up by nuclear processes in their 
interiors. As time passed, many of the stars of this and following 
stellar generations returned the processed matter to their 
surroundings at the ends of their lives, either during violent 
supernova explosions or via strong "stellar winds". In this way, the 
interstellar gas in the Milky Way system has ever since been 
continuously enriched with heavier elements. Stars of later 
generations like our Sun now contain those elements produced by their 
ancestors and we are indeed ourselves made up of them.

Consequently, the early (and hence, old) stars in the Milky Way mainly
differ from younger stars by containing very small amounts of such 
elements.

Hunting the earliest stars
--------------------------
Have some of those earliest stars survived to our days? In theory, at 
least, it would be possible that some of the lighter ones - having the 
longest lifetimes - are still around. But if so, where are they?

During the past three decades, astronomers have desperately tried to 
find bona-fide representatives of the very first stellar generation(s) 
in the Milky Way, i.e. stars with no or, at most, extremely low 
abundance of elements other than hydrogen and helium. The researchers 
usually refer to such objects as Population III stars, the other two 
populations being stars with heavy-element abundances like the Sun 
(Population I) or somewhat less (Population II) [3].

The Hamburg/ESO survey
----------------------
Now, a group of astronomers from Germany, Sweden, Australia, Brazil 
and the USA [2] has found a giant star that has a concentration of 
heavy elements 200,000 times lower than the Sun, or about 20 times 
less than the previous "record" for this kind of star. It thus 
provides the researchers with a unique window towards the early stages 
of the formation of the Milky Way and a fine opportunity to study 
stellar gas with a composition close to that produced during the Big 
Bang.

This is one important outcome of a systematic search for the most
metal-deficient stars that is currently being carried out at Hamburger
Sternwarte [4]. Over a period of more than 10 years, a large 
collection of photographic pictures of the southern sky were obtained 
with the ESO 1-m Schmidt Telescope, a wide-angle telescope at the La 
Silla observatory in Chile that has now been decommissioned. Thanks to 
a large glass prism in the front of the telescope, every object in the 
observed sky field - stars as well as galaxies - was imaged as a small 
spectrum, providing a first rough idea about its type and composition.

The main aim of this "Hamburg/ESO survey" (with Dieter Reimers, 
Associate Director of the Hamburger Sternwarte, as Principal 
Investigator and Lutz Wisotzki, now at Astrophysikalisches Institut 
Potsdam, Germany, as Project Scientist) was to find quasars 
(particularly active centres of galaxies), a task that was 
accomplished most successfully, cf. e.g., ESO PR 10/97 and ESO PR 
08/00 (Report F).

A very welcome by-product of this survey has been a rich harvest of 
very metal-poor stars. This part of the project is led by Norbert 
Christlieb, also from the Hamburg Observatory, and now on sabbatical 
leave at the Research School of Astronomy and Astrophysics of the 
Australian National University (Canberra, Australia).

Using fast computers and advanced pattern-recognition software to 
analyze the photographic exposures and thus to sift through millions 
of registered stellar spectra, about 8000 candidates for very 
metal-poor stars were found.  These stars are now being scrutinized 
spectroscopically one-by-one with many medium-sized telescopes all 
over the world. Confirmed candidates are then observed with the 
largest telescopes in the world in order to obtain very detailed 
spectra (of high spectral resolution), which allow the astronomers to 
determine their chemical composition accurately.

The very metal-deficient star HE 0107-5240

ESO PR Photo 25a/02              ESO PR Photo 25b/02
[Preview - JPEG: 400 x 458 pix   [Preview - JPEG: 494 x 400 pix
 - 86k                            - 55k
[Normal - JPEG: 800 x 915 pix    [Normal - JPEG: 987 x 800 pix 
 - 648k]                          - 216k]

(continued)

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