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

 Caption: PR Photo 25a/02 shows a small sky field with the very
 metal-deficient star HE 0107-5240 at the centre (reproduced from the
 Digital Sky Survey [STScI Digitized Sky Survey, (C) 1993, 1994, AURA, 
 Inc.  all rights reserved - cf. http://archive.eso.org/dss/dss]). PR 
 Photo 25b/02 displays a comparison of a region of the spectrum of the 
 Sun (top) with that of CD -38 245, the previously most iron-deficient 
 star known (2nd from top), the new record-holder HE 0107-5240 (3rd 
 from top), and a (hypothetical) Population III star [4], consisting 
 only of elements produced in the Big Bang, i.e. hydrogen and helium, 
 and traces of lithium.  As can be seen, the spectral absorption lines 
 become progressively weaker with decreasing content of heavier 
 elements. While there is 1 iron atom for every 31,000 hydrogen atoms 
 in the atmosphere of the Sun, in HE 0107-5240 this ratio is about 
 200,000 times smaller, or only 1 iron atom for every 6.8 billion 
 hydrogen atoms! The two spectra in the middle show that HE 0107-5240 
 is indeed much more metal-poor than the previous record-holder CD -38 
 245 - the iron (Fe) lines in the spectrum of HE 0107-5240 are weaker 
 (or absent) and the Nickel (Ni) line is not visible at all.

One of these stars has been designated HE 0107-5240 ("HE" stands for
Hamburg/ESO Survey, and the number denotes the approximate position of 
the star on the sky). It is about ten thousand times fainter than the 
faintest stars that can be seen with the unaided eye. It is located in 
the direction of the southern constellation Phoenix, at a distance of 
about 36,000 light-years.

This star was observed in December 2001 with the UV-Visual Echelle
Spectrograph (UVES) on the 8.2-m VLT KUEYEN telescope at the ESO 
Paranal Observatory (Chile). From these spectra, Norbert Christlieb 
and his colleagues at the Dept. of Astronomy and Space Physics, 
University of Uppsala (Sweden) and at the Munich University 
Observatory (Germany) were able to determine the chemical composition 
of the star.

The implications
----------------
HE 0107-5240 turns out to be the most metal-poor star known to date.

"This is, in a way, the closest we have ever come to the conditions 
directly after the Big Bang by studying stars", says Norbert 
Christlieb. "But obviously, a lot must have happened between the Big 
Bang and the formation of this star. In spite of its extreme 
metal-poorness, it evidently contains some metals, and they were most 
probably formed in a even earlier, massive star that exploded as a 
supernova".

Bengt Gustafsson from the University of Uppsala, who lead the chemical
analysis jointly with Christlieb, adds that "this star also has an
abnormally large content of carbon and nitrogen. Those elements may 
possibly have been formed by nuclear reactions with helium and 
hydrogen deep inside the star and subsequently transported upwards to 
the stellar surface where they can now be observed. It is also 
possible that a neigbouring star at the end of its life 'polluted' our 
star by transferring some of its enriched material to HE 0107-5240 at 
that moment. The ongoing observations with UVES will help us to decide 
which scenario is the most probable."

Renewed hope to find first-generation stars
-------------------------------------------
The mass of HE 0107-5240 is about 80% of that of the Sun. This 
discovery thus clearly demonstrates that stars with masses slightly 
less than the Sun can form from very metal-poor gas. This is 
unexpected, as most current theoretical calculations indicate that it 
is very difficult to form low-mass stars shortly after the Big Bang, 
because metals are needed to efficiently cool gas clouds as they 
contract into stars. But now HE 0107-5240 reveals that Nature has 
found a way to achieve the necessary cooling. It therefore appears 
that many of the model calculations must be refined.

Equally important: if a star like HE 0107-5240, with about 0.8 solar 
mass and 1/200,000 of the metal content of the Sun, did indeed form in 
the early Universe, then it should also have been possible for 
low-mass Population III stars to form. If so, they would have survived 
until today.

This implies that there is new hope to find them by means of large,
systematic searches like the Hamburg/ESO Survey. Until now, follow-up
spectroscopic observations - which are necessarily quite 
time-consuming - have only been made of about one-quarter of the 8000 
low-metal-abundance candidate stars identified in that survey. It is 
therefore not excluded that a bona-fide Population III star may 
eventually be found in the course of this programme.

More information
----------------
The information presented in this Press Release is based on a research
article ("A stellar relic from the early Milky Way" by Norbert 
Christlieb et al.) that appears in the research journal "Nature" on 
October 31, 2002.

Notes

[1]: This press release is issued in coordination between ESO and 
Hamburger Sternwarte in Germany.

[2]: The team consists of Norbert Christlieb (Hamburger Sternwarte,
University of Hamburg, Germany; on sabbatical leave at the Research 
School of Astronomy and Astrophysics, Mount Stromlo Observatory, 
Australia), Michael S. Bessell (Research School of Astronomy and 
Astrophysics, Mount Stromlo Observatory, Australia), Timothy C. Beers 
(Department of Physics and Astronomy, Michigan State University, East 
Lansing, USA), Bengt Gustafsson, Paul S. Barklem, Torgny Karlsson, 
Michelle Mizuno-Wiedner (Department of Astronomy and Space Physics, 
University of Uppsala, Sweden), Andreas Korn (University Observatory 
Munich, Germany) and Silvia Rossi (Instituto de Astronomia, Geofisica 
e Ciencias Atmosfericas, Universidade de Sao Paulo, Brazil).

[3]: Most stars in the Milky Way galaxy move within the disk, and for 
most of these, 1 to 2 percent of their mass consists of chemical 
elements that are heavier than hydrogen and helium; this is also the 
case for the Sun, which at 4.6 billion years is about one third of the 
age of our galaxy.  There exists, however, another population of stars 
for which the heavy-element abundance is only 1/10 - 1/1000 of that of 
the Sun. Those stars are found in globular clusters, but most move in 
a huge swarm around the disk, in the halo of the Galaxy. These "halo 
stars" were born when the Milky Way galaxy was young and their motions 
still carry the imprint of the process by which our galaxy formed, 
when gravity brought the gas together and the first stars appeared. 
The "halo stars" are said to belong to "Population II",
in contrast to 
the younger stars in the disk (like the Sun) that are referred to as 
"Population I" stars. But what is then the origin of the small amount 
of heavy elements in Population II stars? There must have been 
supernovae and other exploding stars in the very early (or even pre-) 
Milky Way gas, out of which Population II stars were formed. This 
first (still hypothetical) stellar generation has been named 
"Population III".  There have been many attempts to find Population 
III stars, which are then presumably totally void of metals, but those 
searches have not succeeded so far.

[4]: Astronomers refer to elements heavier than hydrogen and helium as
"metals". Stars with a low abundance of heavier elements are thus 
referred to as "metal-poor" stars.

Contact

Norbert Christlieb
p.t. RSAA, Australian National Observatory
Mount Stromlo Observatory
Weston, ACT, Australia
Phone (office): +61-2-6842-6250 (until 1 November)
Phone (office): +61-2-6125-0286 (after 1 November)
email: nchristlieb{at}hs.uni-hamburg.de

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