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.

Stellar generations in the Milky Way galaxy

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   [ESO PR Photo 25b/02]  ESO PR Photo
                         25a/02                                25b/02






 [Preview - JPEG: 400 x 458 pix - 86k  [Preview - JPEG: 494 x 400 pix - 55k
 [Normal - JPEG: 800 x 915 pix -       [Normal - JPEG: 987 x 800 pix -
 648k]                                 216k]








 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@hs.uni-hamburg.de