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ASTROPHYSICS: EARLY STARS AND THE BIG BANG
The following points are made by Roger Cayrel (Nature 2005 434:838):=20
1) Identifying stars born at the beginning of the era of stellar =
formation proved a long and frustrating task. In 2002, however, =
Christlieb et al [1] reported the discovery of the first such "relic of =
the dawn of time". New work [2] announces the discovery of a second. But =
why are these two objects so remarkable? And why does it help to have =
two instead of one?
2) Some 13.7 billion years ago, the Universe was much simpler than it is =
today. It consisted of a uniform, hot gas showing only small =
fluctuations in temperature and density, and containing no large =
structures -- no galaxies, stars or planets. For the first 15 minutes of =
its existence, the temperature and density of this hot gas were high =
enough to allow the nuclear reactions necessary for the production of =
the lightest chemical elements. Heavier elements, such as the metals, =
were not produced in this first flurry of nucleosynthetic activity. =
After the first 15 minutes, the Universe's rapid expansion put an end to =
conditions that favored nucleosynthesis and nothing more happened to the =
nuclear composition of the Universe for about 200 million years. The =
ingredients of this frozen primordial soup -- principally, the light =
elements lithium and helium, as well as deuterium, a heavy isotope of =
hydrogen -- are fairly well known from both theory and experiment[3].
3) A second opportunity for nuclear activity arose only when the =
original fluctuations of the early Universe had grown sufficiently large =
for haloes of dark matter to begin to form. This triggered gravitational =
instabilities and the collapse of conventional baryonic matter into =
clouds of gas, from which stars then formed [4]; in the cores of these =
stars, both the temperature and density reached values that again made =
nuclear reactions possible. Practically all the heavier elements, from =
carbon to uranium -- the elements from which later solid planets and =
organic life formed -- were synthesized in these first stars.
4) The search for stars with a composition reflecting that of these =
first stars has been going on for the past 25 years [5]. Before the =
discovery of the two "relics" (HE0107-5240 by Christlieb et al [1] and =
HE1327-2326 by Frebel et al .[2]), the lowest proportion, with respect =
to hydrogen, of stellar-made elements in the oldest stars was about a =
ten-thousandth of that observed in the Sun -- a tiny amount, but, =
crucially, not zero. This finding seemed to support the theory that =
matter from the Big Bang was unable to fragment into stellar masses that =
were small enough for those stars still to be shining today: massive =
stars burn their fuel faster, and a star with a mass greater than around =
nine-tenths of the mass of the Sun would have exhausted its nuclear =
energy supply by now. Long-lived stars could thus be born only from =
interstellar gas already enriched in products of nucleosynthesis that =
had been expelled at the end of the evolution of earlier stars -- either =
through a violent event such as a supernova, or through less dramatic =
mass loss, as is caused for example by stellar winds. If true, this =
would have ruled out any hope of finding a star with the primordial mix.
5) The discovery of HE0107-5240 and HE1327-2326, which have iron =
abundances respectively 200,000 and 300,000 times smaller than that of =
the Sun, is therefore of great significance. However, despite their =
impressively low iron content -- well below the previous record, which =
stood for some 20 years before their discovery -- both stars contain a =
proportion of carbon that is only 25 times smaller than that of the Sun. =
And the deficiency of the various elements between carbon and iron in =
the periodic table increases steadily with increasing atomic number.
References (abridged):
1. Christlieb, N. et al. Nature 419, 904-906 (2002)
2. Frebel, A. et al. Nature 434, 871-873 (2005)
3. Coc, A., Vangioni-Flam, E., Descouvemont, P., Adahchour, A. & Angulo, =
C. Astrophys. J. 600, 544-552 (2004)
4. Bromm, V. & Larson, R. B. Annu. Rev. Astron. Astrophys. 42, 79-118 =
(2004)
5. Beers, T. C., Preston, G. W. & Shectman, S. A. Astron. J. 103, =
1987-2034 (1992)
From ScienceWeek
http://scienceweek.com/2005/sw050603-1.htm
Comment:
The carbon abundance in these stars is a problem for the theory. Many =
more such stars need to be located before a statistical variation can be =
ruled out ie even rare, unusual or unexpected star types occur with some =
frequency (probability of occurrence >0). Searching for stars of a =
particular type at a particular distance does not preclude stars of the =
that type being found at some other distance eg much closer.
--=20
Posted by
Robert Karl Stonjek
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<DIV>ASTROPHYSICS: EARLY STARS AND THE BIG BANG</DIV>
<DIV> </DIV>
<DIV>The following points are made by Roger Cayrel (Nature 2005 =
434:838): </DIV>
<DIV> </DIV>
<DIV>1) Identifying stars born at the beginning of the era of stellar =
formation=20
proved a long and frustrating task. In 2002, however, Christlieb et al =
[1]=20
reported the discovery of the first such "relic of the dawn of time". =
New work=20
[2] announces the discovery of a second. But why are these two objects =
so=20
remarkable? And why does it help to have two instead of one?</DIV>
<DIV> </DIV>
<DIV>2) Some 13.7 billion years ago, the Universe was much simpler than =
it is=20
today. It consisted of a uniform, hot gas showing only small =
fluctuations in=20
temperature and density, and containing no large structures -- no =
galaxies,=20
stars or planets. For the first 15 minutes of its existence, the =
temperature and=20
density of this hot gas were high enough to allow the nuclear reactions=20
necessary for the production of the lightest chemical elements. Heavier=20
elements, such as the metals, were not produced in this first flurry of=20
nucleosynthetic activity. After the first 15 minutes, the Universe's =
rapid=20
expansion put an end to conditions that favored nucleosynthesis and =
nothing more=20
happened to the nuclear composition of the Universe for about 200 =
million years.=20
The ingredients of this frozen primordial soup -- principally, the light =
elements lithium and helium, as well as deuterium, a heavy isotope of =
hydrogen=20
-- are fairly well known from both theory and experiment[3].</DIV>
<DIV> </DIV>
<DIV>3) A second opportunity for nuclear activity arose only when the =
original=20
fluctuations of the early Universe had grown sufficiently large for =
haloes of=20
dark matter to begin to form. This triggered gravitational instabilities =
and the=20
collapse of conventional baryonic matter into clouds of gas, from which =
stars=20
then formed [4]; in the cores of these stars, both the temperature and =
density=20
reached values that again made nuclear reactions possible. Practically =
all the=20
heavier elements, from carbon to uranium -- the elements from which =
later solid=20
planets and organic life formed -- were synthesized in these first =
stars.</DIV>
<DIV> </DIV>
<DIV>4) The search for stars with a composition reflecting that of these =
first=20
stars has been going on for the past 25 years [5]. Before the discovery =
of the=20
two "relics" (HE0107-5240 by Christlieb et al [1] and HE1327-2326 by =
Frebel et=20
al .[2]), the lowest proportion, with respect to hydrogen, of =
stellar-made=20
elements in the oldest stars was about a ten-thousandth of that observed =
in the=20
Sun -- a tiny amount, but, crucially, not zero. This finding seemed to =
support=20
the theory that matter from the Big Bang was unable to fragment into =
stellar=20
masses that were small enough for those stars still to be shining today: =
massive=20
stars burn their fuel faster, and a star with a mass greater than around =
nine-tenths of the mass of the Sun would have exhausted its nuclear =
energy=20
supply by now. Long-lived stars could thus be born only from =
interstellar gas=20
already enriched in products of nucleosynthesis that had been expelled =
at the=20
end of the evolution of earlier stars -- either through a violent event =
such as=20
a supernova, or through less dramatic mass loss, as is caused for =
example by=20
stellar winds. If true, this would have ruled out any hope of finding a =
star=20
with the primordial mix.</DIV>
<DIV> </DIV>
<DIV>5) The discovery of HE0107-5240 and HE1327-2326, which have iron =
abundances=20
respectively 200,000 and 300,000 times smaller than that of the Sun, is=20
therefore of great significance. However, despite their impressively low =
iron=20
content -- well below the previous record, which stood for some 20 years =
before=20
their discovery -- both stars contain a proportion of carbon that is =
only 25=20
times smaller than that of the Sun. And the deficiency of the various =
elements=20
between carbon and iron in the periodic table increases steadily with =
increasing=20
atomic number.</DIV>
<DIV> </DIV>
<DIV>References (abridged):</DIV>
<DIV> </DIV>
<DIV>1. Christlieb, N. et al. Nature 419, 904-906 (2002)</DIV>
<DIV> </DIV>
<DIV>2. Frebel, A. et al. Nature 434, 871-873 (2005)</DIV>
<DIV> </DIV>
<DIV>3. Coc, A., Vangioni-Flam, E., Descouvemont, P., Adahchour, A. =
&=20
Angulo, C. Astrophys. J. 600, 544-552 (2004)</DIV>
<DIV> </DIV>
<DIV>4. Bromm, V. & Larson, R. B. Annu. Rev. Astron. Astrophys. 42, =
79-118=20
(2004)</DIV>
<DIV> </DIV>
<DIV>5. Beers, T. C., Preston, G. W. & Shectman, S. A. Astron. J. =
103,=20
1987-2034 (1992)</DIV>
<DIV> </DIV>
<DIV>From ScienceWeek<BR><A=20
href=3D"http://scienceweek.com/2005/sw050603-1.htm">http://scienceweek.co=
m/2005/sw050603-1.htm</A></DIV>
<DIV> </DIV>
<DIV>Comment:<BR>The carbon abundance in these stars is a problem for =
the=20
theory. Many more such stars need to be located before a =
statistical=20
variation can be ruled out ie even rare, unusual or unexpected star =
types occur=20
with some frequency (probability of occurrence >0). Searching =
for stars=20
of a particular type at a particular distance does not preclude stars of =
the=20
that type being found at some other distance eg much closer.</DIV>
<DIV><BR>-- <BR>Posted by<BR>Robert Karl Stonjek<BR></DIV></BODY></HTML>
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