ON THE FIRST GENERATION OF STARS
The following points are made by Timothy C. Beers (Science 2005 309:390):
1) The very first stars that formed after the Big Bang, some 13 to 14
billion years ago, are likely to have been quite massive and extremely
short-lived; no examples are expected to remain in the Universe today.
However, they may have left behind their "calling cards" by producing a
distinctive distribution of elements recorded in the atmospheres of
long-lived stars that formed just after these massive progenitors. Stars
that are extremely iron poor (hyper metal-poor stars) are believed to be
very old, and are thus possible candidates for second-generation stars.
2) Iwamoto et al. [1] have described a model that attempts to account for
the elemental abundances in two hyper metal-poor stars. The stars, HE
0107-5240 [2] and HE 1327-2326 [3], contain less than 1/100,000 of the iron
observed in the Sun. Furthermore, they are both greatly enhanced, relative
to the Sun, in the light elements carbon, nitrogen, and oxygen (for HE
0107-5240; studies of the oxygen abundance in HE 1327-2326 are under way);
these are the most important elements for the formation of life, at least of
the form with which we are familiar.
3) Star formation in the Milky Way and throughout the present Universe is
poorly understood. This is because it takes place in a complex environment
where one has to account for the effects of the elements produced by
previous generations of stars, the influence of magnetic fields, and star
formation-triggering events such as shocks from nearby supernovae. In the
very early Universe, the physics of star formation is thought to have been
much simpler, because only hydrogen, helium, and a small amount of lithium
were present; stars most likely formed via radiative cooling by molecules
involving these elements.
4) Modern computational models of early star formation predict that most
stars that formed in the early Universe were probably quite massive, on the
order of several hundred times the mass of the Sun. Such stars burn their
fuel extremely rapidly (within a few million years after their birth) and
then explode. Astronomers are uncertain which elements might form in these
very massive stars during their explosive death throes, but current
calculations indicate that they should eject large amounts of iron and only
small amounts of carbon [4,5].
References (abridged):
1. N. Iwamoto, H. Umeda, N. Tominaga, K. Nomoto, K. Maeda, Science 309, 451
(2005)
2. N. Christlieb et al., Nature 419, 904 (2005)
3. A. Frebel et al., Nature 434, 871 (2005)
4. H. Umeda, K. Nomoto, Astrophys. J. 565, 385 (2002)
5. A. Heger, S. E. Woosley, Astrophys. J. 567, 532 (2002)
Science http://www.sciencemag.org
From ScienceWeek
http://scienceweek.com/2005/sw050812-2.htm
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Posted by
Robert Karl Stonjek
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