ScienceWeek

ASTROPHYSICS: 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

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ASTROPHYSICS: EARLY STARS AND THE BIG BANG

The following points are made by Roger Cayrel (Nature 2005 434:838):

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)

Nature http://www.nature.com/nature

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ASTROPHYSICS: STAR FORMATION HISTORY OF THE UNIVERSE

The following points are made by Alan Heavens (American Scientist 2005 93:36):

1) More than 9000 billion billion (9 10^(21)) stars have been formed in the observable Universe since it began 13.7 billion years ago. Despite the apparent wealth of stars in the sky, current cosmological models suggest that the Universe was quite dark for much of its first billion years. During these dark ages, the Universe contained clouds of gas and dark matter, but little else -- the first stars did not form until several hundred million years had passed. Once the cosmic star-making machinery got going it seems to have churned out stars at a prodigious rate.

2) Differing rates of star formation provide clues about the physical circumstances in which star birth takes place. These "physical circumstances" are, of course, the galaxies, and the rate at which stars were made is intimately related to how the galaxies were formed.

3) Ever since 1917, when Vesto Slipher (1875-1969) noticed that the spectral lines of a galaxy's light emissions are routinely shifted toward longer ("redder") wavelengths, we have had firm evidence that the Universe is expanding. The nature of this expansion depends on how much matter the Universe contains, and cosmologists have been trying to determine its density for decades now. Recent observations of exploding stars, the large-scale structure of the Universe and the fireball radiation left over from the Big Bang -- called the "cosmic microwave background" -- have shown us that the universe does not have enough matter to stop the expansion. Furthermore, the matter it does contain appears to be made of some rather strange stuff. Only 4 percent of the density is made up of ordinary matter --neutrons, protons, electrons and so on -- the kind that makes up people, planets and pixels. Another 23 percent is dark matter, an extraordinary and unknown material that is not seen on Earth. And 73 percent is in dark energy, an even more bizarre substance that has a "repulsive gravity" that is causing the Universe to accelerate its expansion.

4) Despite the admittedly mysterious circumstances, we know quite well in broad outline how the Universe formed the structures we see. Observations of the cosmic microwave background, which dates to when the Universe was only 300,000 years old, show that the Universe was not quite uniform in its early phases. These observations have been made with a number of ground-, balloon-and satellite-based experiments, most famously with the Cosmic Background Explorer in the early 1990s, and now at higher resolution with the Wilkinson Microwave Anisotropy Probe. These experiments revealed small irregularities in the density of the early Universe. The irregularities also happened to be unstable: Denser-than-average regions had slightly stronger gravity, and so pulled matter in to form "clumps." In this way dense objects formed over time through a combination of gentle accretion and the merger of smaller units.

5) Ironically, to form something hot like a star, gas has to cool first. Large clouds of gas can support themselves by pressure if they are hot, but if the gas cools the pressure is removed and the cloud collapses. On the smallest scales, the final stages of collapse will heat the cloud, and if the temperature reaches some millions of degrees, nuclear reactions may start and the object becomes a star. On large scales, we believe the process of collapse operates bottom-up: Relatively "small" objects (around a million times the mass of the sun) collapse first. As the dense gas cools, it fragments into small clouds that ultimately collapse into stars.

6) These systems of many stars are drawn together by gravity into larger systems, and these larger systems themselves may coalesce. And so the process goes, building larger systems in a hierarchical fashion, through small galaxies, large galaxies, galaxy groups and finally galaxy clusters and superclusters. Today the largest structures that have fully collapsed are clusters of galaxies containing dark matter, gas and star-filled galaxies that total some hundreds of thousands of billions of solar masses.[1-3]

References (abridged):

1. Heavens, A. F., B. D. Panter, R. Jimenez and J. S. Dunlop. 2004. The star-formation history of the Universe from the stellar populations of nearby galaxies. Nature 428:625-627

2. Jimenez, R., J. MacDonald, J. Dunlop, P. Padoan and J. Peacock, J. 2004. Synthetic stellar populations: Single stellar populations, stellar interior models and primordial proto-galaxies. Monthly Notices of the Royal Astronomical Society 349:240-254

3. Bunker, A. J., E. R. Stanway, R. S. Ellis, and R. G. McMahon. In press. The star formation rate of the Universe at z~6 from the Hubble Ultra-Deep Field. Monthly Notices of the Royal Astronomical Society

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