Turkish Journal of Physics




For decades large-scale cosmic structures have been modeled as the gravitational amplification of small-density perturbations of the cosmological recombination epoch of the Big Bang. In astrophysics, cosmological nucleosynthesis is considered responsible for the production of the pristine gas, which should be found in the first-generation stars in the form of hydrogen and helium as the main constituents. In the later type of second-generation stars, hydrogen is converted into helium by the CN-cycle reactions, in which heavier elements are produced. These elements are believed to enrich the intergalactic medium by possible star bursts at the last stages of evolution. Stability criteria in the stellar evolutionary models pointed out that first-generation stars should be massive and live long enough for the nucleosynthesis of the natural elements, heavier than hydrogen and helium. Initially, they were expected to be very faint and blue to be observed spectroscopically. Nowadays, more and more metal-deficient star observations, made possible by the new era of space telescopes, are interpreted as the discovery of a primordial footprint of the initially pure gas. These new data are combined with the astrophysical models to review the predictability, mass, and chemical composition with regard to stability and existence of the first-generation stars.

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