Population II star
setÃ¢re-ye porineš-e II
Fr.: étoiles de population II
A member of a population of relatively old stars, containing a small fraction of → metals, found mainly in the → halo of the Galaxy and in → globular clusters.
→ population; II, Roman number 2; → star.
Population III star
setâre-ye porineš-e III
Fr.: étoile de population III
A member of the first generation of stars, formed out of pristine gas, enriched by → primordial nucleosynthesis alone. The material from which these stars formed consisted mostly of hydrogen and helium. Because neutral hydrogen clouds were free of dust, their cooling mechanism was drastically ineffective. As a result, these star forming clouds had a much higher temperature than in the present epoch, and their → Jeans mass was much higher. Therefore, these first generation of stars were principally massive, with a typical mass scale of order of about 100 Msun. Population III stars started forming about 300 million years after the → Big Bang at → redshifts between 50 and 6, when the Universe had between 1 and 5% of its present age. These stars were probably responsible for the → reionization of the Universe. Given their high mass, they lived only a few million years ending with either a → pair-instability supernova phase or a direct collapse to a → black hole. Population III stars thus initiated the chemical enrichment of the Universe and opened the way to more normal modes of star formation, namely → Population II. Some models predict a bimodal → initial mass function for the first stars, allowing also for solar mass stars. See also → extremely metal-poor star.
→ population; III, Roman number 3; → star.