Metallicity - Population II Stars

Population II Stars

Population II, or metal-poor stars, are those with relatively little metal. The idea of a relatively small amount must be kept in perspective as even metal-rich astronomical objects contain low quantities of any element other than hydrogen or helium; metals constitute only a tiny percentage of the overall chemical makeup of the universe, even 13.7 billion years after the Big Bang. However, metal-poor objects are even more primitive. These objects formed during an earlier time of the universe. Intermediate Population II stars are common in the bulge near the centre of our galaxy; whereas Population II stars found in the galactic halo are older and thus more metal-poor. Globular clusters also contain high numbers of Population II stars. It is believed that Population II stars created all the other elements in the periodic table, except the more unstable ones. An interesting characteristic of Population II stars is that despite their lower overall metallicity, they often have a higher relative abundance of alpha elements (O, Si, Ne, etc.) compared to Population I stars; current theory suggests this is the result of Type II supernovae being more important contributors to the interstellar medium at the time of their formation, while Type Ia supernovae metal enrichment came later in the universe's evolution.

Scientists have targeted these oldest stars in several different surveys, including the HK objective-prism survey of Timothy C. Beers et al. and the Hamburg-ESO survey of Norbert Christlieb et al., originally started for faint quasars. Thus far, they have uncovered and studied in detail about ten very metal-poor stars (as Sneden's Star, Cayrel's Star, BD +17° 3248) and two of the oldest stars known to date and also the oldest star: HE0107-5240 and HE1327-2326 and HE 1523-0901. Less extreme in their metal deficiency, but nearer and brighter and hence longer known, are HD 122563 (a red giant) and HD 140283 (a subgiant).