Metal-poor
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Metal-poor is a term that is used to describe the chemical make up of an object. Typically the chemicals which are most often referenced in astronomy are hydrogen and helium, with all others being referred to as metals. In astronomy circles, a "metal" refers to any element heavier than helium. Metals make up only a small percentage of the chemical make up of the universe, even 13.7 billion years after the Big Bang.
Shortly after the Big Bang, the universe was comprised entirely of hydrogen, helium, and trace amounts of deuterium and lithium. These elements were created during the era of nucleosynthesis, (which lasted from .0001 - 3s after the Big Bang.) These primitive atomic structures made up the primordial material from which the first stars were born.
Metal-poor objects are those which contain relatively small amounts of the elements heavier than helium. The idea of a "relatively" small amount must be kept in mind because even metal-rich objects contain very small amounts of any element other than hydrogen; however, metal-poor objects are even more primitive. These objects formed during earlier times in the universe. The first stars, referred to as Population III, were incredibly massive and, during their lives, they created the elements up to iron in a process known as nucleosynthesis. These large stars had spectacular deaths through supernovae and created all the elements heavier than iron. This "polluted" the universe with metals.
The next generation of stars were born out of these materials left by the supernova outbursts. Thus, the most metal-poor objects were born the earliest. As subsequent generations of stars were born, they became more metal-enriched, as the gaseous clouds from which they formed because more metal-rich. The next generation of stars are known as Population II and are the earliest stars which have been directly observed. In the past couple years, a team of scientists have been targetting these oldest stars in the Hamburg-ESO survey. Thus far, they have uncovered two of the oldest stars known to date: HE0107-5240 and HE1327- 2326. Despite their incredibly low metal content, these stars are still a part of Population II.
Stars which are metal-poor are classified as such because they have a low metallicity. Metallicity is a measure of the amount of hydrogen to other elements. Iron is a very common element used to determine the metallicity of a star. Standard nomenclature often uses [Fe/H] which is a logarithmic ratio of the iron to hydrogen in a star compared with the iron to hyrdogen ratio of the Sun. Stars of a metallicity below -6 are considered Population III. Thus far, none have been found.