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In astronomy and physical cosmology, the metallicity or ''Z'' is the fraction of mass of a star or other kind of astronomical object, beyond hydrogen (X) and helium (Y). Most of the physical matter in the universe is in the form of hydrogen and helium, so astronomers conveniently use the blanket term "metals" to refer to all other elements.〔 For example, stars or nebulae that are relatively rich in carbon, nitrogen, oxygen, and neon would be "metal-rich" in astrophysical terms, even though those elements are non-metals in chemistry. This term should not be confused with the usual physical definition of solid metals. Metallicity within stars and other astronomical objects is an approximate estimation of their chemical abundances that change over time by the mechanisms of stellar evolution,〔 〕 and therefore provide an indication of their age.〔 〕 In cosmological terms, the universe is also chemically evolving. According to the Big Bang Theory, the early universe first consisted of hydrogen and helium, with trace amounts of lithium and beryllium, but no heavier elements. Through the process of stellar evolution, where stars at the end of their lives discard most of their mass by stellar winds or explode as supernovae, the metal content of the Galaxy and the universe increases.〔 〕 It is postulated that older generations of stars generally have lower metallicities than those of younger generations.〔 〕 Observed changes in the chemical abundances of different types of stars, based on the spectral peculiarities that were later attributed to metallicity, led astronomer Walter Baade in 1944 to propose the existence of two different populations of stars.〔 〕 These became commonly known as Population I (metal-rich) and Population II (metal-poor) stars. A third stellar population was introduced in 1978, known as Population III stars.〔 〕〔 〕 These extremely metal-poor stars were theorised to have been the 'first-born' stars created in the universe. ==Definition== Stellar composition, as determined by spectroscopy, is usually simply defined by the parameters X, Y and Z. Here X is the fractional percentage of hydrogen, Y is the fractional percentage of helium, and all the remaining chemical elements as the fractional percentage, Z. It is simply defined as; : In most stars, nebulae and other astronomical sources, hydrogen and helium are the two dominant elements. The hydrogen mass fraction is generally expressed as where is the total mass of the system and the fractional mass of the hydrogen it contains. Similarly, the helium mass fraction is denoted as . The remainder of the elements are collectively referred to as 'metals', and the metallicity—the mass fraction of elements heavier than helium—can be calculated as : For the Sun, these parameters are often assumed to have the following approximate values, although recent research shows that lower values for might be more appropriate: : = 0.25 |- | Metallicity || |} The metallicity of many astronomical objects cannot be measured directly. Instead, proxies are used to obtain an indirect estimate. For example, an observer might measure the iron content of a galaxy (for example using the brightness of an iron emission line) directly, then compare that value with models to estimate the total metallicity. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「metallicity」の詳細全文を読む スポンサード リンク
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