|
In 1944, Walter Baade categorized groups of stars within the Milky Way from their spectra. Two main divisions were defined as Population I and II, with another division known as Population III added in 1978. Later these differences were shown to be significant, dividing them by their chemical composition or metallicity, the proportion of stellar matter made up of the "heavier chemical elements", beyond hydrogen and helium. By coincidence, each Population group definition has decreasing metal content and increasing age. Hence, the first stars in the universe (low metal content) were deemed Population III, and recent stars (high metallicity) are Population I. ==Stellar populations== Observation of the spectra of stars has revealed that the metallicity of older stars have fewer heavy elements compared to the Sun. This immediately suggests that metallicity has evolved through the generations of stars by the process of stellar evolution. On the current cosmological models, the matter created in the Big Bang was mostly hydrogen and helium, with only a very tiny fraction of light elements like lithium and beryllium. After this, when the universe cooled sufficiently, the first stars were born as extremely metal-poor Population III stars. Without metals, it is postulated that their stellar masses were hundreds of times that of the Sun. In turn, these massive stars evolved very quickly, and their nucleosynthetic processes quickly created the first 26 elements (up to iron in the periodic table). Current theoretical stellar models show that most high-mass Population III stars quickly exhausted their fuel and exploded in extremely energetic pair-instability supernovae. Those explosions would have thoroughly dispersed their material, ejecting metals into the interstellar medium (ISM), to be incorporated into the later generations of stars. Their destruction suggests that no galactic high-mass Population III stars should be observable. However, some Population III stars might be seen in high-redshift galaxies whose light originated during the earlier history of the universe. None have been discovered. Stars too massive to produce pair-instability supernovae would have collapsed into black holes through a process known as photodisintegration, but some matter may have escaped during this process in the form of relativistic jets, and this could have "sprayed" the first metals into the universe. It has been proposed that recent supernovae SN 2006gy and SN 2007bi may have been pair-instability supernovae in which such super-massive Population III stars exploded. It has been speculated that these stars could have formed relatively recently in dwarf galaxies containing primordial metal-free interstellar matter; past supernovae in these galaxies could have ejected their metal-rich contents at speeds high enough for them to escape the galaxy, keeping the metal content of the galaxy very low. The oldest observed stars, known as Population II, have very low metallicities; as subsequent generations of stars were born they became more metal-enriched, as the gaseous clouds from which they formed received the metal-rich dust manufactured by previous generations. As those stars died, they returned metal-enriched material to the interstellar medium via planetary nebulae and supernovae, enriching further the nebulae out of which the newer stars formed. These youngest stars, including the Sun, therefore have the highest metal content, and are known as Population I stars. ===Population I stars=== Population I, or metal-rich stars, are young stars with the highest metallicity out of all three populations. The Earth's Sun is an example of a metal-rich star. These are common in the spiral arms of the Milky Way galaxy. Generally, the youngest stars, the extreme Population I, are found farther toward the center of a galaxy, and intermediate Population I stars are farther out. The Sun is considered an intermediate Population I star. Population I stars have regular elliptical orbits of the galactic centre, with a low relative velocity. It was hypothesised that the high metallicity of Population I stars makes them more likely to possess planetary systems than the other two populations, because planets, particularly terrestrial planets, are thought to be formed by the accretion of metals. However, observations of the Kepler data-set have found smaller planets around stars with a range of metallicities, while only larger, potential gas giant planets are concentrated around stars with relatively higher metallicity - a finding that has implications for theories of gas giant formation.〔Buchhave, L.A. et al. (2012) An abundance of small exoplanets around stars with a wide range of metallicities. Nature 486:375–377〕 Between the intermediate Population I and the Population II stars comes the intermediary disc population. ===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 percentages of any element other than hydrogen or helium; metals constitute only a tiny percentage of the overall chemical makeup of the universe, even 13.8 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 I 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 ratio of alpha elements (O, Si, Ne, etc.) relative to Fe as 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, whereas Type Ia supernovae metal enrichment came later in the universe's evolution.〔Wolfe, Gawiser, Prochaska, "DAMPED Lyalpha SYSTEMS", Annu. Rev. Astron. Astrophys. 2005. 43: 861–918 http://ned.ipac.caltech.edu/level5/Sept05/Wolfe/Wolfe3.html〕 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 (such as Sneden's Star, Cayrel's Star, BD +17° 3248) and three of the oldest stars known to date: HE0107-5240, HE1327-2326 and HE 1523-0901. Caffau's star was identified as the most metal-poor star yet when it was found in 2012 using Sloan Digital Sky Survey data. However, in February 2014 the discovery of an even lower metallicity star was announced, SMSS J031300.36-670839.3 located with the aid of SkyMapper astronomical survey data. 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). 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「Stellar population」の詳細全文を読む スポンサード リンク
|