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In astronomy and physical cosmology, the metallicity or Z, is the proportion of its matter making up the chemical elements in stars or other kinds of astronomical objects, excluding their 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 "metal" to describe 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 in 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 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 with 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 then assumed, that older generations of stars generally have lower metallicities than the current younger generation stars.
Observed changes in the chemical abundances of different types of stars, based on the spectral peculiarities that were later attributed to metallicity, lead astronomer Walter Baade in 1944 to propose the existence of different populations of stars in galaxies. These became commonly known as Population I and Population II stars. Another kind of stellar population was introduced in 1978, known as Population III stars,  appearing for the first time in a paper title in 1980. These extremely metal poor stars were theorised to have been the first born stars created in the Universe.
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
Description Solar value Hydrogen mass fraction Helium mass fraction Metallicity
For many astronomical objects the metallicity 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.
The metallicity is often expressed as "[Fe/H]", which represents the logarithm of the ratio of a star's iron abundance compared to that of the Sun (iron is not the most abundant heavy element, but it is among the easiest to measure with spectral data in the visible spectrum). The formula for the logarithm is expressed thus:
where and are the number of iron and hydrogen atoms per unit of volume respectively. The unit often used for metallicity is the "dex" which is a (now-deprecated) contraction of 'decimal exponent'. By this formulation, stars with a higher metallicity than the Sun have a positive logarithmic value, whereas those with a lower metallicity than the Sun have a negative value. The logarithm is based on powers of 10; stars with a value of +1 have ten times the metallicity of the Sun (101). Conversely, those with a value of −1 have one tenth (10−1), whereas those with −2 have a hundredth (10−2), and so on. Young Population I stars have significantly higher iron-to-hydrogen ratios than older Population II stars. Primordial Population III stars are estimated to have a metallicity of less than −6.0, that is, less than a millionth of the abundance of iron which is found in the Sun.
This same sort of notation is used to express differences in the individual elements from the solar proportion. For example, the notation "[O/Fe]" represents the difference in the logarithm of the star's oxygen abundance compared to that of the Sun and the logarithm of the star's iron abundance compared to the Sun:
The point of this notation is that if a mass of gas is diluted with pure hydrogen, then its [Fe/H] value will decrease (because there are fewer iron atoms per hydrogen atom after the dilution), but for all other elements X, the [X/Fe] ratios will remain unchanged. By contrast, if a mass of gas is polluted with some amount of pure oxygen, then its [Fe/H] will remain unchanged but its [O/Fe] ratio will increase. In general, a given stellar nucleosynthetic process alters the proportions of only a few elements or isotopes, so a star or gas sample with nonzero [X/Fe] values may be showing the signature of particular nuclear processes.
Relation between Z and [Fe/H]
These two ways of expressing the metallic content of a star are related through the equation:
where [M/H] is the star's total metal abundance (i.e. all elements heavier than helium) defined as a more general expression than the one for [Fe/H]:
The iron abundance and the total metal abundance are often assumed to be related through a constant A as:
where A assumes values between 0.9 and 1. Using the formulas presented above, the relation between Z and [Fe/H] can finally be written as:
Stellar populations were categorized as I and II in 1944 by Walter Baade, then later in 1978, was added Population III. By coincidence, each group has decreasing metal content and increasing age. Hence the first stars in the Universe (low metal content) were population III, and recent stars (high metallicity) are population I.
Observation of the spectra of stars has revealed that the older stars have fewer heavy elements compared to the Sun. This 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 could be formed. In turn, these massive stars evolved very quickly, whose nucleosynthesis 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 no galactic high-mass Population III stars are observable. However, some Population III stars might be seen in high-redshift galaxies whose light originated during the earlier history of the Universe, remaining as a future area of astronomical research. 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 the nebulae out of which the newer stars formed ever further. 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 in and intermediate Population I stars are farther out, etc. 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.
Between the intermediate populations I and II 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.
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).
Population III stars
Population III, or extremely metal-poor stars (EMP), are a hypothetical extinct population of extremely massive and hot stars with virtually no metals, except possibly for intermixing ejecta from other nearby Pop III supernovae. Their existence is inferred from cosmology, but they have not yet been observed directly. Indirect evidence for their existence has been found in a gravitationally lensed galaxy in a very distant part of the Universe. They are also thought to be components of faint blue galaxies. Their existence is proposed to account for the fact that heavy elements, which could not have been created in the Big Bang, are observed in quasar emission spectra, as well as the existence of faint blue galaxies. It is believed that these stars triggered a period of reionization. UDFy-38135539, a galaxy recently discovered, is believed to have been a part of this process. Some theories hold that there were two generations of Population III stars.
Current theory is divided on whether the first stars were very massive or not - theories proposed in 2009 and 2011 suggest the first star groups might have consisted of a massive star surrounded by several smaller stars. One theory, which seems to be borne out by computer models of star formation, is that with no heavy elements and a much warmer interstellar medium from the Big Bang, it was easy to form stars with much greater total mass than the ones visible today. Typical masses for Population III stars would be expected to be about several hundred solar masses, which is much larger than the current stars. Analysis of data on extremely low-metallicity Population II stars such as HE0107-5240, which are thought to contain the metals produced by Population III stars, suggest that these metal-free stars had masses of 20 to 130 solar masses instead. On the other hand, analysis of globular clusters associated with elliptical galaxies suggests pair-instability supernovae, which are typically associated with very massive stars, were responsible for their metallic composition. This also explains why there have been no low-mass stars with zero metallicity observed, although models have been constructed for smaller Population III stars. Clusters containing zero-metallicity red dwarfs or brown dwarfs (possibly created by pair-instability supernovae) have been proposed as dark matter candidates, but searches for these and other MACHOs through gravitational microlensing have produced negative results.
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