In astronomy and physical cosmology, the metallicity (also designated Z) of an object is the proportion of its matter made up of chemical elements other than hydrogen and helium. Because stars, which comprise most of the visible matter in the universe, are composed mostly of hydrogen and helium, astronomers use for convenience the blanket term "metal" to describe all other elements collectively. Thus, a nebula 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 definition of "metal"; metallic bonds are impossible within stars, and the very strongest chemical bonds are only possible in the outer layers of cool K and M stars. Earth-like chemistry therefore has little or no relevance in stellar interiors.
The metallicity of an astronomical object may provide an indication of its age. When the universe first formed, according to the Big Bang theory, it consisted almost entirely of hydrogen which, through primordial nucleosynthesis, created a sizeable proportion of helium and only trace amounts of lithium and beryllium and no heavier elements. Therefore, older stars have lower metallicities than younger stars such as our Sun.
In most astronomical systems, hydrogen and helium are the dominant elements. The hydrogen mass fraction is generally expressed as where is the total mass of the system and the 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 are categorized as I, II, and III, with each group having decreasing metal content and increasing age. The populations were named in the order they were discovered, which is the reverse of the order of their formation. Thus, the first stars in the universe (low metal content) were population III, and recent stars (high metallicity) are population I.
Although older stars do have fewer heavy elements, the fact that all stars observed have some heavier elements poses something of a puzzle, and the current explanation for this proposes the existence of hypothetical metal-free Population III stars in the early universe. Soon after the Big Bang, without metals, it is believed that only stars with masses hundreds of times that of the Sun could be formed; near the end of their lives these stars would have created the first 26 elements up to iron in the periodic table via nucleosynthesis.
Because of their high mass, current stellar models show that Population III stars would have soon exhausted their fuel and exploded in extremely energetic pair-instability supernovae. Those explosions would have thoroughly dispersed their material, ejecting metals throughout the universe to be incorporated into the later generations of stars that are observed today. The high mass of the first stars is used to explain why, as of 2010[update], no Population III stars have been observed. Because they were all destroyed in supernovae in the early universe, Population III stars should only be seen in faraway galaxies whose light originated much earlier in the history of the universe, and searching for these stars or establishing their nonexistence (thereby invalidating the current model) is an active area of research in astronomy. Stars too massive to produce pair-instability supernovae would have collapsed into black holes through a process known as photodisintegration, but some matter escapes during this process in the form of relativistic jets, and this could have "sprayed" the first metals into the universe. Though Population III stars have been and remain the goal of a number of searches for such stars, none have been definitely identified.
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 next generation of stars was born out of those materials left by the death of the first. 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.
Across the Milky Way, metallicity is higher in the galactic center and decreases as one moves outwards. The gradient in metallicity is attributed to the density of stars in the galactic center: there are more stars in the centre of the galaxy and so, over time, more metals have been returned to the interstellar medium and incorporated into new stars. By a similar mechanism, larger galaxies tend to have a higher metallicity than their smaller counterparts. In the case of the Magellanic Clouds, two small irregular galaxies orbiting the Milky Way, the Large Magellanic Cloud has a metallicity about forty percent that of the Milky Way, whereas the Small Magellanic Cloud has a metallicity about ten per cent that of the Milky Way. Of the stars we see nearby in our Milky Way around us, Population II stars are rare, and Population I stars make up the vast majority of visible stars bright enough to see with the unaided eye. The globular star clusters in the Milky Way are the most prominent representatives of Population II.
Population I stars
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. 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.
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 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 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 (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. Caffau's star has been identified as the most metal-poor star found as of 2012[update], initially highlighted in an automated search of Sloan Digital Sky 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 metal-free stars, are a hypothetical extinct population of extremely massive and hot stars with virtually no surface metals, except for a small quantity of metals formed in the Big Bang, such as lithium-7. These stars are believed to have been formed in the early universe. 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. 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 MACHO's through gravitational microlensing have produced negative results.
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