In astronomy and physical cosmology, the metallicity of an object is the proportion of its matter made up of chemical elements other than hydrogen and helium. Since stars, which comprise most of the visible matter in the universe, are composed mostly of hydrogen and helium, astronomers, for convenience's sake, use 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 nonmetals in conventional 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. Normal 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.
While 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 involves proposing the existence of Population III stars. Without metals, it is believed that only incredibly massive stars could be formed, and near the end of their lives, created the first 26 elements up to iron in the periodic table via nucleosynthesis. Because of their high mass, the current model is that they subsequently all died in spectacular supernovae that completely dispersed their material, ejecting those elements throughout the universe, and forming later generations of stars, with heavy elements, that we see today. The high mass of the first stars is used to explain why, as of 2007, 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 far away 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.
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 centre and decreases as one moves outwards. The gradient in metallicity is attributed to the density of stars in the galactic centre: 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 of about forty per cent of the Milky Way, while the Small Magellanic Cloud has a metallicity of about ten per cent of the Milky Way.
where and is the number of iron and hydrogen atoms per unit of volume respectively. By this formulation, stars with a higher metallicity than the Sun have a positive logarithmic value, while those with a lower metallicity than the Sun have a negative value. The logarithm is based on powers of ten; stars with a value of +1 have ten times the metallicity of the Sun (101), while those with +2 have a hundred (10²) and those with +3 have a thousand (10³). Conversely, those with a value of -1 have one tenth (10 -1), while 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 (since 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.
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, since 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.
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 CS22892-052, CS31082-001, BD +17° 3248) and two of the oldest stars known to date: HE0107-5240 and HE1327- 2326.
Population III or metal-free stars (they contained metals at the very end of their lifetimes - they are said to be metal-free because the metals exist in the core and are unobservable) are a hypothetical population of extremely massive and hot stars with virtually no metal content, 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. They have not yet been observed directly, but indirect evidence for their existence has been found in a gravitationally lensed galaxy in the very distant universe. They are also thought to be components of faint blue galaxies. Their existence is necessary 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.
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 from the Big Bang, it was easy to form stars with much more 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 low-metallicity Population II stars, which are thought to contain the metals produced by Population III stars, suggest that these metal-free stars had masses of 10 to 100 solar masses instead. This also explains why there have been no low-mass stars with zero metallicity observed. Confirmation of these theories awaits the launch of NASA's James Webb Space Telescope. New spectroscopic surveys, such as SEGUE or SDSS-II, may also locate Population III stars.
The most massive star that can form today is about 110 solar masses with a possible extreme maximum upper limit of 150 solar masses; a more massive protostar would blow itself apart during the initial ignition of nuclear reactions. Without enough carbon, oxygen, and nitrogen in the core, however, the CNO cycle could not begin and the star would not destroy itself so readily. Direct fusion through the proton-proton chain does not proceed quickly enough to produce the copious amounts of energy such a star would need to support its immense bulk. The end result would be the star collapsing into a black hole without ever actually shining properly. This is why astronomers consider Population III to be something of a mystery—by all rights they should not exist, yet they are necessary for an explanation of the quasar observations.
If these stars were able to form properly, their lifespan would be extremely short, certainly less than one million years. As they can no longer form today, viewing one would require us to look to the very edges of the observable universe, since the time it takes light to reach Earth from great distances is extremely great, it is possible to see "back in time" by looking farther away. Seeing to this distance while still being able to resolve a star could prove difficult, even for the James Webb Space Telescope.
Volker Bromm, Richard B. Larson (2004), THE FIRST STARS, Annual Reviews of Astronomy and Astrophysics, vol. 42, pp. 79–118.