Metallicity

In astronomy, the metallicity of an object is the proportion of its matter made up of chemical elements other than hydrogen and helium. All heavier elements are described in astronomy as "metals".

The metallicity of an object can give an indication of its age. When the universe first formed (according to the big bang), it consisted almost entirely of hydrogen which, through primordial nucleosynthesis, created a sizeable proportion of helium and only trace amounts of lithium. As a result, the oldest stars have very low metallicities. As the age of the universe increases, so does its metal content and consequently the metallicty of stars, due to nucleosynthesis in stars, the return of metal-enriched material to the interstellar medium (ISM) via planetary nebulae and supernovae, and the consequent incorporation of that enriched material in new stars through star formation.

The metallicity of the Sun is approximately 1.6 per cent by mass. For other stars, the metallicity is often expressed as ^*, which represents the logarithm of the ratio of a star's iron abundance compared to that of the Sun. Young Population I stars, like the Sun, have significantly higher metallicities than older Population II stars, which formed when there was a lower metal content in the universe. The very first stars, referred to as Population III, are estimated to have a metallicity of less that −6.0 (that is, less than a millionth of the abundance of iron that is found in the Sun). As of 2005, no Population III stars have been found.

Across the Milky Way, metallicity is higher in the galactic centre and decreases moving 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 the lifetime of the galaxy, more metals have been returned to the ISM and incorporated into new stars. By a similar mechanism, larger galaxies tend to have higher metallicities than smaller ones. In the case of the Magellanic Clouds, two small irregular galaxies orbiting the Milky Way, the Large Magellanic Cloud has a metallicity about 40 per cent of the Milky Way, while the Small Magellanic Cloud has a metallicity about 10 per cent of the Milky Way.

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See also