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MetallicityIn astronomy, the metallicity of an object is the proportion of its matter made up of 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, it consisted almost entirely of hydrogen and helium (with only trace amounts of lithium), and so the oldest stars have very low metallicities. As the age of the universe increases, so does its metal content, due to nucleosynthesis in stars, and the return of metal-enriched material to the interstellar medium (ISM) via planetary nebulae and supernovae. The sun's metallicity is approximately 1.6 per cent by mass. For other stars in the galaxy, the metallicity is often expressed as Fe/H, which represents the logarithm of the ratio of the star's iron abundance to that of the sun's. 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 <-6.0. Currently, no Population III stars have been found. Across the galaxy, metallicity is higher in the centre and decreases moving outwards. This is because there are more stars in the centre of the galaxy and so over its lifetime, more metals have been returned to the ISM. Similarly, 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 galactic value, while the Small Magellanic Cloud has a metallicity about 10 per cent of the galactic value.
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