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In 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, 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.
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 CloudThe Large Magellanic Cloud (also known as LMC) is a dwarf galaxy that is, in some sense, in orbit around our own Milky Way galaxy. It is at a distance of about fifty kiloparsecs (50,000 parsecs, or 160,000 light years). It has about 1/20 the diameter of o has a metallicity about 40 per cent of the galactic value, while the Small Magellanic CloudGlobular Cluster 47 Tucanae appears in the upper part of the image. The Small Magellanic Cloud SMC —also known as NGC 292 is an irregular galaxy in orbit around the Milky Way Galaxy. At a distance of about 200,000 light years, it is one of the Milky Way's has a metallicity about 10 per cent of the galactic value.
