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. (This terminology is used differently to the usual meaning of the word 'metal', since on the grandest of scales the universe is overwhelmingly composed of hydrogen and helium, astronomers label all the heavier elements "metal"). For example, a nebula rich in carbon compounds would be called "metal-rich", even though carbon is not considered a metal in other contexts.
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A grouping by Walter Baade , which contains the red giants, and refers to older stars, found principally in elliptical galaxies, the centres of spiral galaxies and in globular clusters. They are deficient in in metals .
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