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 of stars made by the German astronomer Walter Baade (c1944), indicating the younger stars, which are relatively rich in metals . They are found in the arms of spiral galaxies, associated with clouds of interstellar matter. The Sun is of this type.
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