Stellar population

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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There are two approximate stellar populations, known as I and II. Population II stars are found in globular clusters,elliptical galaxies and also within the galactic nucleus of spiral galaxies. Population II stars are old stars with a low metal content. Population I stars are young stars with a high metal content and are found in the spiral arms of spiral galaxies.