In astronomy and physical cosmology, the metallicity of an object's the proportion of its matter made up of chemical elements other than hydrogen and helium. Since stars, which comprise most of the visible matter in the universe, are composed mostly of hydrogen and helium, astronomers, for convenience's sake, use the blanket term "metal" to describe all other elements collectively. Thus, a nebula rich in carbon, nitrogen, oxygen, and neon would be "metal rich" in astrophysical terms even though… (More on Metallicity)