In astronomy an 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 should not be confused with the usual meaning of the word 'metal'. Since on the grandest of scales normal (i.e. atomic) matter in the universe is composed overwhelmingly of hydrogen and helium, astronomers label all the heavier elements "metal".Metallic bonds are impossible within stars, and all but the strongest chemical bonds are impossible in all but the coolest layers of "cool" K and M stars, so normal chemistry has little or no relevance in stellar interiors. A nebula rich in carbon, nitrogen, oxygen, and neon would be "metal rich" in astrophysical terms even though those elements are nonmetals in conventional chemistry.