In
astronomy and
physical cosmology, the
metallicity or
Z is the fraction of mass of a star or other kind of astronomical object that is not in
hydrogen (
X) or
helium (
Y). Most of the physical matter in the
universe is in the form of hydrogen and helium, so
astronomers use the word "metals" as a convenient short term for "all elements except hydrogen and helium". This usage is distinct from the usual physical definition of a solid
metal. The astronomical usage is claimed to be justified because in the high-temperature and pressure environment of a star, atoms do not undergo
chemical reactions and effectively have no
chemical properties, including that of being a metal as usually understood. For example,
stars and
nebulae with relatively high abundances of
carbon,
nitrogen,
oxygen, and
neon are called "metal-rich" in astrophysical terms, even though those elements are non-metals in chemistry.