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In astronomy and physical cosmology, the metallicity or Z, is the proportion of its matter making up the chemical elements in stars or other kinds of astronomical objects, excluding their hydrogen (X) and helium (Y). Most of the physical matter in the universe is in the form of hydrogen and helium, so astronomers conveniently use the blanket term "metal" to describe all other elements. For example, stars or nebulae that are relatively rich in carbon, nitrogen, oxygen, and neon would be "metal-rich" in astrophysical terms, even though those elements are non-metals in chemistry. This term should not be confused with the usual physical definition of solid metals. Metallicity in stars and other astronomical objects is an approximate estimation of their chemical abundances that change over time by the mechanisms of stellar evolution, and therefore provide an indication of age. In cosmological terms, the universe is also chemically evolving. According to the Big Bang Theory, the early universe first consisted of hydrogen and helium, with trace amounts of lithium and beryllium, but with no heavier elements. Through the process of stellar evolution, where stars at the end of their lives discard most of their mass by stellar winds or explode as supernovae, the metal content of the Galaxy and the Universe increases. It is then assumed, that older generations of stars generally have lower metallicities than the current younger generation stars. Observed changes in the chemical abundances of different types of stars, based on the spectral peculiarities that were later attributed to metallicity, lead astronomer Walter Baade in 1944 to propose the existence of different populations of stars in galaxies. These became commonly known as Population I and Population II stars. Another kind of stellar population was introduced in 1978, known as Population III stars, appearing for the first time in a paper title in 1980. These extremely metal poor stars were theorised to have been the first born stars created in the Universe.