how the last term calculated?
Astronomers use several different methods to describe and approximate metal abundances, depending on the available tools and the object of interest.
Metallicity
Astronomers use several different methods to describe and approximate metal abundances, depending on the available tools and the object of interest. 
how many globular clusters are in the milky way About 150 to 158 currently known what are these? It is a spherical collection of stars that orbits a galactic core. do they contain metal? It consists of Population II stars, to which astronomers refer to as heavier elements as metals and to the proportions of these elements as the metallicity.
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Metallicity [SEP] Common methods of calculation
Astronomers use several different methods to describe and approximate metal abundances, depending on the available tools and the object of interest. Some methods include determining the fraction of mass that is attributed to gas versus metals, or measuring the ratios of the number of atoms of two different elements as compared to the ratios found in the Sun.