Metallicity is a term in astronomy that refers to the proportion of elements in an astronomical object (usually a star) that are other than hydrogen or helium. In astronomy, all elements heavier then hydrogen and helium are collectively referred to with the blanket term "metals". Therefore, a star or a nebula that contained significant amounts of elements such as carbon, oxygen, or nitrogen would be considered "metal rich" even though those same elements are classified as nonmetals in chemistry.
Usually, metallicity is expressed in terms of the relative amounts of iron and hydrogen present in a star, since iron is among the easiest elements to measure with spectral data in the visible spectrum. This is determined by analyzing absorption lines in a stellar spectrum using a spectrometer, compared to the star's solar value. The ratio of the amount of iron to the amount of hydrogen in the object is divided by the ratio of the amount of iron to the amount of hydrogen in the Sun (which has a metallicity of 1.6 percent by mass). This value, expressed as [Fe/H], is calculated using the following logarithmic formula.
Where NFe and NH is the number of iron and hydrogen atoms per unit of volume respectively. Using this formula, a star that has a higher metallicity then our Sun will have a positive logarithmic value, while any star with a lower metallicity then our sun will result in a negative value. Because the logarithm is based on powers of ten; a star with a value of [Fe/H] = -1 will have an abundance of heavy elements (metallicity) one tenth of that of our sun, while a value of [Fe/H] =+1 represents a star with ten times the abundance of heavy elements of our sun. Measurements of thousands of stars have resulted in a generally established range from -4 (extremely metal-poor) to +1 (very metal-rich).
Types of stars according to Metallicity
Population I stars are stars that are considered to be the most metal-rich (or highest metallicity), our sun falls into this category. Although such stars may be as metal-poor as >-1 in comparison to the sun, many have more metallicity then our own sun. Such stars tend to be found either in, or near the plane of a spiral galaxy such as our own Milky Way. Some of these stars are referred to as Extreme Population I stars, found in the spiral arms, class O and B stars, T Tauri stars and other stars just entering the main sequence of stellar evolution are counted among these stars.
Population II stars are metal-poor stars (negative values beyond -1), usually located in the bulge near to the center of spiral galaxies as well as the galactic halo. Such stars are also common in Globular Clusters and make up the overwhelming number of the stellar population in elliptical galaxies. These stars are the reasoned source of most of the elements in the periodic table.
Population III stars are essentially metal-free stars, although some have metals at the end of their lives. Population III stars were considered to be extremely massive and hot but have not been directly observed. Indirect evidence of their existence has been found through looking at gravitationally lensed, and distant faint blue galaxies in the very distant universe.