Metallicity in its general sense is the amount of metals (as per astrophysics, lithium and all heavier elements) in an astronomical object as compared to the whole, i.e., metals plus non-metals, hydrogen and helium. Metallicity is of interest for stars, globular clusters, galaxies, galaxy clusters, molecular clouds, etc. Since heavier elements are synthesized over time, and the universe was nearly all hydrogen at the Big Bang, metallicity expresses an object's age, history, or genesis. A galaxy with high metallicity has lived long enough to gain it and a star with high metallicity may have been formed from gas from previous stars.
In the term metallicity's strictest sense, it is expressed by the letter Z which is defined to be the mass ratio (mass fraction) of metals to all elements, with X representing the mass ratio of hydrogen to all elements and Y similarly for helium, and X + Y + Z = 1.0. The Sun's Z value is still under study but is around .02.
Metallicity of other objects is often expressed differently, in what could be called metal abundance, which relates the ratio of the count of metal atoms to all atoms of the object to the same for the Sun. This is expressed in bracket notation:
[M/H] = log10(Nmetals/NH)body - log10(Nmetals/NH)Sun
where Nmetals is the number of metal atoms and NH is the number of hydrogen atoms. As such, "[M/H] = 0" means "same metal abundance as the Sun".
This is often approximated by measuring the abundance of a specific metal that can be measured in a practical manner, typically iron. Consequently, the abundance specifically of iron (the ratio [Fe/H]) is often used as a proxy to express the metallicity of stars, galaxies, etc. The metallicity Z can be approximated by multiplying [Fe/H] by a number in the .9 to 1.0 range.
Stars can be categorized are into three groups according to metallicity, known as stellar populations:
An age-metallicity relation for stars and galactic clusters, is generally accepted.
In gas, metallicity affects optical thickness: the higher the metallicity, the optically thinner.
The abundances of other metals in stars, etc., are often stated relative to iron, e.g., [Si/Fe] or [O/Fe]. When [Fe/H] is also established, abundances of these elements relative to hydrogen are evident. Oxygen is more common in higher mass stars and carbon in lower mass. A [C/O] of 1.0 is high.
The phrase bulk metallicity has been used to refer to:
Mass-metallicity relations have been observed for various types of objects: galaxy clusters, galaxies, stars, and giant planets. Metallicity is presumed to affect planet formation, i.e., providing solid material for rocky planets, and cores of gas planets. The resulting distribution could affect dynamics: how many planets are within a radius likely to result in impacts, or at a radius more likely to result in ejection from the planetary system.