metallicity

Metallicity in its general sense is the ratio of metals (metals using astrophysics' odd definition: all elements except hydrogen and helium) in an astronomical object as compared to the whole, i.e., compared to metals and non-metals combined. Stars are basically hydrogen and helium, and the small amount of other elements in a star is of high interest; this is also generally true of globular clusters, galaxies, galaxy clusters, molecular clouds, and nearly everything other than rocky planets. Since heavier elements have been synthesized over the course of the universe's lifetime (through fusion associated with stars), and the universe was nearly free of metals immediately after the Big Bang nucleosynthesis, metallicity reflects an object's age, history, or genesis. A main sequence star with high metallicity formed from an interstellar medium that included metals from stellar wind and supernovae of previous stars that produced metals during their post-main-sequence phase, and a galaxy with high metallicity, at minimum, must include portions old enough to have gained that metallicity.

Metallicity can be quantified by the mass fraction of metals to all elements, typically indicated by Z. Similarly, X is used for the mass fraction of hydrogen to all elements and Y similarly for helium, thus X + Y + Z = 1. The Sun's Z value is still under study but is around 0.02.

Metallicity is also often quantified as an abundance ratio (chemical abundance ratio) a ratio of number counts of atoms, using a particular notation (bracket notation) that quantifies such a ratio in terms of the analogous ratio in the Sun; such a metal abundance is indicated by:

[M/H] = log10(Nmetals/NH)body - log10(Nmetals/NH)Sun

where N_metals is the number of metal atoms and N_H is the number of hydrogen atoms. As such, "[M/H] = 0" means "same metal abundance as the Sun". As a proxy for this, the abundance of iron (the ratio [Fe/H]) has often been used, since iron produces revealing spectral lines, and for stars, galaxies, etc. [Fe/H] is sometimes actually cited as the object's metallicity.

While iron abundance has been a common proxy for metallicity, oxygen, a more abundant metal, is also used now, often using an absolute scale sometimes called the 12 scale: an absolute abundance scale (i.e., not scaled to the Sun) which is the dex of the ratio of (in this case) oxygen atoms to each 10¹² hydrogen atoms, often cited as ε_O or (incompatibly) log ε(O).

Stars can be categorized into three groups according to metallicity, known as stellar populations:

• Population I (Pop I, metal-rich population, or MPC) - metal-rich stars - recent stars, e.g., the Sun.
• Population II (Pop II, metal-poor population, or MPP) - metal-poor stars) - such as older Milky-Way stars.
• Population III - metal-free stars, the universe's presumed original stars, with virtually no metallicity.

The descriptions metal-rich (MR) and metal-poor (MP) are common. Also terms such as metal-rich cluster (MRC) metal-poor cluster (MPC) for stellar clusters.

It is generally accepted that there exists a age-metallicity relation (AMR) for stars and groups of coeval stars such as galactic clusters, i.e., some correlation between the age of the star (or group of stars) and its metallicity

In a gas, metallicity affects optical thickness: at a given mass density, the higher the metallicity, the optically thinner.

The abundances of other metals in stars, etc., are often quanitifed as their abundance ratios 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. A common distinction in the specifics of a star's metals is whether the material is alpha rich or alpha poor (greater or lesser amount of alpha elements), a clue to the type of past supernova that produced the star's material, which in turn, provides clues to the galaxy's past history.

The phrase bulk metallicity has been used to refer to:

• The overall metallicity of a star, cloud, or galaxy, etc.
• The average metallicity of a group or class of stars, or clouds, etc.
• The overall metallicity of a planet, or a planet's atmosphere.

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.