Astrophysics (Index)About


(metal fraction of an object)

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., both 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 reflects 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 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 0.02.

Metallicity 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 a 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, which shows spectral lines. 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 0.9 to 1.0 range.

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

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 an age-metallicity relation for stars and groups of coeval stars such as galactic clusters.

In a 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.

Further reading:

Referenced by pages:
blue horizontal branch (BHB)
brown dwarf (BD)
47 Tucanae (47 Tuc)
galactic archaeology
galactic bulge
galaxy age determination
galaxy strangulation
G-dwarf problem
giant planet
giant star
globular cluster (GC)
habitable zone (HZ)
Haro 29
Hayashi limit
Hayashi track
horizontal branch (HB)
HD 133131
helium (He)
hot Jupiter (HJ)
iron (Fe)
I Zwicky 18 (I Zw 18)
Lambda Boötis star (λ Boo)
Lick indices
line blanketing
Lyman-Werner photon
main sequence fitting
mass fraction
Milky Way
main-sequence lifetime (MS lifetime)
Milky Way chemical evolution
NGC 2363
NGC 346
planet demographics
planet formation
post-main-sequence star
red clump (RC)
red-giant branch (RGB)
RR Lyrae variable (RRL)
SBS 0335-052
silicon (Si)
supermassive star (SMS)
SkyMapper Southern Survey (SMSS)
spectral line designation
spectral signature
stellar age determination
stellar kinematics
stellar parameter determination
stellar population
stellar structure
subdwarf (sd)
synthetic photometry
velocity-metallicity relation
weak-line star
Wolf-Lundmark-Melotte (WLM)