Astrophysics (Index)About

metallicity

(Z)
(metal fraction of an object)

Metallicity in its general sense is the ratio of metals (metals as per astrophysics, meaning lithium and all heavier elements) in an astronomical object as compared to the whole, i.e., compared to metals and non-metals (hydrogen and helium) 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 galaxy with high metallicity has lived long enough to gain it and a main sequence star with high metallicity formed from gas that included metals from stellar wind and supernovae of previous stars that produced metals during their post-main-sequence phase.

The term metallicity is often used for a particular quantification of this amount, Z, which is defined to be the mass ratio (mass fraction) of metals to all elements. Similarly, X represents the mass ratio of hydrogen to all elements and Y similarly for helium, and X + Y + Z = 1. The Sun's Z value is still under study but is around 0.02. Metallicity is also often expressed as 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 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.

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 (not tied to the Sun) which is the dex of the ratio of (in this case,) oxygen atoms to each 1012 hydrogen atoms, often cited as εO or (incompatibly) log ε(O).

Stars can be categorized 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 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: 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. 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:

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.


(atoms,measure,metals)
Further reading:
https://en.wikipedia.org/wiki/Metallicity
https://dictionary.obspm.fr/index.php?showAll=1&formSearchTextfield=metallicity
https://www.daviddarling.info/encyclopedia/M/metallicity.html
https://meetings.iac.es/itn-gaia2013/media/Primas2.pdf
http://icc.dur.ac.uk/~tt/Lectures/Galaxies/TeX/lec/node27.html
http://hyperphysics.phy-astr.gsu.edu/hbase/Astro/nucsyn.html#c2
https://astronomy.swin.edu.au/cosmos/M/Metals
http://burro.case.edu/Academics/Astr222/Galaxy/Structure/metals.html

Referenced by pages:
47 Tucanae (47 Tuc)
age-metallicity relation (AMR)
age-velocity-dispersion relation (AVR)
alpha element
alpha-enhanced
asteroseismology
BaSTI
blue horizontal branch (BHB)
brown dwarf (BD)
chemodynamics
evolutionary track
extremely metal poor galaxy (XMPG)
G-dwarf problem
galactic archaeology
galactic bulge
galactic disk
galaxy age determination
galaxy strangulation
galaxy subgroup
giant planet
giant star
globular cluster (GC)
habitable zone (HZ)
Haro 29
Hayashi limit
Hayashi track
HBK
HD 133131
helium (He)
Honda-like star
horizontal branch (HB)
hot Jupiter (HJ)
I Zwicky 18 (I Zw 18)
inside-out growth
iron (Fe)
isochronal fitting
KMOS3D
Lambda Boötis star (λ Boo)
Lick indices
line blanketing
Lyman-Werner photon
M-type star (M)
main sequence fitting
main-sequence lifetime (MS lifetime)
mass fraction
metal
metallicity gradient
Milky Way (MW)
Milky Way chemical evolution
NGC 2363
NGC 346
open cluster (OC)
pair-instability supernova (PISN)
planet demographics
planet formation
Population I (Pop I)
Population II (Pop II)
Population III (Pop III)
post-main-sequence star
protogalaxy
red clump (RC)
red-giant branch (RGB)
RR Lyrae variable (RRL)
s-process
SBS 0335-052
silicon (Si)
SkyMapper Southern Survey (SMSS)
SMSS J2003-1142
spectral line designation
spectral signature
spectral type
SSP
stellar age determination
stellar association
stellar cluster (SC)
stellar halo
stellar kinematics
stellar parameter determination
stellar population
stellar structure
subdwarf (sd)
supermassive star (SMS)
synthetic photometry
thick disk
thin disk
Vega
velocity-metallicity relation
weak-line star
WISE 1534-1043
Wolf-Lundmark-Melotte (WLM)

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