View source for Metallicity

{{Short description|Relative abundance of heavy elements in a star or other astronomical object}}
{{for|metallic and nonmetallic compounds|Metal|Nonmetallic material}}
[[File:A Swarm of Ancient Stars - GPN-2000-000930.jpg|thumb|250px|right|The [[globular cluster]] [[Messier 80|M80]]. Stars in globular clusters are mainly older metal-poor members of [[population&nbsp;II]].]]
In [[astronomy]], '''metallicity''' is the [[Abundance of the chemical elements|abundance]] of [[Chemical element|elements]] present in an object that are heavier than [[hydrogen]] and [[helium]]. Most of the normal currently detectable (i.e. non-[[Dark matter|dark]]) [[matter]] in the universe is either hydrogen or helium, and [[astronomer]]s use the word ''"metals"'' as convenient shorthand for ''"all elements except hydrogen and helium"''. This word-use is distinct from the conventional chemical or physical definition of a [[metal]] as an electrically conducting solid. [[Star]]s and [[nebula]]e with relatively high abundances of heavier elements are called "metal-rich" when discussing metallicity, even though many of those elements are called [[Nonmetal (chemistry)|nonmetal]]s in chemistry.
{{TOC limit|2}}

==Metals in early spectroscopy==
[[Image:Fraunhofer lines.svg|thumb|upright=1.6|Solar spectrum with Fraunhofer lines as it appears visually.]]
In 1802, [[William Hyde Wollaston]]<ref name="eb">Melvyn C. Usselman: [http://www.britannica.com/EBchecked/topic/646649/William-Hyde-Wollaston William Hyde Wollaston] Encyclopædia Britannica, retrieved 31 March 2013</ref> noted the appearance of a number of dark features in the solar spectrum.<ref>William Hyde Wollaston (1802) [http://rstl.royalsocietypublishing.org/content/92/365.full.pdf+html "A method of examining refractive and dispersive powers, by prismatic reflection,"] ''Philosophical Transactions of the Royal Society'', '''92''': 365–380; see especially p. 378.</ref> In 1814, [[Joseph von Fraunhofer]] independently rediscovered the lines and began to systematically study and measure their [[wavelength]]s, and they are now called [[Fraunhofer lines]]. He mapped over 570 lines, designating the most prominent with the letters A through K and weaker lines with other letters.<ref>{{cite book|last=Hearnshaw|first=J.B.|title=The analysis of starlight|date=1986|publisher=[[Cambridge University Press]]|location=Cambridge|isbn=978-0-521-39916-6|page=27}}</ref><ref>Joseph Fraunhofer (1814 - 1815) [https://books.google.com/books?id=2-AAAAAAYAAJ&pg=PA203 "Bestimmung des Brechungs- und des Farben-Zerstreuungs - Vermögens verschiedener Glasarten, in Bezug auf die Vervollkommnung achromatischer Fernröhre"] (Determination of the refractive and color-dispersing power of different types of glass, in relation to the improvement of achromatic telescopes), ''Denkschriften der Königlichen Akademie der Wissenschaften zu München'' (Memoirs of the Royal Academy of Sciences in Munich), '''5''':  193–226; see especially pages 202–205 and the plate following page 226.</ref><ref>{{Cite book
  | last1 = Jenkins  | first1 = Francis A.
  | last2 = White   | first2 = Harvey E.
  | title = Fundamentals of Optics
  | url = https://archive.org/details/fundamentalsopti00jenk  | url-access = limited  | edition = 4th
  | publisher = [[McGraw-Hill]]
  | date = 1981
  | page = [https://archive.org/details/fundamentalsopti00jenk/page/n37 18]
  | isbn = 978-0-07-256191-3
  }}</ref>

About 45 years later, [[Gustav Kirchhoff]] and [[Robert Bunsen]]<ref>See:
*  Gustav Kirchhoff (1859) [https://books.google.com/books?id=CMgAAAAAYAAJ&pg=PA662"Ueber die Fraunhofer'schen Linien"] (On Fraunhofer's lines), ''Monatsbericht der Königlichen Preussische Akademie der Wissenschaften zu Berlin'' (Monthly report of the Royal Prussian Academy of Sciences in Berlin), 662–665.
*  Gustav Kirchhoff (1859) [https://books.google.com/books?id=uksDAAAAYAAJ&pg=RA1-PA251 "Ueber das Sonnenspektrum"] (On the sun's spectrum), ''Verhandlungen des naturhistorisch-medizinischen Vereins zu Heidelberg'' (Proceedings of the Natural History / Medical Association in Heidelberg), '''1''' (7) :  251–255.</ref> noticed that several Fraunhofer lines coincide with characteristic [[emission spectrum|emission lines]] identifies in the spectra of heated chemical elements.<ref>{{cite journal 
 |author= G. Kirchhoff
 |title=Ueber die Fraunhofer'schen Linien
 |journal=Annalen der Physik
 |volume=185 |issue=1 |pages=148–150 |date=1860
 |doi=10.1002/andp.18601850115|bibcode = 1860AnP...185..148K |url=https://zenodo.org/record/1423666
 }}</ref> They inferred that dark lines in the solar spectrum are caused by [[absorption (electromagnetic radiation)|absorption]] by [[chemical element]]s in the solar atmosphere.<ref>{{cite journal 
 |author= G. Kirchhoff
 |title=Ueber das Verhältniss zwischen dem Emissionsvermögen und dem Absorptionsvermögen der Körper für Wärme und Licht
 |trans-title=On the relation between the emissive power and the absorptive power of bodies towards heat and light 
 |journal=Annalen der Physik
 |volume=185 |issue=2 |pages=275–301 |date=1860
 |doi=10.1002/andp.18601850205|bibcode = 1860AnP...185..275K |url=https://zenodo.org/record/1423668|doi-access=free}}</ref>  Their observations<ref>{{Cite web |title=Kirchhoff and Bunsen on Spectroscopy |url=https://www.chemteam.info/Chem-History/Kirchhoff-Bunsen-1860.html |access-date=2024-07-02 |website=www.chemteam.info}}</ref> were in the visible range where the strongest lines come from metals such as sodium, potassium, and iron.<ref>{{Cite web |title=Spectrum analysis in its application to terrestrial substances and the physical constitution of the heavenly bodies : familiarly explained / by H. Schellen ... |url=https://hdl.handle.net/2027/hvd.hn3317?urlappend=%3Bseq=211 |access-date=2024-07-02 |website=HathiTrust | hdl=2027/hvd.hn3317?urlappend=%3Bseq=211 |language=en}}</ref> In the early work on the chemical composition of the sun the only elements that were detected in spectra were hydrogen and various metals,<ref name="Meadows">{{Cite book |last=Meadows |first=A. J. (Arthur Jack) |url=http://archive.org/details/earlysolarphysic0000mead |title=Early solar physics |date=1970 |publisher=Oxford, New York, Pergamon Press |others=Internet Archive |isbn=978-0-08-006653-0}}</ref>{{Rp|pages=23-24}} with the term ''metallic'' frequently used when describing them.<ref name="Meadows" />{{Rp|location=Part 2}} In contemporary usage in astronomy all the extra elements beyond just hydrogen and helium are termed metallic.

==Origin of metallic elements==
{{see also|Stellar nucleosynthesis|Big Bang nucleosynthesis}}
The presence of heavier elements results from stellar nucleosynthesis, where the majority of elements heavier than hydrogen and helium in the Universe (''metals'', hereafter) are formed in the cores of stars as they [[Stellar evolution|evolve]]. Over time, [[stellar wind]]s and [[supernova]]e deposit the metals into the surrounding environment, enriching the [[interstellar medium]] and providing recycling materials for the [[Protostar|birth of new stars]]. It follows that older generations of stars, which formed in the metal-poor [[Chronology of the universe|early Universe]], generally have lower metallicities than those of younger generations, which formed in a more metal-rich Universe.

==Stellar populations==
[[File:Treasures3.jpg|thumb|right|[[Population I]] star [[Rigel]] with [[reflection nebula]] [[IC&nbsp;2118]]]]
Observed changes in the chemical abundances of different types of stars, based on the spectral peculiarities that were later attributed to metallicity, led astronomer [[Walter Baade]] in 1944 to propose the existence of two different [[Stellar population|populations of stars]].<ref>
{{cite journal
 |first=Walter  |last=Baade
 |year=1944
 |title=The Resolution of Messier&nbsp;32, NGC&nbsp;205, and the central region of the Andromeda Nebula
 |journal=Astrophysical Journal
 |volume=100  |pages=121–146
 |doi=10.1086/144650  |bibcode=1944ApJ...100..137B
|doi-access=free
 }}
</ref>
These became commonly known as {{nobr|[[population I]]}} (metal-rich) and {{nobr|[[population II]]}} (metal-poor) stars. A third, earliest [[stellar population]] was hypothesized in 1978, known as {{nobr|[[population III]]}} stars.<ref>
{{cite journal
 |first=M.J.  |last=Rees
 |year=1978
 |title=Origin of pregalactic microwave background
 |journal=Nature
 |volume=275  |issue=5675  |pages=35–37
 |doi=10.1038/275035a0 |bibcode=1978Natur.275...35R
 |s2cid=121250998
}}
</ref><ref>
{{cite journal
 |first1=S.D.M. |last1=White
 |first2=M.J.   |last2=Rees
 |year=1978
 |title=Core condensation in heavy halos - a two-stage theory for galaxy formation and clustering
 |journal=Monthly Notices of the Royal Astronomical Society
 |volume=183  |issue=3  |pages=341–358
 |bibcode=1978MNRAS.183..341W
 |doi=10.1093/mnras/183.3.341  |doi-access=free
}}
</ref><ref>
{{cite journal
 |author1=Puget, J.L. 
 |author2=Heyvaerts, J. 
 |year=1980
 |title=Population&nbsp;III stars and the shape of the cosmological black body radiation
 |journal=Astronomy and Astrophysics
 |volume=83 |issue=3 |pages=L10–L12
 |bibcode=1980A&A....83L..10P
}}
</ref> These "extremely metal-poor" (XMP) stars are theorized to have been the "first-born" stars created in the Universe.

== Common methods of calculation ==

Astronomers use several different methods to describe and approximate metal abundances, depending on the available tools and the object of interest. Some methods include determining the fraction of mass that is attributed to [[gas]] versus metals, or measuring the ratios of the number of atoms of two different elements as compared to the ratios found in the [[Sun]].

=== Mass fraction ===
Stellar composition is often simply defined by the parameters {{mvar|X}}, {{mvar|Y}}, and {{mvar|Z}}. Here {{mvar|X}} represents the mass fraction of [[hydrogen]], {{mvar|Y}} is the mass fraction of [[helium]], and {{mvar|Z}} is the mass fraction of all the remaining chemical elements. Thus

<math display="block"> X + Y + Z = 1 </math>

In most [[stars]], [[nebula]]e, [[H II region|H{{sup|II}} regions]], and other astronomical sources, hydrogen and helium are the two dominant elements. The hydrogen mass fraction is generally expressed as <math>\ X \equiv \tfrac{m_\mathsf{H}}{M}\ ,</math> where {{mvar|M}} is the total mass of the system, and <math>\ m_\mathsf{H}\ </math> is the mass of the hydrogen it contains. Similarly, the helium mass fraction is denoted as <math>\ Y \equiv \tfrac{m_\mathsf{He}}{M} ~.</math> The remainder of the elements are collectively referred to as "metals", and the mass fraction of metals is calculated as

<math display="block"> Z = \sum_{e > \mathsf{He}} \tfrac{m_e}{M} = 1 - X - Y ~.</math>

For the surface of the Sun ([[Astronomical symbols|symbol]] <math>\odot</math>), these parameters are measured to have the following values:<ref name=asplund>
{{cite journal
 |last1=Asplund |first1=Martin |last2=Grevesse |first2=Nicolas
 |last3=Sauval |first3=A. Jacques |last4=Scott |first4=Pat
 |year=2009
 |title=The chemical composition of the Sun
 |journal=[[Annual Review of Astronomy & Astrophysics]]
 |volume=47 |issue=1 |pages=481–522
 |bibcode=2009ARA&A..47..481A
 |doi=10.1146/annurev.astro.46.060407.145222
 |arxiv = 0909.0948  |s2cid=17921922
}}
</ref>

{|class="wikitable"
|-
! Description !! Solar value
|-
| Hydrogen mass fraction || <math>\ X_\odot = 0.7381\ </math>
|-
| Helium mass fraction || <math>\ Y_\odot = 0.2485\ </math>
|-
| Metal mass fraction || <math>\ Z_\odot = 0.0134\ </math>
|}

Due to the effects of [[stellar evolution]], neither the initial composition nor the present day bulk composition of the Sun is the same as its present-day surface composition.

===Chemical abundance ratios===
The overall stellar metallicity is conventionally defined using the total hydrogen content, since its abundance is considered to be relatively constant in the Universe, or the [[iron]] content of the star, which has an abundance that is generally linearly increasing in time in the Universe.<ref>
{{cite journal
 |last1=Hinkel   |first1=Natalie    |last2=Timmes  |first2=Frank
 |last3=Young    |first3=Patrick    |last4=Pagano  |first4=Michael
 |last5=Turnbull |first5=Maggie
 |date=September 2014
 |title=Stellar abundances in the Solar neighborhood: ''The Hypatia Catalog''
 |journal=[[Astronomical Journal]]
 |volume=148 |issue=3 |page= 33
 |doi=10.1088/0004-6256/148/3/54|arxiv= 1405.6719
 |bibcode= 2014AJ....148...54H |s2cid= 119221402
 |url=https://iopscience.iop.org/article/10.1088/0004-6256/148/3/54
}}
</ref>
Hence, iron can be used as a chronological indicator of nucleosynthesis. [[Iron]] is relatively easy to measure with spectral observations in the star's spectrum given the large number of iron lines in the star's spectra (even though oxygen is the [[Abundance of the chemical elements#Universe|most abundant heavy element]] – see [[#H-II-region-anchor|metallicities in H{{sup|II}} regions]] below). The abundance ratio is the [[common logarithm]] of the ratio of a star's iron abundance compared to that of the Sun and is calculated thus:<ref name=Matteucci2001>
{{cite book
 | first=Francesca | last=Matteucci
 | year=2001
 | title=The Chemical Evolution of the Galaxy
 | series=Astrophysics and Space Science Library
 | volume=253  | page=7
 | publisher=Springer Science & Business Media
 | isbn=978-0-7923-6552-5
 | url=https://books.google.com/books?id=PT7O1nS7CksC&pg=PA7
}}
</ref>

<math display="block"> \left[ \frac{ \mathsf{Fe} }{ \mathsf{H} } \right] ~=~ \log_{10}{\left( \frac{N_{\mathsf{Fe}}}{N_{\mathsf{H}} } \right)_\star } -~ \log_{10}{\left(\frac{N_{ \mathsf{Fe}} }{ N_{\mathsf{H}} } \right)_\odot}\ ,</math>

where <math>\ N_{\mathsf{Fe}}\ </math> and <math>\ N_{\mathsf{H}}\ </math> are the number of iron and hydrogen atoms per unit of volume respectively, <math>\odot</math> is the [[Astronomical symbols|standard symbol]] for the Sun, and <math>\star</math> for a star (often omitted below). The unit often used for metallicity is the [[dex (decimal exponent)|dex]], contraction of "decimal exponent".<ref>{{cite book
 | title=A Dictionary of Weights, Measures, and Units
 | first=Donald | last=Fenna | year=2002
 | isbn=9780191078989 | publisher=OUP Oxford
 | url=https://books.google.com/books?id=uBk9DAAAQBAJ&pg=PT92
}}</ref> By this formulation, stars with a higher metallicity than the Sun have a positive [[common logarithm]], whereas those more dominated by hydrogen have a corresponding negative value. For example, stars with a <math>\ \bigl[\tfrac{ \mathsf{Fe} }{ \mathsf{H} } \bigr]_\star\ </math> value of +1 have 10&nbsp;times the metallicity of the Sun ({{10^|+1}}); conversely, those with a <math>\ \bigl[\tfrac{ \mathsf{Fe} }{ \mathsf{H} } \bigr]_\star\ </math> value of &minus;1 have {{sfrac|1|10}}, while those with a <math>\ \bigl[\tfrac{ \mathsf{Fe} }{ \mathsf{H} } \bigr]_\star\ </math> value of 0 have the same metallicity as the Sun, and so on.<ref name="Martin" />

Young population&nbsp;I stars have significantly higher iron-to-hydrogen ratios than older population&nbsp;II stars. Primordial [[Stellar population#Population III stars|population&nbsp;III]] stars are estimated to have metallicity less than &minus;6, a millionth of the abundance of iron in the Sun.<ref name="AJ-20150604">
{{cite journal
 |last1=Sobral   |first1=David      |last2=Matthee    |first2=Jorryt
 |last3=Darvish  |first3=Behnam     |last4=Schaerer   |first4=Daniel
 |last5=Mobasher |first5=Bahram     |last6=Röttgering |first6=Huub J.A.
 |last7=Santos   |first7=Sérgio     |last8=Hemmati    |first8=Shoubaneh
 |display-authors=6
 |date=4 June 2015
 |title=Evidence for pop&nbsp;III-like stellar populations in the most luminous Lyman-α emitters at the epoch of re-ionisation: Spectroscopic confirmation
 |journal=[[The Astrophysical Journal]]
 |volume=808  |issue=2  |page=139
 |doi=10.1088/0004-637x/808/2/139  |s2cid=18471887
 |bibcode=2015ApJ...808..139S  |arxiv = 1504.01734
}}
</ref><ref name="NYT-20150617">
{{cite news
 |last=Overbye |first=Dennis |author-link=Dennis Overbye
 |date=17 June 2015
 |title=Astronomers report finding earliest stars that enriched the cosmos
 |newspaper=[[The New York Times]]
 |url=https://www.nytimes.com/2015/06/18/science/space/astronomers-report-finding-earliest-stars-that-enriched-cosmos.html
 |access-date=17 June 2015
}}
</ref>
The same notation is used to express variations in abundances between other individual elements as compared to solar proportions. For example, the notation <math>\ \bigl[\tfrac{ \mathsf{O} }{ \mathsf{Fe} } \bigr]\ </math> represents the difference in the logarithm of the star's oxygen abundance versus its iron content compared to that of the Sun. In general, a given [[stellar nucleosynthesis|stellar nucleosynthetic]] process alters the proportions of only a few elements or isotopes, so a star or gas sample with certain <math>\ \bigl[\tfrac{ \mathsf{?} }{ \mathsf{Fe} } \bigr]_\star\ </math> values may well be indicative of an associated, studied nuclear process.

===Photometric colors===
Astronomers can estimate metallicities through measured and calibrated systems that correlate [[Photometry (astronomy)|photometric measurements]] and [[Spectroscopy|spectroscopic measurements]] (see also [[Spectrophotometry]]). For example, the [[UBV photometric system|Johnson UVB filters]] can be used to detect an [[ultraviolet]] (UV) excess in stars,<ref>
{{cite journal
 |last1=Johnson |first1=H.L.
 |last2=Morgan  |first2=W.W.
 |date=May 1953
 |title=Fundamental stellar photometry for standards of spectral type on the revised system of the ''Yerkes Spectral Atlas''
 |journal=[[The Astrophysical Journal]]
 |volume=117 |page=313
 |doi=10.1086/145697  |issn=0004-637X
 |bibcode=1953ApJ...117..313J
}}
</ref>
where a smaller UV excess indicates a larger presence of metals that absorb the UV radiation, thereby making the star appear "redder".<ref>
{{cite journal
 |last=Roman |first=Nancy G.
 |date=December 1955
 |title=A catalogue of high-velocity stars
 |journal=The Astrophysical Journal Supplement Series
 |volume=2 |page=195
 |doi=10.1086/190021 |issn=0067-0049
 |bibcode=1955ApJS....2..195R
|doi-access=free
 }}
</ref><ref>
{{cite journal
 |last1=Sandage |first1=A.R. |author1-link=Allan Sandage
 |last2=Eggen   |first2=O.J.
 |date=1959-06-01
 |title=On the existence of subdwarfs in the (MBol, log Te)-diagram
 |journal=[[Monthly Notices of the Royal Astronomical Society]]
 |volume=119 |issue=3 |pages=278–296
 |doi=10.1093/mnras/119.3.278 |doi-access=free
 |issn=0035-8711  |bibcode=1959MNRAS.119..278S
}}
</ref><ref>
{{cite journal
 |last1=Wallerstein |first1=George
 |last2=Carlson |first2=Maurice
 |date=September 1960
 |title=Letter to the Editor: On the ultraviolet excess in G&nbsp;dwarfs
 |journal=[[The Astrophysical Journal]]
 |volume=132  |page=276
 |doi=10.1086/146926  |issn=0004-637X
 |bibcode=1960ApJ...132..276W
}}
</ref>
The UV excess, {{mvar|δ}}(U−B), is defined as the difference between a star's U and B&nbsp;band [[Magnitude (astronomy)|magnitudes]], compared to the difference between U and B&nbsp;band magnitudes of metal-rich stars in the [[Hyades (star cluster)|Hyades cluster]].<ref name=Wildey-Burbidge-etal-1962>
{{Cite journal
 |last1=Wildey   |first1=R.L.
 |last2=Burbidge |first2=E.M.
 |last3=Sandage  |first3=A.R. |author3-link=Allan Sandage
 |last4=Burbidge |first4=G.R.
 |date=January 1962
 |title=On the effect of Fraunhofer lines on u, b, V measurements
 |journal=[[The Astrophysical Journal]]
 |volume=135  |page=94
 |doi=10.1086/147251  |issn=0004-637X
 |bibcode=1962ApJ...135...94W
|doi-access=free
 }}
</ref>
Unfortunately, {{mvar|δ}}(U−B) is sensitive to both metallicity and [[Effective temperature|temperature]]: If two stars are equally metal-rich, but one is cooler than the other, they will likely have different {{mvar|δ}}(U−B) values<ref name=Wildey-Burbidge-etal-1962/> (see also [[Blanketing effect]]<ref>
{{cite journal
 |last1=Schwarzschild |first1=M.
 |last2=Searle        |first2=L.
 |last3=Howard        |first3=R.
 |date=September 1955
 |title=On the colors of subdwarfs
 |journal=[[The Astrophysical Journal]]
 |volume=122  |page=353
 |doi=10.1086/146094  |issn=0004-637X
 |bibcode=1955ApJ...122..353S
|doi-access=free
 }}
</ref><ref>
{{Cite journal |last=Cameron |first=L. M. |date=June 1985 |title=Metallicities and distances of galactic clusters as determined from UBV data – Part&nbsp;Three – Ages and abundance gradients of open clusters |journal=[[Astronomy and Astrophysics]] |volume=147 |page=47 |bibcode=1985A&A...147...47C |issn=0004-6361}}
</ref>).
To help mitigate this degeneracy, a star's B−V&nbsp;[[color index]] can be used as an indicator for temperature. Furthermore, the UV&nbsp;excess and B−V&nbsp;index can be corrected to relate the {{mvar|δ}}(U−B) value to iron abundances.<ref>
{{Cite journal
 |last=Sandage  |first=A.R.  |author-link=Allan Sandage
 |date=December 1969
 |title=New subdwarfs. II. Radial velocities, photometry, and preliminary space motions for 112&nbsp;stars with large proper motion
 |journal=[[The Astrophysical Journal]]
 |volume=158 |page=1115
 |doi=10.1086/150271  |issn=0004-637X
 |bibcode=1969ApJ...158.1115S
|doi-access=free
 }}
</ref><ref>
{{Cite journal
 |last=Carney  |first=B.W.
 |date=October 1979
 |title=Subdwarf ultraviolet excesses and metal abundances
 |journal=[[The Astrophysical Journal]]
 |volume=233  |page=211
 |doi=10.1086/157383 |issn=0004-637X
 |bibcode=1979ApJ...233..211C
|doi-access=free
 }}
</ref><ref>
{{Cite journal
 |last1=Laird  |first1=John B.
 |last2=Carney |first2=Bruce W.
 |last3=Latham |first3=David W.
 |date=June 1988
 |title=A survey of proper-motion stars. III - Reddenings, distances, and metallicities
 |journal=[[The Astronomical Journal]]
 |volume=95 |page=1843
 |doi=10.1086/114782 |issn=0004-6256
 |bibcode=1988AJ.....95.1843L
}}
</ref>

Other [[photometric system]]s that can be used to determine metallicities of certain astrophysical objects include the Strӧmgren system,<ref>
{{cite book
 |last=Strömgren |first=Bengt
 |year=1963
 |edition=original (re-issued 1968)
 |chapter=Quantitative classification methods
 |editor-last=Strand |editor-first=Kaj Aage 
 |title=Basic Astronomical Data: Stars and stellar systems
 |publisher=University of Chicago Press
 |place=Chicago, IL 
 |page=123
 |bibcode=1963bad..book..123S
}}
* 1980 reprint edition: {{oclc|7047642}}, {{ISBN|0-2264-5964-0}}
* 1988 reprint edition: {{ISBN|978-2-2645-9640-6}}
</ref><ref>
{{cite journal
 |last=Crawford |first=L.D.
 |date=1966
 |title=Photo-electric H-beta and U V B Y photometry
 |journal=Spectral Classification and Multicolour Photometry
 |volume=24  |page=170
 |bibcode=1966IAUS...24..170C
}}
</ref>
the Geneva system,<ref>
{{cite journal
 |last1=Cramer |first1=N.
 |last2=Maeder |first2=A. 
 |date=October 1979
 |title=Luminosity and {{mvar|T}}{{sub|eff}} determinations for B-type stars
 |journal=Astronomy and Astrophysics
 |volume=78 |page= 305
 |issn=0004-6361 |bibcode=1979A&A....78..305C
}}
</ref><ref>
{{cite journal
 |last1=Kobi   |first1=D.
 |last2=North  |first2=P.
 |date=November 1990
 |title=A new calibration of the Geneva photometry in terms of Te, log g, (Fe/H) and mass for main sequence A4 to G5&nbsp;stars
 |journal=Astronomy and Astrophysics Supplement Series
 |volume=85  |page=999
 |issn=0365-0138  |bibcode=1990A&AS...85..999K
}}
</ref> the Washington system,<ref>
{{cite journal
 |last=Geisler |first=D.
 |date=1986
 |title=The empirical abundance calibrations for Washington photometry of population&nbsp;II giants
 |journal=Publications of the Astronomical Society of the Pacific
 |volume=98  |issue=606  |page=762
 |doi=10.1086/131822  |doi-access=free
 |bibcode=1986PASP...98..762G  |issn=1538-3873
 |url=http://stacks.iop.org/1538-3873/98/i=606/a=762
}}
</ref><ref>
{{cite journal
 |last1=Geisler |first1=Doug
 |last2=Claria  |first2=Juan J.
 |last3=Minniti |first3=Dante
 |date=November 1991
 |title=An improved metal abundance calibration for the Washington system
 |journal=The Astronomical Journal
 |volume=102  |page=1836
 |doi=10.1086/116008  |issn=0004-6256
 |bibcode=1991AJ....102.1836G
}}
</ref>
and the DDO system.<ref>
{{Cite journal
 |last1=Claria   |first1=Juan J.
 |last2=Piatti   |first2=Andres E.
 |last3=Lapasset |first3=Emilio
 |date=May 1994
 |title=A revised effective-temperature calibration for the DDO photometric system
 |journal=Publications of the Astronomical Society of the Pacific
 |volume=106  |page=436
 |doi=10.1086/133398  |doi-access=free
 |issn=0004-6280  |bibcode=1994PASP..106..436C
}}
</ref><ref>
{{Cite journal
 |last=James  |first=K.A.
 |date=May 1975
 |title=Cyanogen strengths, luminosities, and kinematics of K&nbsp;giant stars
 |journal=The Astrophysical Journal Supplement Series
 |volume=29  |page=161
 |doi=10.1086/190339  |issn=0067-0049
 |bibcode=1975ApJS...29..161J
|doi-access=free
 }}
</ref>

== Metallicities in various astrophysical objects ==

=== Stars ===
At a given mass and age, a metal-poor star will be slightly warmer. {{nobr|[[Population II star]]s'}} metallicities are roughly {{sfrac|1|1000}} to {{sfrac|1|10}} of the Sun's <math>\left(\ \bigl[ \tfrac{ \mathsf{Fe} }{ \mathsf{H} } \bigr]\ = {-3.0}\ ...\ {-1.0}\ \right)\ ,</math>  but the group appears cooler than {{nobr|population I}} overall, as heavy population&nbsp;II stars have long since died. Above 40&nbsp;[[solar mass]]es, metallicity influences how a star will die: Outside the [[pair-instability supernova|pair-instability window]], lower metallicity stars will collapse directly to a black hole, while higher metallicity stars undergo a [[core-collapse supernova|type&nbsp;Ib/c supernova]] and may leave a [[neutron star]].

==== Relationship between stellar metallicity and planets ====
A star's metallicity measurement is one parameter that helps determine whether a star may have a giant [[planet]], as there is a direct correlation between metallicity and the presence of a giant planet. Measurements have demonstrated the connection between a star's metallicity and [[gas giant]] planets, like [[Jupiter]] and [[Saturn]]. The more metals in a star and thus its [[planetary system]] and [[protoplanetary disk]], the more likely the system may have gas giant planets. Current models show that the metallicity along with the correct planetary system temperature and distance from the star are key to planet and [[planetesimal]] formation. For two stars that have equal age and mass but different metallicity, the less metallic star is [[Stellar classification|bluer]]. Among stars of the same color, less metallic stars emit more ultraviolet radiation. The Sun, with [[Solar System#Planets|eight planets]] and nine consensus [[dwarf planets]], is used as the reference, with a <math>\ \bigl[\tfrac{ \mathsf{Fe} }{ \mathsf{H} } \bigr]\ </math> of 0.00.<ref>{{cite web
 |last=Wang
 |first=Ji
 |title=Planet-metallicity correlation - the rich get richer
 |publisher=[[Caltech]]
 |url=http://www.astro.caltech.edu/~jwang/Project4.html
 |access-date=2016-09-28
 |archive-date=2017-07-13
 |archive-url=https://web.archive.org/web/20170713073323/http://www.astro.caltech.edu/~jwang/Project4.html
 |url-status=dead
 }}</ref><ref>
{{cite journal
 |last1=Fischer |first1=Debra A.
 |last2=Valenti |first2=Jeff
 |year=2005
 |title=The planet-metallicity correlation
 |journal=[[The Astrophysical Journal]]
 |volume=622 |issue=2 |page=1102
 |bibcode=2005ApJ...622.1102F
 |doi=10.1086/428383  |doi-access=free
}}
</ref><ref>
{{cite journal
 |last1=Wang    |first1=Ji
 |last2=Fischer |first2=Debra A.
 |year=2013
 |title=Revealing a universal planet-metallicity correlation for planets of different sizes around Solar-type stars
 |journal=[[The Astronomical Journal]]
 |volume=149 |issue=1 |page=14
 |doi=10.1088/0004-6256/149/1/14  |arxiv=1310.7830
 |bibcode=2015AJ....149...14W  |s2cid=118415186
}}
</ref><ref>
{{cite magazine
 |author=Sanders, Ray
 |date=9 April 2012
 |title=When stellar metallicity sparks planet formation
 |magazine=Astrobiology Magazine
 |url=http://www.astrobio.net/news-exclusive/when-stellar-metallicity-sparks-planet-formation/
 
 |archive-url=https://web.archive.org/web/20210507132606/https://www.astrobio.net/news-exclusive/when-stellar-metallicity-sparks-planet-formation/
 |archive-date=2021-05-07
}}
</ref><ref>
{{cite conference
 |title=The G&nbsp;star problem
 |editor1=Hill, Vanessa 
 |editor2=François, Patrick 
 |editor3=Primas, Francesca|editor3-link= Francesca Primas
 |book-title=From Lithium to Uranium: Elemental tracers of early cosmic evolution
 |conference=[[International Astronomical Union|IAU]] Symposium&nbsp;228  
 |series=Proceedings of the [[International Astronomical Union]] Symposia and Colloquia
 |volume=228  |pages=509–511
}} {{citation not found|date=2023-03-20|reason=No such title in IAU Symposium&nbsp;228 proceedings.}}
:Missing article's page numbers are imbedded in:
{{cite conference
 |author=Arimoto, N.
 |date=23–27 May 2005
 |title=Linking the halo to its surroundings
 |place=Paris, France
 |conference=[[International Astronomical Union|IAU]] Symposium&nbsp;228  
 |editor1=Hill, Vanessa 
 |editor2=François, Patrick 
 |editor3=Primas, Francesca|editor3-link= Francesca Primas
 |publication-date=February 2006 
 |book-title=From Lithium to Uranium: Elemental tracers of early cosmic evolution
 |series=Proceedings of the [[International Astronomical Union]] Symposia and Colloquia
 |volume=228 |pages=503–512
 |publisher=[[International Astronomical Union|IAU]] / Cambridge University Press
 |isbn=978-0-52185199-2  |doi=10.1017/S1743921305006344
 |bibcode=2005IAUS..228..503A
|doi-access=free
 }}
</ref>

==== <span class="anchor" id="H-II-region-anchor">H{{sup|II}} regions</span> ====
Young, massive and hot stars (typically of spectral types [[O-type star|O]] and [[B-type main-sequence star|B]]) in [[H II region|H{{sup|II}} regions]] emit [[Ultraviolet|UV photons]] that ionize [[ground-state]] hydrogen atoms, knocking [[electron]]s free; this process is known as [[photoionization]]. The free electrons can [[Collisional excitation|strike]] other atoms nearby, exciting bound metallic electrons into a [[Metastability|metastable state]], which eventually decay back into a ground state, emitting photons with energies that correspond to [[Forbidden mechanism|forbidden lines]]. Through these transitions, astronomers have developed several observational methods to estimate metal abundances in H{{sup|II}} regions, where the stronger the forbidden lines in spectroscopic observations, the higher the metallicity.<ref>{{Cite journal |last1=Kewley |first1=L.J. |last2=Dopita |first2=M.A. |date=September 2002 |title=Using strong lines to estimate abundances in extragalactic H{{sup|II}} regions and starburst galaxies |journal=The Astrophysical Journal Supplement Series |volume=142 |issue=1 |pages=35–52 |doi=10.1086/341326 |issn=0067-0049 |arxiv=astro-ph/0206495 |bibcode=2002ApJS..142...35K |s2cid=16655590 }}</ref><ref>{{Cite journal |last1=Nagao |first1=T. |last2=Maiolino |first2=R. |last3=Marconi |first3=A. |date=2006-09-12 |title=Gas metallicity diagnostics in star-forming galaxies |journal=[[Astronomy & Astrophysics]] |volume=459 |issue=1 |pages=85–101 |doi=10.1051/0004-6361:20065216 |issn=0004-6361 |arxiv=astro-ph/0603580 |bibcode=2006A&A...459...85N|s2cid=16220272 }}</ref> These methods are dependent on one or more of the following: the variety of asymmetrical densities inside H{{sup|II}} regions, the varied temperatures of the embedded stars, and/or the electron density within the ionized region.<ref>{{Cite journal |last=Peimbert |first=Manuel |date=December 1967 |title=Temperature determinations of H{{sup|II}} regions |journal=[[The Astrophysical Journal]] |volume=150 |page=825 |doi=10.1086/149385 |issn=0004-637X |bibcode=1967ApJ...150..825P|doi-access=free }}</ref><ref>{{Cite journal |last=Pagel |first=B.E.J. |date=1986 |title=Nebulae and abundances in galaxies |url=http://stacks.iop.org/1538-3873/98/i=608/a=1009 |journal=[[Publications of the Astronomical Society of the Pacific]] |volume=98 |issue=608 |page=1009 |doi=10.1086/131863 |issn=1538-3873 |bibcode=1986PASP...98.1009P|s2cid=120467036 }}</ref><ref>{{Cite journal |last1=Henry |first1=R.B.C. |last2=Worthey |first2=Guy |date=August 1999 |title=The distribution of heavy elements in spiral and elliptical galaxies |journal=[[Publications of the Astronomical Society of the Pacific]] |volume=111 |issue=762 |pages=919–945 |doi=10.1086/316403 |issn=0004-6280 |arxiv=astro-ph/9904017 |bibcode=1999PASP..111..919H |s2cid=17106463 }}</ref><ref>{{Cite journal |last1=Kobulnicky |first1=Henry A. | last2=Kennicutt | first2=Robert C. Jr. |last3=Pizagno |first3=James L. |date=April 1999 |title=On measuring nebular chemical abundances in distant galaxies using global emission-line spectra |journal=[[The Astrophysical Journal]] |volume=514 |issue=2 |pages=544–557 |doi=10.1086/306987 |issn=0004-637X |arxiv=astro-ph/9811006 |bibcode=1999ApJ...514..544K|s2cid=14643540 }}</ref>

Theoretically, to determine the total abundance of a single element in an H{{sup|II}} region, all transition lines should be observed and summed. However, this can be observationally difficult due to variation in line strength.<ref name=Grazyna-2004-Esteban-etal>{{cite book |last=Grazyna |first=Stasinska |chapter=Abundance determinations in H{{sup|II}} regions and planetary nebulae |year=2004 |title=Cosmochemistry: The melting pot of the elements |editor1=Esteban, C. |editor2=Garcia Lopez, R.J. |editor3=Herrero, A. |editor4=Sanchez, F. |series=Cambridge Contemporary Astrophysics |publisher=Cambridge University Press |pages=115–170 |arxiv=astro-ph/0207500 |bibcode=2002astro.ph..7500S}}</ref><ref>{{Cite journal |last1=Peimbert |first1=Antonio |last2=Peimbert |first2=Manuel |last3=Ruiz |first3=Maria Teresa |date=December 2005 |title=Chemical composition of two H{{sup|II}} regions in NGC&nbsp;6822 based on VLT spectroscopy |journal=[[The Astrophysical Journal]] |volume=634 |issue=2 |pages=1056–1066 |doi=10.1086/444557 |issn=0004-637X |arxiv=astro-ph/0507084 |bibcode=2005ApJ...634.1056P |s2cid=17086551 }}</ref> Some of the most common forbidden lines used to determine metal abundances in H{{sup|II}} regions are from [[oxygen]] (e.g. [O{{sup|II}}] {{mvar|λ}} = (3727, 7318, 7324)&nbsp;Å, and [O{{sup|III}}] {{mvar|λ}} = (4363, 4959, 5007)&nbsp;Å), [[nitrogen]] (e.g. [N{{sup|II}}] {{mvar|λ}} = (5755, 6548, 6584)&nbsp;Å), and [[sulfur]] (e.g. [S{{sup|II}}] {{mvar|λ}} = (6717, 6731)&nbsp;Å and [S{{sup|III}}] {{mvar|λ}} = (6312, 9069, 9531)&nbsp;Å) in the [[Visible spectrum|optical]] spectrum, and the [O{{sup|III}}] {{mvar|λ}} = (52, 88)&nbsp;μm and [N{{sup|III}}] {{mvar|λ}} = 57&nbsp;μm lines in the [[Infrared spectroscopy|infrared]] spectrum. [[Oxygen]] has some of the stronger, more abundant lines in H{{sup|II}} regions, making it a main target for metallicity estimates within these objects. To calculate metal abundances in H{{sup|II}} regions using oxygen [[flux]] measurements, astronomers often use the {{mvar|R}}<sub>23</sub> method, in which

<math display="block">R_{23} = \frac{\ \left[\ \mathsf{O}^\mathsf{II} \right]_{3727~\AA} + \left[\ \mathsf{O}^\mathsf{III} \right]_{4959~\AA + 5007~\AA}\ }{\Bigl[\ \mathsf{ H}_\mathsf{\beta} \Bigr]_{4861 ~\AA} }\ ,</math>

where <math>\ \left[\ \mathsf{O}^\mathsf{II} \right]_{3727~\AA} + \left[\ \mathsf{O}^\mathsf{III} \right]_{4959~\AA + 5007~\AA}\ </math> is the sum of the fluxes from oxygen [[Spectral line|emission lines]] measured at the [[rest frame]] {{mvar|λ}} = (3727, 4959 and 5007)&nbsp;Å wavelengths, divided by the flux from the [[Balmer series]] H{{sub|β}} emission line at the rest frame {{mvar|λ}} = 4861&nbsp;Å wavelength.<ref>{{cite journal |last1=Pagel |first1=B.E.J. |last2=Edmunds |first2=M.G. |last3=Blackwell |first3=D.E. |last4=Chun |first4=M.S. |last5=Smith |first5=G. |date=1979-11-01 |title=On the composition of H{{sup|II}} regions in southern galaxies – I. NGC 300 and 1365 |journal=[[Monthly Notices of the Royal Astronomical Society]] |volume=189 |issue=1 |pages=95–113 |doi=10.1093/mnras/189.1.95 |issn=0035-8711 |bibcode=1979MNRAS.189...95P|doi-access=free }}</ref>
This ratio is well defined through models and observational studies,<ref>{{Cite journal |last1=Dopita |first1=M.A. |last2=Evans |first2=I.N. |date=August 1986 |title=Theoretical models for H{{sup|II}} regions. II - The extragalactic H{{sup|II}} region abundance sequence |journal=The Astrophysical Journal |language=en |volume=307 |page=431 |doi=10.1086/164432 |issn=0004-637X |bibcode=1986ApJ...307..431D|doi-access=free }}</ref><ref>{{Cite journal |last=McGaugh |first=Stacy S. |date=October 1991 |title=H{{sup|II}} region abundances - Model oxygen line ratios |journal=[[The Astrophysical Journal]] |volume=380 |page=140 |doi=10.1086/170569 |issn=0004-637X |bibcode=1991ApJ...380..140M|doi-access=free }}</ref><ref>{{Cite journal |last=Pilyugin |first=L.S. |date=April 2001 |title=On the oxygen abundance determination in H{{sup|II}} regions |url=https://www.aanda.org/articles/aa/ps/2001/14/aa10396.ps.gz |journal=[[Astronomy & Astrophysics]] |volume=369 |issue=2 |pages=594–604 |doi=10.1051/0004-6361:20010079 |issn=0004-6361 |arxiv=astro-ph/0101446 |bibcode=2001A&A...369..594P|s2cid=54527173 }}</ref> but caution should be taken, as the ratio is often degenerate, providing both a low and high metallicity solution, which can be broken with additional line measurements.<ref>{{Cite journal |last1=Kobulnicky |first1=Henry A. |last2=Zaritsky |first2=Dennis |date=1999-01-20 |title=Chemical Properties of Star-forming Emission-Line Galaxies atz=0.1–0.5 |journal=[[The Astrophysical Journal]] |volume=511 |issue=1 |pages=118–135 |doi=10.1086/306673 |issn=0004-637X |arxiv=astro-ph/9808081 |bibcode=1999ApJ...511..118K |s2cid=13094276 }}</ref>
Similarly, other strong forbidden line ratios can be used, e.g. for sulfur, where<ref>{{Cite journal |last1=Diaz |first1=A.I. |last2=Perez-Montero |first2=E. |date=2000-02-11 |title=An empirical calibration of nebular abundances based on the sulphur emission lines |journal=[[Monthly Notices of the Royal Astronomical Society]] |volume=312 |issue=1 |pages=130–138 |doi=10.1046/j.1365-8711.2000.03117.x |doi-access=free |issn=0035-8711 |arxiv=astro-ph/9909492 |bibcode=2000MNRAS.312..130D |s2cid=119504048 }}</ref>

<math display="block">S_{23} = \frac{\ \left[\ \mathsf{S}^\mathsf{II} \right]_{6716~\AA + 6731~\AA} + \left[\ \mathsf{S}^\mathsf{III} \right]_{9069~\AA + 9532~\AA}\ }{\Bigl[\ \mathsf{H}_\mathsf{\beta} \Bigr]_{4861 ~\AA} } ~.</math>

Metal abundances within H{{sup|II}} regions are typically less than 1%, with the percentage decreasing on average with distance from the [[Galactic Center]].<ref name=Grazyna-2004-Esteban-etal/><ref>{{Cite journal |last1=Shaver |first1=P.A. |last2=McGee |first2=R.X. |last3=Newton |first3=L.M. |last4=Danks |first4=A.C. |last5=Pottasch |first5=S.R. |date=1983-09-01 |title=The galactic abundance gradient |journal=[[Monthly Notices of the Royal Astronomical Society]] |volume=204 |issue=1 |pages=53–112 |doi=10.1093/mnras/204.1.53 |issn=0035-8711 |bibcode=1983MNRAS.204...53S|doi-access=free }}</ref><ref>{{Cite journal |last1=Afflerbach |first1=A. |last2=Churchwell |first2=E. |last3=Werner |first3=M. W. |date=1997-03-20 |title=Galactic abundance gradients from infrared fine-structure lines in compact H{{sup|II}} regions |journal=[[The Astrophysical Journal]] |volume=478 |issue=1 |pages=190–205 |doi=10.1086/303771 |doi-access=free |issn=0004-637X |bibcode=1997ApJ...478..190A }}</ref><ref>{{cite book |last1=Pagel |first1=J. |last2=Bernard |first2=E. |year=1997 |title=Nucleosynthesis and Chemical Evolution of Galaxies |page=392 |publisher=Cambridge University Press |bibcode=1997nceg.book.....P  |isbn=978-0-521-55061-1}}</ref><ref>{{cite journal |last1=Balser |first1=Dana S. |last2=Rood |first2=Robert T. |last3=Bania |first3=T.M. |last4=Anderson |first4=L.D. |title=H{{sup|II}} region metallicity distribution in the Milky Way disk |date=2011-08-10 |journal=[[The Astrophysical Journal]] |volume=738 |issue=1 |page=27 |doi=10.1088/0004-637X/738/1/27 |issn=0004-637X |arxiv=1106.1660 |bibcode=2011ApJ...738...27B|s2cid=119252119 }}</ref>

==See also==
* [[Cosmos Redshift 7]], a galaxy that reportedly contains Population&nbsp;III stars
* [[Galaxy formation and evolution]]
* [[GRB 090423]], the most distant seen, presumably from a low-metallicity progenitor
* [[Metallicity distribution function]]
{{Clear}}

==References==
{{reflist|25em|
|refs=

<ref name=Martin>
{{cite web
 |author=Martin, John C.
 |title=What we learn from a star's metal content
 |series=New analysis RR&nbsp;Lyrae kinematics in the solar neighborhood
 |publisher=[[University of Illinois, Springfield]]
 |url=https://edocs.uis.edu/jmart5/www/rrlyrae/metals.htm
 |access-date=7 September 2005 
 |archive-url=https://web.archive.org/web/20141009051221/https://edocs.uis.edu/jmart5/www/rrlyrae/metals.htm
 |archive-date=2014-10-09
}}
</ref>

}} <!-- end "refs=" -->

{{refbegin|25em|small=y}}
* {{cite journal
 |author1=Salvaterra, R.
 |author2=Ferrara, A.
 |author3=Schneider, R. |author3-link=Raffaella Schneider
 |year=2004
 |title=Induced formation of primordial low-mass stars
 |journal=[[New Astronomy (journal)|New Astronomy]]
 |volume=10 |issue=2 |pages=113–120
 |doi=10.1016/j.newast.2004.06.003 |bibcode=2004NewA...10..113S
 |arxiv=astro-ph/0304074|citeseerx=10.1.1.258.923 |s2cid=15085880
}}
* {{cite journal
 |author1=Heger, A.
 |author2=Woosley, S.E.
 |year=2002
 |title=The nucleosynthetic signature of population&nbsp;III
 |journal=[[Astrophysical Journal]]
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 |bibcode=2002ApJ...567..532H  |doi=10.1086/338487
 |arxiv=astro-ph/0107037  |s2cid=16050642
}}
{{refend}}

==Further reading==
* {{cite book
 |first1=Karl F. |last1=Kuhn
 |first2=Theo |last2=Koupelis
 |year=2004
 |title=Quest of the Universe |edition=Fourth
 |publisher=Jones and Bartlett
 |place=Canada
 |isbn=0-7637-0810-0
 |page=593
}}
* {{cite journal
 |last1=Bromm |first1=Volker
 |last2=Larson |first2=Richard B.
 |year=2004
 |title=The first stars
 |journal=[[Annual Review of Astronomy and Astrophysics]]
 |volume=42 |issue=1 |pages=79–118
 |arxiv=astro-ph/0311019 |bibcode=2004ARA&A..42...79B
 |s2cid=119371063  |doi=10.1146/annurev.astro.42.053102.134034
}}

{{Star}}
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