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Metallicity

The globular cluster M80. Stars in globular clusters are mainly older metal-poor

members of Population II.

In astronomy and physical cosmology, the

metallicity of an object is the proportion of

its matter made up of chemical elements

other than hydrogen and helium. Since stars,

which comprise most of the visible matter in

the universe, are composed mostly of

hydrogen and helium, astronomers use for

convenience the blanket term "metal" to

describe all other elements collectively.[1]

Thus, a nebula rich in carbon, nitrogen,

oxygen, and neon would be "metal-rich" in

astrophysical terms even though those

elements are non-metals in chemistry. This

term should not be confused with the usual

definition of "metal"; metallic bonds are

impossible within stars, and the very

strongest chemical bonds are only possible

in the outer layers of cool K and M stars.

Normal chemistry therefore has little or no

relevance in stellar interiors.

The metallicity of an astronomical object

may provide an indication of its age. When

the universe first formed, according to the Big Bang theory, it consisted almost entirely of hydrogen which, through

primordial nucleosynthesis, created a sizeable proportion of helium and only trace amounts of lithium and beryllium

and no heavier elements. Therefore, older stars have lower metallicities than younger stars such as our Sun.

Populations III, II, and I

Stellar populations are categorized as I, II, and III, with each group having decreasing metal content. The

populations were named in the order they were discovered, which is the reverse of the order they were created. Thus,

the first stars in the universe (low metal content) were population III, and recent stars (high metallicity) are

population I.

While older stars do have fewer heavy elements, the fact that all stars observed have some heavier elements poses

something of a puzzle, and the current explanation for this proposes the existence of hypothetical metal-free

Population III stars in the early universe. Soon after the Big Bang, without metals, it is believed that only stars with

masses hundreds of times that of the Sun could be formed; near the end of their lives these stars would have created

the first 26 elements up to iron in the periodic table via nucleosynthesis.

Because of their high mass, current stellar models show that Population III stars would have soon exhausted their

fuel and exploded in extremely energetic pair-instability supernovae. Those explosions would have thoroughly

dispersed their material, ejecting metals throughout the universe to be incorporated into the later generations of stars

that are observed today. The high mass of the first stars is used to explain why, as of 2010, no Population III stars

have been observed. Because they were all destroyed in supernovae in the early universe, Population III stars should

only be seen in far away galaxies whose light originated much earlier in the history of the universe, and searching for

these stars or establishing their nonexistence (thereby invalidating the current model) is an active area of research in

http://en.wikipedia.org/w/index.php?title=Pair-instability_supernovahttp://en.wikipedia.org/w/index.php?title=Nucleosynthesishttp://en.wikipedia.org/w/index.php?title=Periodic_tablehttp://en.wikipedia.org/w/index.php?title=Ironhttp://en.wikipedia.org/w/index.php?title=Stellar_evolutionhttp://en.wikipedia.org/w/index.php?title=Big_Banghttp://en.wikipedia.org/w/index.php?title=Berylliumhttp://en.wikipedia.org/w/index.php?title=Lithiumhttp://en.wikipedia.org/w/index.php?title=Primordial_nucleosynthesishttp://en.wikipedia.org/w/index.php?title=Big_Banghttp://en.wikipedia.org/w/index.php?title=Metallic_bondhttp://en.wikipedia.org/w/index.php?title=Metalhttp://en.wikipedia.org/w/index.php?title=Neonhttp://en.wikipedia.org/w/index.php?title=Oxygenhttp://en.wikipedia.org/w/index.php?title=Nitrogenhttp://en.wikipedia.org/w/index.php?title=Carbonhttp://en.wikipedia.org/w/index.php?title=Nebulahttp://en.wikipedia.org/w/index.php?title=Universehttp://en.wikipedia.org/w/index.php?title=Heliumhttp://en.wikipedia.org/w/index.php?title=Hydrogenhttp://en.wikipedia.org/w/index.php?title=Chemical_elementhttp://en.wikipedia.org/w/index.php?title=Physical_cosmologyhttp://en.wikipedia.org/w/index.php?title=Astronomyhttp://en.wikipedia.org/w/index.php?title=File:A_Swarm_of_Ancient_Stars_-_GPN-2000-000930.jpghttp://en.wikipedia.org/w/index.php?title=Messier_80

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astronomy.

It has been proposed that recent supernovae SN2006gy and SN 2007bi may have been pair-instability supernovae in

which such super-massive Population III stars exploded. It has been speculated that these stars could have formed

relatively recently in dwarf galaxies containing primordial metal-free interstellar matter; past supernovae in these

galaxies could have ejected their metal-rich contents at speeds high enough for them to escape the galaxy, keeping

the metal content of the galaxy very low

[2]

.The next generation of stars was born out of those materials left by the death of the first. The oldest observed stars,

known as Population II, have very low metallicities;[3]

as subsequent generations of stars were born they became

more metal-enriched, as the gaseous clouds from which they formed received the metal-rich dust manufactured by

previous generations. As those stars died, they returned metal-enriched material to the interstellar medium via

planetary nebulae and supernovae, enriching the nebulae out of which the newer stars formed ever further. These

youngest stars, including the Sun, therefore have the highest metal content, and are known as Population I stars.

Across the Milky Way, metallicity is higher in the galactic centre and decreases as one moves outwards. The

gradient in metallicity is attributed to the density of stars in the galactic centre: there are more stars in the centre of

the galaxy and so, over time, more metals have been returned to the interstellar medium and incorporated into new

stars. By a similar mechanism, larger galaxies tend to have a higher metallicity than their smaller counterparts. In the

case of the Magellanic Clouds, two small irregular galaxies orbiting [see note about newest research [4] the Milky

Way, the Large Magellanic Cloud has a metallicity of about forty per cent of the Milky Way, while the Small

Magellanic Cloud has a metallicity of about ten per cent of the Milky Way.

Calculation

The metallicity of the Sun is approximately 1.6 percent by mass. For other stars, the metallicity is often expressed as

"[Fe/H]", which represents the logarithm of the ratio of a star's iron abundance compared to that of the Sun (iron is

not the most abundant heavy element, but it is among the easiest to measure with spectral data in the visible

spectrum) The formula for the logarithm is expressed thus:

where and is the number of iron and hydrogen atoms per unit of volume respectively. The unit often used

for metallicity is the "dex" which is a (now-deprecated) contraction of decimal exponent[5]

. By this formulation,

stars with a higher metallicity than the Sun have a positive logarithmic value, while those with a lower metallicity

than the Sun have a negative value. The logarithm is based on powers of ten; stars with a value of +1 have ten times

the metallicity of the Sun (101). Conversely, those with a value of -1 have one tenth (10

1), while those with -2 have

a hundredth (102

), and so on.[6]

Young Population I stars have significantly higher iron-to-hydrogen ratios than

older Population II stars. Primordial Population III stars are estimated to have a metallicity of less than 6.0, that is,

less than a millionth of the abundance of iron which is found in the Sun.

This same sort of notation is used to express differences in the individual elements from the solar proportion. For

example, the notation "[O/Fe]" represents the difference in the logarithm of the star's oxygen abundance compared to

that of the Sun and the logarithm of the star's iron abundance compared to the Sun:

The point of this notation is that if a mass of gas is diluted with pure hydrogen, then its [Fe/H] value will decrease

(since there are fewer iron atoms per hydrogen atom after the dilution), but for all other elements X, the [X/Fe] ratios

will remain unchanged. By contrast, if a mass of gas is polluted with some amount of pure oxygen, then its [Fe/H]

http://en.wikipedia.org/w/index.php?title=Powers_of_tenhttp://en.wikipedia.org/w/index.php?title=Logarithmhttp://en.wikipedia.org/w/index.php?title=Small_Magellanic_Cloudhttp://en.wikipedia.org/w/index.php?title=Small_Magellanic_Cloudhttp://en.wikipedia.org/w/index.php?title=Large_Magellanic_Cloudhttp://news.bbc.co.uk/2/hi/science/nature/6249421.stmhttp://en.wikipedia.org/w/index.php?title=Orbithttp://en.wikipedia.org/w/index.php?title=Irregular_galaxyhttp://en.wikipedia.org/w/index.php?title=Magellanic_Cloudshttp://en.wikipedia.org/w/index.php?title=Galactic_centrehttp://en.wikipedia.org/w/index.php?title=Milky_Wayhttp://en.wikipedia.org/w/index.php?title=Sunhttp://en.wikipedia.org/w/index.php?title=Planetary_nebulahttp://en.wikipedia.org/w/index.php?title=Interstellar_mediumhttp://en.wikipedia.org/w/index.php?title=Cosmic_dusthttp://en.wikipedia.org/w/index.php?title=Gashttp://en.wikipedia.org/w/index.php?title=Interstellar_matterhttp://en.wikipedia.org/w/index.php?title=Dwarf_galaxieshttp://en.wikipedia.org/w/index.php?title=Pair-instability_supernovahttp://en.wikipedia.org/w/index.php?title=SN_2007bihttp://en.wikipedia.org/w/index.php?title=SN2006gy

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will remain unchanged but its [O/Fe] ratio will increase. In general, a given stellar nucleosynthetic process alters the

proportions of only a few elements or isotopes, so a star or gas sample with nonzero [X/Fe] values may be showing

the signature of particular nuclear processes.

Population I stars

Populations I and II

Population I or metal-rich stars are

those young stars whose metallicity is

highest. The Earth's Sun is an example

of a metal-rich star. These are common

in the spiral arms of the Milky Way

galaxy.

Generally, the youngest stars, the

extreme Population I, are found farther

in and intermediate Population I stars

are farther out, etc. The Sun isconsidered an intermediate Population

I star. Population I stars have regular

elliptical orbits of the galactic centre,

with a low relative velocity. The high

metallicity of Population I stars makes

them more likely to possess planetary

systems than the other two populations, since planets, particularly terrestrial planets, are thought to be formed by the

accretion of metals.[7]

Between the intermediate populations I and II comes the intermediary disc population.

Population II stars

Population II, or metal-poor stars, are those with relatively little metal. The idea ofa relatively small amountmust

be kept in perspective as even metal-rich astronomical objects contain low quantities of any element other than

hydrogen or helium; metals constitute only a tiny percentage of the overall chemical makeup of the universe, even

13.7 billion years after the Big Bang. However, metal-poor objects are even more primitive. These objects formed

during an earlier time of the universe. Intermediate Population II stars are common in the bulge near centre of the

galaxy; whereas Population II stars found in the galactic halo are older and thus more metal-poor. Globular clusters

also contain high numbers of Population II stars.[8]

It is believed that Population II stars created all the other

elements in the periodic table, except the more unstable ones.

Scientists have targeted these oldest stars in several different surveys, including the HK objective-prism survey of

Timothy C. Beers et al. and the Hamburg-ESO survey of Norbert Christlieb et al., originally started for faint quasars.

Thus far, they have uncovered and studied in detail about ten very metal-poor stars (as CS22892-052, CS31082-001,

BD +17 3248) and two of the oldest stars known to date: HE0107-5240 and HE1327-2326. Less extreme in their

metal deficiency, but nearer and brighter and hence longer known, are HD 122563 (a red giant) and HD 140283 (a

subdwarf).

http://en.wikipedia.org/w/index.php?title=Subdwarfhttp://en.wikipedia.org/w/index.php?title=HD_140283http://en.wikipedia.org/w/index.php?title=Red_gianthttp://en.wikipedia.org/w/index.php?title=HD_122563http://en.wikipedia.org/w/index.php?title=HE1327-2326http://en.wikipedia.org/w/index.php?title=HE0107-5240http://en.wikipedia.org/w/index.php?title=BD_%2B17%C2%B0_3248http://en.wikipedia.org/w/index.php?title=CS31082-001_%28star%29http://en.wikipedia.org/w/index.php?title=CS22892-052_%28star%29http://en.wikipedia.org/w/index.php?title=Quasarshttp://en.wikipedia.org/w/index.php?title=Norbert_Christliebhttp://en.wikipedia.org/w/index.php?title=European_Southern_Observatoryhttp://en.wikipedia.org/w/index.php?title=Timothy_C._Beershttp://en.wikipedia.org/w/index.php?title=Periodic_tablehttp://en.wikipedia.org/w/index.php?title=Chemical_elementhttp://en.wikipedia.org/w/index.php?title=Globular_clustershttp://en.wikipedia.org/w/index.php?title=Galactic_spheroid%23Galactic_spheroidhttp://en.wikipedia.org/w/index.php?title=Bulge_%28astronomy%29http://en.wikipedia.org/w/index.php?title=Accretion_%28astrophysics%29http://en.wikipedia.org/w/index.php?title=Terrestrial_planethttp://en.wikipedia.org/w/index.php?title=Planetshttp://en.wikipedia.org/w/index.php?title=Planetary_systemhttp://en.wikipedia.org/w/index.php?title=Planetary_systemhttp://en.wikipedia.org/w/index.php?title=Relative_velocityhttp://en.wikipedia.org/w/index.php?title=Elliptical_orbithttp://en.wikipedia.org/w/index.php?title=Milky_Wayhttp://en.wikipedia.org/w/index.php?title=Spiral_armhttp://en.wikipedia.org/w/index.php?title=Earthhttp://en.wikipedia.org/w/index.php?title=File:Starpop.svghttp://en.wikipedia.org/w/index.php?title=Stellar_nucleosynthesis

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Metallicity 4

Population III stars

Possible glow of Population III stars imaged by NASA's Spitzer Space Telescope.

Credit: NASA / JPL-Caltech / A. Kashlinsky (GSFC)

Simulated image of the first stars, 400 million years after the Big Bang.

Population III or metal-free stars were a

population of extremely massive and hot

stars with virtually no surface metals, except

for a small quantity of metals formed in the

Big Bang, such as Lithium-7. These stars

are believed to have been formed in the

early universe. They have not yet been

observed directly, but indirect evidence for

their existence has been found in a

gravitationally lensed galaxy in the very

distant part of the universe.[9]

They are also

thought to be components of faint blue

galaxies. Their existence is proposed to

account for the fact that heavy elements,

which could not have been created in the

Big Bang, are observed in quasar emission

spectra, as well as the existence of faint blue

galaxies.[10]

It is believed that these stars

triggered a period of reionization.

Current theory is divided on whether the

first stars were very massive or not. One

theory, which seems to be borne out by

computer models of star formation, is that

with no heavy elements from the Big Bang,

it was easy to form stars with much more total mass than the ones visible today. Typical masses for Population III

stars would be expected to be about several hundred solar masses, which is much larger than the current stars.

Analysis of data on low-metallicity Population II stars, which are thought to contain the metals produced by

Population III stars, suggest that these metal-free stars had masses of 10 to 100 solar masses instead. This also

explains why there have been no low-mass stars with zero metallicity observed. Confirmation of these theories

awaits the launch of NASA's James Webb Space Telescope. New spectroscopic surveys, such as SEGUE or

SDSS-II, may also locate Population III stars.

See also Abundance of the chemical elements

Sources

Page 593-In Quest of the Universe Fourth Edition Karl F. Kuhn Theo Koupelis. Jones and Bartlett Publishers

Canada. 2004. ISBN 0-7637-0810-0

Volker Bromm, Richard B. Larson (2004), THE FIRST STARS, Annual Reviews of Astronomy and Astrophysics,

vol. 42, pp. 79118.

[1] John C. Martin. "What we learn from a star's metal content" (https://edocs.uis.edu/jmart5/www/rrlyrae/metals.htm).New Analysis RR

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http://www.newscientist.com/article/mg20527470.900-primordial-giant-the-star-that-time-forgot.htmlhttp://www.newscientist.com/article/mg20527470.900-primordial-giant-the-star-that-time-forgot.htmlhttps://edocs.uis.edu/jmart5/www/rrlyrae/metals.htmhttp://en.wikipedia.org/w/index.php?title=Abundance_of_the_chemical_elementshttp://en.wikipedia.org/w/index.php?title=Sloan_Digital_Sky_Survey%23SDSS-IIhttp://en.wikipedia.org/w/index.php?title=Sloan_Digital_Sky_Survey%23Sloan_Extension_for_Galactic_Understanding_and_Exploration_.28SEGUE.29http://en.wikipedia.org/w/index.php?title=Spectroscopyhttp://en.wikipedia.org/w/index.php?title=James_Webb_Space_Telescopehttp://en.wikipedia.org/w/index.php?title=Solar_masshttp://en.wikipedia.org/w/index.php?title=Star_formationhttp://en.wikipedia.org/w/index.php?title=Reionizationhttp://en.wikipedia.org/w/index.php?title=Emission_spectrumhttp://en.wikipedia.org/w/index.php?title=Emission_spectrumhttp://en.wikipedia.org/w/index.php?title=Quasarhttp://en.wikipedia.org/w/index.php?title=Faint_blue_galaxyhttp://en.wikipedia.org/w/index.php?title=Faint_blue_galaxyhttp://en.wikipedia.org/w/index.php?title=Gravitationally_lensed_galaxyhttp://en.wikipedia.org/w/index.php?title=Isotopes_of_lithium%23Lithium-7http://en.wikipedia.org/w/index.php?title=Big_Banghttp://en.wikipedia.org/w/index.php?title=File:NASA-WMAP-first-stars.jpghttp://en.wikipedia.org/w/index.php?title=Big_Banghttp://en.wikipedia.org/w/index.php?title=File:Ssc2005-22a1.jpghttp://en.wikipedia.org/w/index.php?title=Goddard_Space_Flight_Centerhttp://en.wikipedia.org/w/index.php?title=A._Kashlinskyhttp://en.wikipedia.org/w/index.php?title=Caltechhttp://en.wikipedia.org/w/index.php?title=JPLhttp://en.wikipedia.org/w/index.php?title=Spitzer_Space_Telescopehttp://en.wikipedia.org/w/index.php?title=NASA

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[3] Lauren J. Bryant. "What Makes Stars Tick" (http://www.indiana. edu/~rcapub/v27n1/tick.shtml).Indiana University Research &

Creative Activity. . Retrieved September 7, 2005.

[4] http:/ /news.bbc.co.uk/2/hi/science/nature/6249421. stm

[5] A Dictionary of Units of Measurement (http://www.unc.edu/~rowlett/units/dictD.html)

[6] John C. Martin: What we learn from a star's metal content. (https://edocs.uis.edu/jmart5/www/rrlyrae/metals.htm)

[7] Charles H. Lineweaver (2000). "An Estimate of the Age Distribution of Terrestrial Planets in the Universe: Quantifying Metallicity as a

Selection Effect" (http://arxiv. org/abs/astro-ph/0012399). University of New South Wales. . Retrieved 2006-07-23.

[8] T. S. van Albada, Norman Baker (1973). "On the Two Oosterhoff Groups of Globular Clusters".Astrophysical Journal185: 477

498.

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[9] R. A. E. Fosbury et al. (2003). "Massive Star Formation in a Gravitationally Lensed H II Galaxy at z = 3.357" (http://iopscience.iop. org/

0004-637X/596/2/797/pdf/0004-637X_596_2_797. pdf).Astrophysical Journal596 (1): 797809. doi:10.1086/378228. .

[10] A. Heger, S. E. Woosley (2002). "The Nucleosynthetic Signature of Population III" (http://adsabs.harvard. edu/cgi-bin/

nph-bib_query?bibcode=2002ApJ. ..567. .532H).Astrophysical Journal567 (1): 532543. doi:10.1086/338487. .

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