Metallicity
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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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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
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Canada. 2004. ISBN 0-7637-0810-0
Volker Bromm, Richard B. Larson (2004), THE FIRST STARS, Annual Reviews of Astronomy and Astrophysics,
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[3] Lauren J. Bryant. "What Makes Stars Tick" (http://www.indiana. edu/~rcapub/v27n1/tick.shtml).Indiana University Research &
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[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
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