Metallicity
Overview
 
In astronomy
Astronomy
Astronomy is a natural science that deals with the study of celestial objects and phenomena that originate outside the atmosphere of Earth...

 and physical cosmology
Physical cosmology
Physical cosmology, as a branch of astronomy, is the study of the largest-scale structures and dynamics of the universe and is concerned with fundamental questions about its formation and evolution. For most of human history, it was a branch of metaphysics and religion...

, the metallicity (also called Z) of an object is the proportion of its matter made up of chemical element
Chemical element
A chemical element is a pure chemical substance consisting of one type of atom distinguished by its atomic number, which is the number of protons in its nucleus. Familiar examples of elements include carbon, oxygen, aluminum, iron, copper, gold, mercury, and lead.As of November 2011, 118 elements...

s other than hydrogen
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...

 and helium
Helium
Helium is the chemical element with atomic number 2 and an atomic weight of 4.002602, which is represented by the symbol He. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas that heads the noble gas group in the periodic table...

. Since stars, which comprise most of the visible matter in the universe
Universe
The Universe is commonly defined as the totality of everything that exists, including all matter and energy, the planets, stars, galaxies, and the contents of intergalactic space. Definitions and usage vary and similar terms include the cosmos, the world and nature...

, are composed mostly of hydrogen and helium, astronomers use for convenience the blanket term "metal" to describe all other elements collectively. Thus, a nebula
Nebula
A nebula is an interstellar cloud of dust, hydrogen gas, helium gas and other ionized gases...

 rich in carbon
Carbon
Carbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...

, nitrogen
Nitrogen
Nitrogen is a chemical element that has the symbol N, atomic number of 7 and atomic mass 14.00674 u. Elemental nitrogen is a colorless, odorless, tasteless, and mostly inert diatomic gas at standard conditions, constituting 78.08% by volume of Earth's atmosphere...

, oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...

, and neon
Neon
Neon is the chemical element that has the symbol Ne and an atomic number of 10. Although a very common element in the universe, it is rare on Earth. A colorless, inert noble gas under standard conditions, neon gives a distinct reddish-orange glow when used in either low-voltage neon glow lamps or...

 would be "metal-rich" in astrophysical terms even though those elements are non-metals in chemistry.
Encyclopedia
In astronomy
Astronomy
Astronomy is a natural science that deals with the study of celestial objects and phenomena that originate outside the atmosphere of Earth...

 and physical cosmology
Physical cosmology
Physical cosmology, as a branch of astronomy, is the study of the largest-scale structures and dynamics of the universe and is concerned with fundamental questions about its formation and evolution. For most of human history, it was a branch of metaphysics and religion...

, the metallicity (also called Z) of an object is the proportion of its matter made up of chemical element
Chemical element
A chemical element is a pure chemical substance consisting of one type of atom distinguished by its atomic number, which is the number of protons in its nucleus. Familiar examples of elements include carbon, oxygen, aluminum, iron, copper, gold, mercury, and lead.As of November 2011, 118 elements...

s other than hydrogen
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...

 and helium
Helium
Helium is the chemical element with atomic number 2 and an atomic weight of 4.002602, which is represented by the symbol He. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas that heads the noble gas group in the periodic table...

. Since stars, which comprise most of the visible matter in the universe
Universe
The Universe is commonly defined as the totality of everything that exists, including all matter and energy, the planets, stars, galaxies, and the contents of intergalactic space. Definitions and usage vary and similar terms include the cosmos, the world and nature...

, are composed mostly of hydrogen and helium, astronomers use for convenience the blanket term "metal" to describe all other elements collectively. Thus, a nebula
Nebula
A nebula is an interstellar cloud of dust, hydrogen gas, helium gas and other ionized gases...

 rich in carbon
Carbon
Carbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...

, nitrogen
Nitrogen
Nitrogen is a chemical element that has the symbol N, atomic number of 7 and atomic mass 14.00674 u. Elemental nitrogen is a colorless, odorless, tasteless, and mostly inert diatomic gas at standard conditions, constituting 78.08% by volume of Earth's atmosphere...

, oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...

, and neon
Neon
Neon is the chemical element that has the symbol Ne and an atomic number of 10. Although a very common element in the universe, it is rare on Earth. A colorless, inert noble gas under standard conditions, neon gives a distinct reddish-orange glow when used in either low-voltage neon glow lamps or...

 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
Metal
A metal , is an element, compound, or alloy that is a good conductor of both electricity and heat. Metals are usually malleable and shiny, that is they reflect most of incident light...

"; metallic bond
Metallic bond
Metallic bonding is the electrostatic attractive forces between the delocalized electrons, called conduction electrons, gathered in an "electron sea", and the positively charged metal ions...

s 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
Big Bang
The Big Bang theory is the prevailing cosmological model that explains the early development of the Universe. According to the Big Bang theory, the Universe was once in an extremely hot and dense state which expanded rapidly. This rapid expansion caused the young Universe to cool and resulted in...

 theory, it consisted almost entirely of hydrogen which, through primordial nucleosynthesis, created a sizeable proportion of helium and only trace amounts of lithium
Lithium
Lithium is a soft, silver-white metal that belongs to the alkali metal group of chemical elements. It is represented by the symbol Li, and it has the atomic number 3. Under standard conditions it is the lightest metal and the least dense solid element. Like all alkali metals, lithium is highly...

 and beryllium
Beryllium
Beryllium is the chemical element with the symbol Be and atomic number 4. It is a divalent element which occurs naturally only in combination with other elements in minerals. Notable gemstones which contain beryllium include beryl and chrysoberyl...

 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 and increasing age. 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
Big Bang
The Big Bang theory is the prevailing cosmological model that explains the early development of the Universe. According to the Big Bang theory, the Universe was once in an extremely hot and dense state which expanded rapidly. This rapid expansion caused the young Universe to cool and resulted in...

, 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
Stellar evolution
Stellar evolution is the process by which a star undergoes a sequence of radical changes during its lifetime. Depending on the mass of the star, this lifetime ranges from only a few million years to trillions of years .Stellar evolution is not studied by observing the life of a single...

 these stars would have created the first 26 elements up to iron
Iron
Iron is a chemical element with the symbol Fe and atomic number 26. It is a metal in the first transition series. It is the most common element forming the planet Earth as a whole, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust...

 in the periodic table
Periodic table
The periodic table of the chemical elements is a tabular display of the 118 known chemical elements organized by selected properties of their atomic structures. Elements are presented by increasing atomic number, the number of protons in an atom's atomic nucleus...

 via nucleosynthesis
Nucleosynthesis
Nucleosynthesis is the process of creating new atomic nuclei from pre-existing nucleons . It is thought that the primordial nucleons themselves were formed from the quark–gluon plasma from the Big Bang as it cooled below two trillion degrees...

.

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 supernova
Pair-instability supernova
A pair-instability supernova occurs when pair production, the production of free electrons and positrons in the collision between atomic nuclei and energetic gamma rays, reduces thermal pressure inside a supermassive star's core...

e. 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, , 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 faraway 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 astronomy. Stars too massive to produce pair-instability supernovae would have collapsed into black hole
Black hole
A black hole is a region of spacetime from which nothing, not even light, can escape. The theory of general relativity predicts that a sufficiently compact mass will deform spacetime to form a black hole. Around a black hole there is a mathematically defined surface called an event horizon that...

s through a process known as photodisintegration
Photodisintegration
Photodisintegration is a physical process in which an extremely high energy gamma ray interacts with an atomic nucleus and causes it to enter an excited state, which immediately decays by emitting a subatomic particle. A single proton or neutron is effectively knocked out of the nucleus by the...

, but some matter escapes during this process in the form of relativistic jet
Relativistic jet
Relativistic jets are extremely powerful jets of plasma which emerge from presumed massive objects at the centers of some active galaxies, notably radio galaxies and quasars. Their lengths can reach several thousand or even hundreds of thousands of light years...

s, and this could have "sprayed" the first metals into the universe.

It has been proposed that recent supernovae SN 2006gy
SN 2006gy
SN 2006gy was an extremely energetic supernova, sometimes referred to as a hypernova or quark-nova, that was discovered on September 18, 2006. It was first observed by Robert Quimby and P. Mondol, and then studied by several teams of astronomers using facilities that included the Chandra, Lick, and...

 and SN 2007bi
SN 2007bi
SN 2007bi was an extremely energetic supernova discovered early in 2007 by the international Nearby Supernova Factory based at the U.S. Department of Energy's Lawrence Berkeley National Laboratory. The precursor star is estimated to have had 200 solar masses at the time of its formation and around...

 may have been pair-instability supernova
Pair-instability supernova
A pair-instability supernova occurs when pair production, the production of free electrons and positrons in the collision between atomic nuclei and energetic gamma rays, reduces thermal pressure inside a supermassive star's core...

e 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.

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; as subsequent generations of stars were born they became more metal-enriched, as the gas
Gas
Gas is one of the three classical states of matter . Near absolute zero, a substance exists as a solid. As heat is added to this substance it melts into a liquid at its melting point , boils into a gas at its boiling point, and if heated high enough would enter a plasma state in which the electrons...

eous clouds from which they formed received the metal-rich dust
Cosmic dust
Cosmic dust is a type of dust composed of particles in space which are a few molecules to 0.1 µm in size. Cosmic dust can be further distinguished by its astronomical location; for example: intergalactic dust, interstellar dust, interplanetary dust and circumplanetary dust .In our own Solar...

 manufactured by previous generations. As those stars died, they returned metal-enriched material to the interstellar medium
Interstellar medium
In astronomy, the interstellar medium is the matter that exists in the space between the star systems in a galaxy. This matter includes gas in ionic, atomic, and molecular form, dust, and cosmic rays. It fills interstellar space and blends smoothly into the surrounding intergalactic space...

 via planetary nebula
Planetary nebula
A planetary nebula is an emission nebula consisting of an expanding glowing shell of ionized gas ejected during the asymptotic giant branch phase of certain types of stars late in their life...

e and supernovae, enriching the nebulae out of which the newer stars formed ever further. These youngest stars, including the Sun
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...

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

Across the Milky Way
Milky Way
The Milky Way is the galaxy that contains the Solar System. This name derives from its appearance as a dim un-resolved "milky" glowing band arching across the night sky...

, 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
Magellanic Clouds
The two Magellanic Clouds are irregular dwarf galaxies visible in the southern hemisphere, which are members of our Local Group and are orbiting our Milky Way galaxy...

, two small irregular galaxies
Irregular galaxy
An irregular galaxy is a galaxy that does not have a distinct regular shape, like a spiral or an elliptical galaxy. The shape of an irregular galaxy is uncommon – they do not fall into any of the regular classes of the Hubble sequence, and they are often chaotic in appearance, with neither a...

 orbit
Orbit
In physics, an orbit is the gravitationally curved path of an object around a point in space, for example the orbit of a planet around the center of a star system, such as the Solar System...

ing (see note about newest research http://news.bbc.co.uk/2/hi/science/nature/6249421.stm) the Milky Way, the Large Magellanic Cloud
Large Magellanic Cloud
The Large Magellanic Cloud is a nearby irregular galaxy, and is a satellite of the Milky Way. At a distance of slightly less than 50 kiloparsecs , the LMC is the third closest galaxy to the Milky Way, with the Sagittarius Dwarf Spheroidal and Canis Major Dwarf Galaxy lying closer to the center...

 has a metallicity of about forty per cent of the Milky Way, while the Small Magellanic Cloud
Small Magellanic Cloud
The Small Magellanic Cloud is a dwarf galaxy. It has a diameter of about 7,000 light-years and contains several hundred million stars. It has a total mass of approximately 7 billion times the mass of our Sun....

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

Calculation

The metallicity of the Sun is approximately 1.8 percent by mass. For other stars, the metallicity is often expressed as "[Fe/H]", which represents the logarithm
Logarithm
The logarithm of a number is the exponent by which another fixed value, the base, has to be raised to produce that number. For example, the logarithm of 1000 to base 10 is 3, because 1000 is 10 to the power 3: More generally, if x = by, then y is the logarithm of x to base b, and is written...

 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 are 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. 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
Powers of Ten
Powers of Ten is a 1968 American documentary short film written and directed by Charles and Ray Eames. The film depicts the relative scale of the Universe in factors of ten . The film is an adaptation of the book Cosmic View by Dutch educator Kees Boeke, and more recently is the basis of a new...

; 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 (10−2), and so on. 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] will remain unchanged but its [O/Fe] ratio will increase. In general, a given stellar nucleosynthetic
Stellar nucleosynthesis
Stellar nucleosynthesis is the collective term for the nuclear reactions taking place in stars to build the nuclei of the elements heavier than hydrogen. Some small quantity of these reactions also occur on the stellar surface under various circumstances...

 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


Population I or metal-rich stars are those young stars whose metallicity is highest. The Earth
Earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...

's Sun is an example of a metal-rich star. These are common in the spiral arms of the Milky Way
Milky Way
The Milky Way is the galaxy that contains the Solar System. This name derives from its appearance as a dim un-resolved "milky" glowing band arching across the night sky...

 galaxy.

Generally, the youngest stars, the extreme Population I, are found farther in and intermediate Population I stars are farther out, etc. The Sun is considered an intermediate Population I star. Population I stars have regular elliptical orbits of the galactic centre, with a low relative velocity
Relative velocity
In non-relativistic kinematics, relative velocity is the vector difference between the velocities of two objects, as evaluated in terms of a single coordinate system....

. The high metallicity of Population I stars makes them more likely to possess planetary system
Planetary system
A planetary system consists of the various non-stellar objects orbiting a star such as planets, dwarf planets , asteroids, meteoroids, comets, and cosmic dust...

s than the other two populations, since planets, particularly terrestrial planet
Terrestrial planet
A terrestrial planet, telluric planet or rocky planet is a planet that is composed primarily of silicate rocks or metals. Within the Solar System, the terrestrial planets are the inner planets closest to the Sun...

s, are thought to be formed by the accretion
Accretion (astrophysics)
In astrophysics, the term accretion is used for at least two distinct processes.The first and most common is the growth of a massive object by gravitationally attracting more matter, typically gaseous matter in an accretion disc. Accretion discs are common around smaller stars or stellar remnants...

 of metals.

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 of a relatively small amount must 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
Bulge (astronomy)
In astronomy, a bulge is a tightly packed group of stars within a larger formation. The term almost exclusively refers to the central group of stars found in most spiral galaxies...

 near the centre of our galaxy
Milky Way
The Milky Way is the galaxy that contains the Solar System. This name derives from its appearance as a dim un-resolved "milky" glowing band arching across the night sky...

; 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. It is believed that Population II stars created all the other elements
Chemical element
A chemical element is a pure chemical substance consisting of one type of atom distinguished by its atomic number, which is the number of protons in its nucleus. Familiar examples of elements include carbon, oxygen, aluminum, iron, copper, gold, mercury, and lead.As of November 2011, 118 elements...

 in the periodic table
Periodic table
The periodic table of the chemical elements is a tabular display of the 118 known chemical elements organized by selected properties of their atomic structures. Elements are presented by increasing atomic number, the number of protons in an atom's atomic nucleus...

, 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
European Southern Observatory
The European Southern Observatory is an intergovernmental research organisation for astronomy, supported by fifteen countries...

 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 and also the oldest star: HE0107-5240
HE0107-5240
|-! style="background-color: #FFFFC0;" colspan="2" | Astrometry|- style="vertical-align: top;"| Apparent magnitude | 15.86|- style="vertical-align: top;"| Distance | 36.000 Ly...

 and HE1327-2326
HE1327-2326
HE1327-2326, discovered in 2005 by Anna Frebel and collaborators, is the star with the lowest known iron abundance to date. The star is a member of Population II, with an iron to hydrogen ratio , or metallicity, of -5.6. This number indicates that its iron content is 300,000 times less than that of...

 and HE 1523-0901
HE 1523-0901
HE 1523-0901 is the designation given to a red giant star located in the Milky Way galaxy approximately seven and a half light-millennia from Earth. It is thought to be a second generation Population II, or metal-poor, star . The star was found in the sample of bright metal-poor halo stars from the...

. Less extreme in their metal deficiency, but nearer and brighter and hence longer known, are HD 122563
HD 122563
HD 122563 is an extremely metal poor red giant star, and the brightest metal poor star in the sky. Its low heavy element content was first recognized by spectroscopic analysis in 1963...

 (a red giant
Red giant
A red giant is a luminous giant star of low or intermediate mass in a late phase of stellar evolution. The outer atmosphere is inflated and tenuous, making the radius immense and the surface temperature low, somewhere from 5,000 K and lower...

) and HD 140283 (a subgiant).

Population III stars

Population III, or metal-free stars is a hypothetical extinct population of extremely massive and hot stars with virtually no surface metals, except for a small quantity of metals formed in the Big Bang
Big Bang
The Big Bang theory is the prevailing cosmological model that explains the early development of the Universe. According to the Big Bang theory, the Universe was once in an extremely hot and dense state which expanded rapidly. This rapid expansion caused the young Universe to cool and resulted in...

, such as lithium-7. These stars are believed to have been formed in the early universe. Their existence is inferred from cosmology
Cosmology
Cosmology is the discipline that deals with the nature of the Universe as a whole. Cosmologists seek to understand the origin, evolution, structure, and ultimate fate of the Universe at large, as well as the natural laws that keep it in order...

, but they have not yet been observed directly. Indirect evidence for their existence has been found in a gravitationally lensed galaxy in a very distant part of the universe. 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
Quasar
A quasi-stellar radio source is a very energetic and distant active galactic nucleus. Quasars are extremely luminous and were first identified as being high redshift sources of electromagnetic energy, including radio waves and visible light, that were point-like, similar to stars, rather than...

 emission spectra
Emission spectrum
The emission spectrum of a chemical element or chemical compound is the spectrum of frequencies of electromagnetic radiation emitted by the element's atoms or the compound's molecules when they are returned to a lower energy state....

, as well as the existence of faint blue galaxies. It is believed that these stars triggered a period of reionization
Reionization
In Big Bang cosmology, reionization is the process that reionized the matter in the universe after the "dark ages," and is the second of two major phase changes of gas in the universe. As the majority of baryonic matter is in the form of hydrogen, reionization usually refers to the reionization of...

. UDFy-38135539
UDFy-38135539
UDFy-38135539 is the Hubble Ultra Deep Field identifier for a galaxy which has been calculated to have a light travel time of 13.1 billion years with a present comoving distance of around 30 billion light-years...

, a galaxy recently discovered, is believed to have been a part of this process.

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
Star formation
Star formation is the process by which dense parts of molecular clouds collapse into a ball of plasma to form a star. As a branch of astronomy star formation includes the study of the interstellar medium and giant molecular clouds as precursors to the star formation process and the study of young...

, 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 mass
Solar mass
The solar mass , , is a standard unit of mass in astronomy, used to indicate the masses of other stars and galaxies...

es, which is much larger than the current stars. Analysis of data on extremely low-metallicity Population II stars such as HE0107-5240
HE0107-5240
|-! style="background-color: #FFFFC0;" colspan="2" | Astrometry|- style="vertical-align: top;"| Apparent magnitude | 15.86|- style="vertical-align: top;"| Distance | 36.000 Ly...

, which are thought to contain the metals produced by Population III stars, suggest that these metal-free stars had masses of 20 to 130 solar masses instead. On the other hand, analysis of globular clusters associated with elliptical galaxies suggests pair-instability supernova
Pair-instability supernova
A pair-instability supernova occurs when pair production, the production of free electrons and positrons in the collision between atomic nuclei and energetic gamma rays, reduces thermal pressure inside a supermassive star's core...

e were responsible for their metallic composition. This also explains why there have been no low-mass stars with zero metallicity observed, although models have been constructed for smaller Population III stars. Clusters containing zero-metallicity red dwarfs or brown dwarfs (possibly created by pair-instability supernovae) have been proposed as dark matter
Dark matter
In astronomy and cosmology, dark matter is matter that neither emits nor scatters light or other electromagnetic radiation, and so cannot be directly detected via optical or radio astronomy...

 candidates, but there is disagreement on this theory. Confirmation of these theories awaits the launch of NASA's James Webb Space Telescope
James Webb Space Telescope
The James Webb Space Telescope , previously known as Next Generation Space Telescope , is a planned next-generation space telescope, optimized for observations in the infrared. The main technical features are a large and very cold 6.5 meter diameter mirror, an observing position far from Earth,...

. New spectroscopic
Spectroscopy
Spectroscopy is the study of the interaction between matter and radiated energy. Historically, spectroscopy originated through the study of visible light dispersed according to its wavelength, e.g., by a prism. Later the concept was expanded greatly to comprise any interaction with radiative...

 surveys, such as SEGUE or SDSS-II, may also locate Population III stars.

A recent theory suggests the first star groups may have consisted of a massive star surrounded by several smaller stars.

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
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