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In1944,Walter Baade categorized groups of stars within theMilky Way intostellar populations.In the abstract of the article by Baade, he recognizes thatJan Oort originally conceived this type of classification in1926.[1]
Baade observed that bluer stars were strongly associated with the spiral arms, and yellow stars dominated near the centralgalactic bulge and withinglobular star clusters.[2] Two main divisions were deemedpopulation I andpopulation II stars, with another newer, hypothetical division calledpopulation III added in 1978.
Among the population types, significant differences were found with their individual observed stellar spectra. These were later shown to be very important and were possibly related to star formation, observedkinematics,[3] stellar age, and evengalaxy evolution in bothspiral andelliptical galaxies. These three simple population classes usefully divided stars by their chemical composition, ormetallicity.[4][5][3] Inastrophysics nomenclaturemetal refers to all elements heavier thanhelium, including chemicalnon-metals such as oxygen.[6]
By definition, each population group shows the trend where lower metal content indicates higher age of stars. Hence, the first stars in the universe (very low metal content) were deemedpopulation III, old stars (low metallicity) aspopulation II, and recent stars (high metallicity) aspopulation I.[7] TheSun is considered population I, a recent star with a relatively high 1.4% metallicity.
Observation ofstellar spectra has revealed that stars older than the Sun have fewer heavy elements compared with the Sun.[3] This immediately suggests that metallicity has evolved through the generations of stars by the process ofstellar nucleosynthesis.
Under current cosmological models, all matter created in theBig Bang was mostlyhydrogen (75%) andhelium (25%), with only a very tiny fraction consisting of other light elements such aslithium andberyllium.[8] When the universe had cooled sufficiently, the first stars were born as population III stars, without any contaminating heavier metals. This is postulated to have affected their structure so that their stellar masses became hundreds of times more than that of the Sun. In turn, these massive stars also evolved very quickly, and theirnucleosynthetic processes created the first 26 elements (up toiron in theperiodic table).[9]
Many theoretical stellar models show that most high-mass population III stars rapidly exhausted their fuel and likely exploded in extremely energeticpair-instability supernovae. Those explosions would have thoroughly dispersed their material, ejecting metals into the interstellar medium (ISM), to be incorporated into the later generations of stars. Their destruction suggests that no galactic high-mass population III stars should be observable.[10] However, some population III stars might be seen in high-redshift galaxies whose light originated during the earlier history of the universe.[11] Scientists have found evidence ofan extremely small ultra metal-poor star, slightly smaller than the Sun, found in a binary system of the spiral arms in theMilky Way. The discovery opens up the possibility of observing even older stars.[12]
Stars too massive to produce pair-instability supernovae would have likely collapsed intoblack holes through a process known asphotodisintegration. Here some matter may have escaped during this process in the form ofrelativistic jets, and this could have distributed the first metals into the universe.[13][14][a]
The oldest stars observed thus far,[10] known as population II, have very low metallicities;[16][7] as subsequent generations of stars were born, they became more metal-enriched, as thegaseous clouds from which they formed received the metal-richdust manufactured by previous generations of stars from population III.
As those population II stars died, they returned metal-enriched material to theinterstellar medium viaplanetary nebulae and supernovae, enriching further the nebulae, out of which the newer stars formed. These youngest stars, including theSun, therefore have the highest metal content, and are known as population I stars.
Population I stars are young stars with the highest metallicity out of all three populations and are more commonly found in thespiral arms of theMilky Way galaxy. TheSun is considered as an intermediate population I star, while the sun-likeμ Arae is much richer in metals.[17] (The termmetal-rich is used to describe stars with a significantly higher metallicity than the Sun; higher than can be explained by measurement error.)
Population I stars usually have regularelliptical orbits of theGalactic Center, with a lowrelative velocity. It was earlier hypothesized that the high metallicity of population I stars makes them more likely to possessplanetary systems than the other two populations, becauseplanets, particularlyterrestrial planets, are thought to be formed by theaccretion of metals.[18] However, observations of theKepler Space Telescope data have found smaller planets around stars with a range of metallicities, while only larger, potential gas giant planets are concentrated around stars with relatively higher metallicity – a finding that has implications for theories of gas-giant formation.[19] Between the intermediate population I and the population II stars comes the intermediate disc population.
Population II, or metal-poor, stars are those with relatively little of the elements heavier than helium. These objects were formed during an earlier time of the universe. Intermediate population II stars are common in thebulge near the centre of theMilky Way, whereas population II stars found in thegalactic halo are older and thus more metal-deficient.Globular clusters also contain high numbers of population II stars.[20]
A characteristic of population II stars is that despite their lower overall metallicity, they often have a higher ratio ofalpha elements (elements produced by thealpha process, likeoxygen andneon) relative toiron (Fe) as compared with population I stars; current theory suggests that this is the result oftype II supernovas being more important contributors to theinterstellar medium at the time of their formation, whereastype Ia supernova metal-enrichment came at a later stage in the universe's development.[21]
Scientists have targeted these oldest stars in several different surveys, including the HK objective-prism survey ofTimothy C. Beerset al.[22] and the Hamburg-ESO survey of Norbert Christlieb et al.,[23] originally started for faintquasars. Thus far, they have uncovered and studied in detail about ten ultra-metal-poor (UMP) stars (such asSneden's Star,Cayrel's Star,BD +17° 3248) and three of the oldest stars known to date:HE 0107-5240,HE 1327-2326 andHE 1523-0901.Caffau's star was identified as the most metal-poor star yet when it was found in 2012 usingSloan Digital Sky Survey data. However, in February 2014 the discovery of an even lower-metallicity star was announced,SMSS J031300.36-670839.3 located with the aid ofSkyMapper astronomical survey data. Less extreme in their metal deficiency, but nearer and brighter and hence longer known, areHD 122563 (ared giant) andHD 140283 (asubgiant).
Population III stars[24] are a hypothetical population of extremely massive, luminous and hot stars with virtually no"metals", except possibly for intermixing ejecta from other nearby, early population III supernovae. The term was first introduced by Neville J. Woolf in 1965.[25][26] Such stars are likely to have existed in the very early universe (i.e., at high redshift) and may have started the production ofchemical elements heavier thanhydrogen, which are needed for the later formation ofplanets andlife as we know it.[27][28]
The existence of population III stars is inferred fromphysical cosmology, but they have not yet been observed directly. Indirect evidence for their existence has been found in agravitationally lensed galaxy in a very distant part of the universe.[29] Their existence may account for the fact that heavy elements – which could not have been created in theBig Bang – are observed inquasaremission spectra.[9] They are also thought to be components offaint blue galaxies. These stars likely triggered the universe's period ofreionization, a majorphase transition of the hydrogen gas composing most of the interstellar medium. Observations of the galaxyUDFy-38135539 suggest that it may have played a role in this reionization process. TheEuropean Southern Observatory discovered a bright pocket of early population stars in the very bright galaxyCosmos Redshift 7 from the reionization period around 800 million years after the Big Bang, atz = 6.60. The rest of the galaxy has some later redder population II stars.[27][30] Some theories hold that there were two generations of population III stars.[31]
Current theory is divided on whether the first stars were very massive or not. One possibility is that these stars were much larger than current stars: several hundredsolar masses, and possibly up to 1,000 solar masses. Such stars would be very short-lived and last only 2–5 million years.[32] Such large stars may have been possible due to the lack of heavy elements and a much warmerinterstellar medium from the Big Bang.[citation needed] Conversely, theories proposed in 2009 and 2011 suggest that the first star groups might have consisted of a massive star surrounded by several smaller stars.[33][34][35] The smaller stars, if they remained in the birth cluster, would accumulate more gas and could not survive to the present day, but a 2017 study concluded that if a star of 0.8 solar masses (M☉) or less was ejected from its birth cluster before it accumulated more mass, it could survive to the present day, possibly even in our Milky Way galaxy.[36]
Analysis of data of extremely low-metallicity population II stars such asHE 0107-5240, which are thought to contain the metals produced by population III stars, suggest that these metal-free stars had masses of 20~130 solar masses.[37] On the other hand, analysis ofglobular clusters associated withelliptical galaxies suggestspair-instability supernovae, which are typically associated with very massive stars, were responsible for theirmetallic composition.[38] This also explains why there have been no low-mass stars with zerometallicity observed, despite models constructed for smaller population III stars.[39][40] Clusters containing zero-metallicityred dwarfs orbrown dwarfs (possibly created by pair-instability supernovae[16]) have been proposed asdark matter candidates,[41][42] but searches for these types ofMACHOs throughgravitational microlensing have produced negative results.[citation needed]
Population III stars are considered seeds of black holes in the early universe. Unlike high-massblack hole seeds, such asdirect collapse black holes, they would have produced light ones. If they could have grown to larger than expected masses, then they could have beenquasi-stars, other hypothetical seeds of heavy black holes which would have existed in the early development of the Universe before hydrogen and helium were contaminated by heavier elements.
Detection of population III stars is a goal of NASA'sJames Webb Space Telescope.[43]
On 8 December 2022, astronomers reported the possible detection of Population III stars, in a high-redshift galaxy called RX J2129–z8He II.[44][45]
The two types of stellar populations had been recognized among the stars of our own galaxy by Oort as early as 1926.
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