"Star generation" redirects here. For the process by which molecular clouds collapse and form stars, see
Star formation.
In
1944,
Walter Baade categorized groups of stars within the
Milky Way into stellar populations.
In the abstract of the article by Baade, he recognizes that
Jan Oort originally conceived this type of classification in
1926.[1]
Baade observed that bluer stars were strongly associated with the spiral arms, and yellow stars dominated near the central
galactic bulge and within
globular star clusters.[2] Two main divisions were defined as population I and population II, with another newer, hypothetical division called population 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, observed
kinematics,[3] stellar age, and even
galaxy evolution in both
spiral and
elliptical galaxies. These three simple population classes usefully divided stars by their chemical composition or
metallicity.[4][5][3]
By definition, each population group shows the trend where decreasing metal content indicates increasing age of stars. Hence, the first stars in the universe (very low metal content) were deemed population III, old stars (low metallicity) as population II, and recent stars (high metallicity) as population I.[6] The
Sun is considered population I, a recent star with a relatively high 1.4% metallicity. Note that astrophysics nomenclature considers any element heavier than
helium to be a "metal", including chemical
non-metals such as oxygen.[7]
Stellar development
Observation of
stellar 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 of
stellar nucleosynthesis.
Formation of the first stars
Under current cosmological models, all matter created in the
Big Bang was mostly
hydrogen (75%) and
helium (25%), with only a very tiny fraction consisting of other light elements such as
lithium and
beryllium.[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 their
nucleosynthetic processes created the first 26 elements (up to
iron in the
periodic table).[9]
Many theoretical stellar models show that most high-mass population III stars rapidly exhausted their fuel and likely exploded in extremely energetic
pair-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 of
an extremely small ultra metal-poor star, slightly smaller than the Sun, found in a binary system of the spiral arms in the
Milky 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 into
black holes through a process known as
photodisintegration. Here some matter may have escaped during this process in the form of
relativistic jets, and this could have distributed the first metals into the universe.[13][14][a]
Formation of the observed stars
The oldest stars observed thus far,[10] known as population II, have very low metallicities;[16][6] 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 of stars from population III.
As those population II stars died, they returned metal-enriched material to the
interstellar medium via
planetary nebulae and supernovae, enriching further the nebulae, out of which the newer stars formed. These youngest stars, including the
Sun, therefore have the highest metal content, and are known as population I stars.
Chemical classification by Baade
Population I stars
Population I, or metal-rich, stars are young stars with the highest metallicity out of all three populations and are more commonly found in the
spiral arms of the
Milky Way galaxy. The
Sun is an example of a metal-rich star and is considered as an intermediate population I star, while the sun-like
μ Arae is much richer in metals.[17]
Population I stars usually have regular
elliptical orbits of the
Galactic Center, with a low
relative velocity. It was earlier hypothesized that the high metallicity of population I stars makes them more likely to possess
planetary systems than the other two populations, because
planets, particularly
terrestrial planets, are thought to be formed by the
accretion of metals.[18] However, observations of the
Kepler 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 stars
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 the
bulge near the centre of the
Milky Way, whereas population II stars found in the
galactic 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 of "
alpha elements" (elements produced by the
alpha process, like
oxygen and
neon) relative to
iron (Fe) as compared with population I stars; current theory suggests that this is the result of
type II supernovas being more important contributors to the
interstellar medium at the time of their formation, whereas
type 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 of
Timothy C. Beerset al.[22] and the Hamburg-
ESO survey of Norbert Christlieb et al.,[23] originally started for faint
quasars. Thus far, they have uncovered and studied in detail about ten ultra-metal-poor (UMP) stars (such as
Sneden's Star,
Cayrel's Star,
BD +17° 3248) and three of the oldest stars known to date:
HE 0107-5240,
HE 1327-2326 and
HE 1523-0901.
Caffau's star was identified as the most metal-poor star yet when it was found in 2012 using
Sloan 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 of
SkyMapper astronomical survey data. Less extreme in their metal deficiency, but nearer and brighter and hence longer known, are
HD 122563 (a
red giant) and
HD 140283 (a
subgiant).
Population III stars
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 of
chemical elements heavier than
hydrogen, which are needed for the later formation of
planets and
life as we know it.[27][28]
The existence of population III stars is inferred from
physical cosmology, 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.[29] Their existence may account for the fact that heavy elements – which could not have been created in the
Big Bang – are observed in
quasaremission spectra.[9] They are also thought to be components of
faint blue galaxies. These stars likely triggered the universe's period of
reionization, a major
phase transition of the hydrogen gas composing most of the interstellar medium. Observations of the galaxy
UDFy-38135539 suggest that it may have played a role in this reionization process. The
European Southern Observatory discovered a bright pocket of early population stars in the very bright galaxy
Cosmos Redshift 7 from the reionization period around 800 million years after the Big Bang, at z = 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 hundred
solar 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 warmer
interstellar 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 as
HE 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 of
globular clusters associated with
elliptical galaxies suggests
pair-instability supernovae, which are typically associated with very massive stars, were responsible for their
metallic composition.[38] 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.[39][40] Clusters containing zero-metallicity
red dwarfs or
brown dwarfs (possibly created by pair-instability supernovae[16]) have been proposed as
dark matter candidates,[41][42] but searches for these types of
MACHOs through
gravitational microlensing have produced negative results.[citation needed]
Population II stars are considered seeds of black holes in the early universe but unlike high-mass
black hole seeds like
direct collapse black holes they would have produced light ones, if they could have grown to larger than expected masses then they could have been
quasi-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.
^It has been proposed that recent supernovae
SN 2006gy and
SN 2007bi may have been
pair-instability supernovae where such super-massive population III stars exploded. Clark (2010) speculates that these stars could have formed relatively recently in
dwarf galaxies, since they contain mainly primordial, metal-free
interstellar matter. Past supernovae in these small galaxies could have ejected their metal-rich contents at speeds high enough for them to escape the galaxy, keeping the small galaxies' metal content very low.[15]
^
ab
Sobral, David; Matthee, Jorryt; Darvish, Behnam; Schaerer, Daniel; Mobasher, Bahram; Röttgering, Huub J.A.; Santos, Sérgio; Hemmati, Shoubaneh (4 June 2015). "Evidence for Pop III-like stellar populations in the most luminous Lyman-α emitters at the epoch of re-ionisation: Spectroscopic confirmation". The Astrophysical Journal. 808 (2): 139.
arXiv:1504.01734.
Bibcode:
2015ApJ...808..139S.
doi:
10.1088/0004-637x/808/2/139.
S2CID18471887.
^Gibson, Carl H.; Nieuwenhuizen, Theo M.; Schild, Rudolph E. (2013). "Why are so many primitive stars observed in the Galaxy halo". Journal of Cosmology. 22: 10163.
arXiv:1206.0187.
Bibcode:
2013JCos...2210163G.
^Wang, Xin; et al. (8 December 2022). "A strong He II λ1640 emitter with extremely blue UV spectral slope at z=8.16: presence of Pop III stars?".
arXiv:2212.04476 [
astro-ph.GA].
Gibson, B. K.; et al. (2013).
"Review: Galactic Chemical Evolution"(PDF). Publications of the Astronomical Society of Australia. Archived from
the original(PDF) on 20 January 2021. Retrieved 17 April 2018.