Editing Stellar population (section)

==Stellar development==
Observation of [[Astronomical spectroscopy#Stars and their properties|stellar spectra]] has revealed that stars older than the Sun have fewer heavy elements compared with the Sun.<ref name=Gibson-etal-2013/> 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]].<ref name=Cyburt-etal-2016/> When the universe had cooled sufficiently, the first stars were born as population&nbsp;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 [[nucleosynthesis|nucleosynthetic]] processes created the first 26&nbsp;elements (up to [[iron]] in the [[periodic table]]).<ref name=Heger-Woosley-2002/>

Many theoretical stellar models show that most high-mass population&nbsp;III stars rapidly exhausted their fuel and likely exploded in extremely energetic [[pair-instability supernova]]e. 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&nbsp;III stars should be observable.<ref name=Schlaufman-Thompson-Casey-2018/> However, some population&nbsp;III stars might be seen in high-[[redshift]] galaxies whose light originated during the earlier history of the universe.<ref name=hao2013/>  Scientists have found evidence of [[2MASS J18082002−5104378|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.<ref name=scinews/>

Stars too massive to produce pair-instability supernovae would have likely collapsed into [[black hole]]s through a process known as [[photodisintegration]]. Here some matter may have escaped during this process in the form of [[relativistic jet]]s, and this could have distributed the first metals into the universe.<ref name=fryer2001/><ref name=heger2003/>{{efn|It has been proposed that recent supernovae [[SN&nbsp;2006gy]] and [[SN&nbsp;2007bi]] may have been [[pair-instability supernova]]e where such super-massive population&nbsp;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.<ref name=Clark-2010/>}}

===Formation of the observed stars===

The oldest stars observed thus far,<ref name=Schlaufman-Thompson-Casey-2018/> known as population&nbsp;II, have very low metallicities;<ref name=Salvaterra-Ferrara-Schneider-2004/><ref name=Byant-2005/> as subsequent generations of stars were born, they became more metal-enriched, as the [[gas]]eous clouds from which they formed received the metal-rich [[cosmic dust|dust]] manufactured by previous generations of stars from population&nbsp;III.

As those population&nbsp;II stars died, they returned metal-enriched material to the [[interstellar medium]] via [[planetary nebula]]e 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&nbsp;I stars.