Editing Reionization (section)

===Population III stars===
[[Image:NASA-WMAP-first-stars.jpg|thumb|upright=1.4|Simulated image of the first stars, 400 million years after the Big Bang.]] [[Stellar population#Population III stars|Population III stars]] were the earliest stars, which had no elements more massive than hydrogen or [[helium]]. During [[Nucleosynthesis#Big Bang nucleosynthesis|Big Bang nucleosynthesis]], the only elements that formed aside from hydrogen and helium were trace amounts of [[lithium]]. Yet quasar spectra have revealed the presence of heavy elements in the [[intergalactic medium]] at an early era. [[Supernova]] explosions produce such heavy elements, so hot, large, Population III stars which will form supernovae are a possible mechanism for reionization. While they have not been directly observed, they are consistent according to models using numerical simulation<ref name="popIII_sim">{{cite journal |author=Gnedin |first1=Nickolay |last2=Ostriker |first2=Jeremiah |name-list-style=amp |date=1997 |title=Reionization of the Universe and the Early Production of Metals |journal=Astrophysical Journal |volume=486 |issue=2 |pages=581–598 |arxiv=astro-ph/9612127 |bibcode=1997ApJ...486..581G |doi=10.1086/304548 |s2cid=5758398}}</ref> and current observations.<ref name="qso_z">{{cite arXiv | author=Limin Lu | date=1998 | title=The Metal Contents of Very Low Column Density Lyman-alpha Clouds: Implications for the Origin of Heavy Elements in the Intergalactic Medium | eprint=astro-ph/9802189 |display-authors=etal}}</ref> A [[Gravitational lens|gravitationally lensed]] galaxy also provides indirect evidence of Population III stars.<ref>{{cite journal |author=Fosbury |first=R. A. E. |display-authors=etal |year=2003 |title=Massive Star Formation in a Gravitationally Lensed H II Galaxy at z = 3.357 |journal=Astrophysical Journal |volume=596 |issue=1 |pages=797–809 |arxiv=astro-ph/0307162 |bibcode=2003ApJ...596..797F |doi=10.1086/378228 |s2cid=17808828}}</ref> Even without direct observations of Population III stars, they are a compelling source. They are more efficient and effective ionizers than Population II stars, as they emit more ionizing photons,<ref name="popII_vs_popIII">{{cite journal |author=Tumlinson |first=Jason |display-authors=etal |date=2002 |title=Cosmological Reionization by the First Stars: Evolving Spectra of Population III |journal=ASP Conference Proceedings |volume=267 |pages=433–434 |bibcode=2002ASPC..267..433T}}</ref> and are capable of reionizing hydrogen on their own in some reionization models with reasonable [[initial mass function]]s.<ref>{{cite journal |author=Venkatesan |first=Apama |display-authors=etal |date=2003 |title=Evolving Spectra of Population III Stars: Consequences for Cosmological Reionization |journal=Astrophysical Journal |volume=584 |issue=2 |pages=621–632 |arxiv=astro-ph/0206390 |bibcode=2003ApJ...584..621V |doi=10.1086/345738 |s2cid=17737785}}</ref> As a consequence, Population III stars are currently considered the most likely energy source to initiate the reionization of the universe,<ref name="popIII_HII">{{cite journal |author=Alvarez |first=Marcelo |display-authors=etal |year=2006 |title=The H II Region of the First Star |journal=Astrophysical Journal |volume=639 |issue=2 |pages=621–632 |arxiv=astro-ph/0507684 |bibcode=2006ApJ...639..621A |doi=10.1086/499578 |s2cid=12753436}}</ref> though other sources are likely to have taken over and driven reionization to completion.

In June 2015, astronomers reported evidence for [[Stellar population#Population III stars|Population III stars]] in the [[Cosmos Redshift 7]] [[galaxy]] at {{math|''z'' {{=}} 6.60}}. 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 element]]s heavier than [[hydrogen]] that are needed for the later formation of [[planet]]s and [[life]] as we know it.<ref name="AJ-20150604">{{cite journal |last1=Sobral |first1=David |last2=Matthee |first2=Jorryt |last3=Darvish |first3=Behnam |last4=Schaerer |first4=Daniel |last5=Mobasher |first5=Bahram |last6=Röttgering |first6=Huub J. A. |last7=Santos |first7=Sérgio |last8=Hemmati |first8=Shoubaneh |title=Evidence For POPIII-Like Stellar Populations In The Most Luminous LYMAN-α Emitters At The Epoch Of Re-Ionisation: Spectroscopic Confirmation |date=4 June 2015 |journal=[[The Astrophysical Journal]] |doi=10.1088/0004-637x/808/2/139 |bibcode=2015ApJ...808..139S |volume=808 |issue=2 |pages=139|arxiv = 1504.01734 |s2cid=18471887 }}</ref><ref name="NYT-20150617">{{cite news |last=Overbye |first=Dennis |author-link=Dennis Overbye |title=Astronomers Report Finding Earliest Stars That Enriched Cosmos |url=https://www.nytimes.com/2015/06/18/science/space/astronomers-report-finding-earliest-stars-that-enriched-cosmos.html |date=17 June 2015 |work=[[The New York Times]] |access-date=17 June 2015 }}</ref>