Editing Timeline of the universe (section)

=== Reionization ===
* c. 100 million years: Gravitational collapse: ordinary matter particles fall into the structures created by dark matter. [[Reionization]] begins: smaller ([[stars]]) and larger non-linear structures ([[quasars]]) begin to take shape – their [[ultraviolet]] light ionizes remaining neutral gas.
* 200–300 million years: First stars begin to shine: Because many are [[Population III stars]] (some [[Population II stars]] are accounted for at this time) they are much bigger and hotter and their life cycle is fairly short. Unlike later generations of stars, these stars are metal free. [[Reionization]] begins, with the absorption of certain wavelengths of light by neutral hydrogen creating [[Gunn–Peterson trough]]s. The resulting ionized gas (especially free electrons) in the [[Warm–hot intergalactic medium|intergalactic medium]] causes some [[Reionization#CMB anisotropy and polarization|scattering]] of light, but with much lower opacity than before recombination due the expansion of the universe and clumping of gas into galaxies.
* 200 million years: The [[List of oldest stars|oldest-known star]] (confirmed) – [[SMSS J031300.36−670839.3]], forms.
* 300 million years: First large-scale astronomical objects, [[protogalaxies]] and [[quasars]] may have begun forming. As Population III stars continue to burn, [[stellar nucleosynthesis]] operates – stars burn mainly by fusing hydrogen to produce more helium in what is referred to as the [[main sequence]]. Over time these stars are forced to fuse helium to produce [[carbon]], [[oxygen]], [[silicon]] and other heavy elements up to [[iron]] on the periodic table. These elements, when seeded into neighbouring gas clouds by [[supernova]], will lead to the formation of more [[Population II]] stars (metal poor) and [[gas giants]].
* 320 million years (''z'' = 13.3): [[HD1]], the oldest-known spectroscopically-confirmed [[galaxy]], forms.<ref>{{Cite web |last=Simion @Yonescat |first=Florin |title=Scientists have spotted the farthest galaxy ever |url=https://ras.ac.uk/news-and-press/news/scientists-have-spotted-farthest-galaxy-ever |access-date=2023-07-13 |website=The Royal Astronomical Society |date=6 April 2022 |language=en}}</ref>
* 380 million years: [[UDFj-39546284]] forms, current record holder for unconfirmed oldest-known [[quasar]].<ref name="Space-20121212">{{cite web |last=Wall |first=Mike |title=Ancient Galaxy May Be Most Distant Ever Seen|url=http://www.space.com/18879-hubble-most-distant-galaxy.html |date=12 December 2012|publisher=[[Space.com]] |access-date=12 December 2012 }}</ref>
* 600 million years: [[HE 1523-0901]], the oldest star found producing [[neutron capture]] elements forms, marking a new point in ability to detect stars with a telescope.<ref name="h152309">{{cite journal|url=https://authors.library.caltech.edu/16647/ |author=Collaborative |title=Discovery of HE 1523–0901 |journal=Astrophysical Journal Letters |volume=660 |pages=L117–L120 |publisher=CaltechAUTHORS |date=11 April 2007 |access-date=19 February 2019 }}</ref>
* 630 million years (''z'' = 8.2): [[GRB 090423]], the oldest [[gamma-ray burst]] recorded suggests that supernovas may have happened very early on in the evolution of the Universe<ref>{{cite web|title=GRB 090423 goes Supernova in a galaxy, far, far away|url=http://www.zimbio.com/member/paulano123/articles/6044508/GRB+090423+goes+Supernova+galaxy+far+far+away|archive-url=https://archive.today/20130105130128/http://www.zimbio.com/member/paulano123/articles/6044508/GRB+090423+goes+Supernova+galaxy+far+far+away|url-status=dead|archive-date=5 January 2013|work= Zimbio|access-date=23 February 2010}}</ref>