Editing Sun (section)

==== Photosphere ====
{{Main|Photosphere}}
[[File:Highest resolution photo of Sun (NSF) as of January 20, 2020.jpg|thumb|alt=A false-colour image of the solar photosphere|The photosphere is structured by convection cells referred to as ''[[Solar granule|granules]]''.]]

The visible surface of the Sun, the photosphere, is the layer below which the Sun becomes [[opacity (optics)|opaque]] to visible light.<ref name=Abhyankar1977 /> Photons produced in this layer escape the Sun through the transparent solar atmosphere above it and become solar radiation, sunlight. The change in opacity is due to the decreasing amount of [[Hydrogen anion|H<sup>−</sup> ions]], which absorb visible light easily.<ref name=Abhyankar1977 /> Conversely, the visible light perceived is produced as electrons react with hydrogen atoms to produce H<sup>−</sup> ions.<ref name="Gibson">{{Cite book |last=Gibson |first=Edward G. |date=1973 |title=The Quiet Sun (NASA SP-303) |publisher=NASA |asin=B0006C7RS0}}</ref><ref name="Shu">{{Cite book |last=Shu |first=F. H. |title=The Physics of Astrophysics |volume=1 |publisher=University Science Books |year=1991 |isbn=978-0-935702-64-4}}</ref>

The photosphere is tens to hundreds of kilometres thick, and is slightly less opaque than air on Earth. Because the upper part of the photosphere is cooler than the lower part, an image of the Sun appears brighter in the centre than on the edge or ''limb'' of the solar disk, in a phenomenon known as ''[[limb darkening]]''.<ref name="Abhyankar1977" /> The spectrum of sunlight has approximately the spectrum of a [[black-body]] radiating at {{convert|5,772|K|F}},<ref name="IAU2015resB3"/> interspersed with atomic [[absorption line]]s from the tenuous layers above the photosphere. The photosphere has a particle density of ~10<sup>23</sup>&nbsp;m<sup>−3</sup> (about 0.37% of the particle number per volume of [[Earth's atmosphere]] at sea level). The photosphere is not fully ionised—the extent of ionisation is about 3%, leaving almost all of the hydrogen in atomic form.<ref>{{cite journal |last1=Rast |first1=M. |last2=Nordlund |first2=Å. |last3=Stein |first3=R. |last4=Toomre |first4=J. |date=1993 |title=Ionization Effects in Three-Dimensional Solar Granulation Simulations |journal=[[The Astrophysical Journal Letters]] |volume=408 |issue=1 |page=L53–L56 |bibcode=1993ApJ...408L..53R |doi=10.1086/186829 |doi-access=free}}</ref>

The coolest layer of the Sun is a temperature minimum region extending to about {{val|500|u=km}} above the photosphere, and has a temperature of about {{val|4100|u=K|fmt=commas}}.<ref name="Abhyankar1977">{{Cite journal |last=Abhyankar |first=K. D. |date=1977 |title=A Survey of the Solar Atmospheric Models |url=http://prints.iiap.res.in/handle/2248/510 |url-status=live |journal=[[Bulletin of the Astronomical Society of India]] |volume=5 |pages=40–44 |bibcode=1977BASI....5...40A |archive-url=https://web.archive.org/web/20200512151641/http://prints.iiap.res.in/handle/2248/510 |archive-date=12 May 2020 |access-date=12 July 2009}}</ref> This part of the Sun is cool enough to allow for the existence of simple molecules such as [[carbon monoxide]] and water.<ref name="Solanki1994">{{Cite journal |last1=Solanki |first1=S. K. |last2=Livingston |first2=W. |last3=Ayres |first3=T. |date=1994 |title=New Light on the Heart of Darkness of the Solar Chromosphere |journal=[[Science (journal)|Science]] |pmid=17748350 |volume=263 |issue=5143 |pages=64–66 |bibcode=1994Sci...263...64S |doi=10.1126/science.263.5143.64 |s2cid=27696504}}</ref>