Editing Sun (section)

=== After core hydrogen exhaustion ===
<!-- [[End of the Sun]] redirects to this section, please fix that if renaming this section. Thanks! -->
[[File:Sun red giant.svg|thumb|left|The size of the current Sun (now in the [[main sequence]]) compared to its estimated size during its red-giant phase in the future|alt=See caption]]
The Sun does not have enough mass to explode as a [[supernova]]. Instead, when it runs out of hydrogen in the core in approximately 5&nbsp;billion years, core hydrogen fusion will stop, and there will be nothing to prevent the core from contracting. The release of gravitational potential energy will cause the luminosity of the Sun to increase, ending the main sequence phase and leading the Sun to expand over the next billion years: first into a [[subgiant]], and then into a [[red giant]].<ref name="carroll_ostlie" /><ref>{{cite web |first=Nola Taylor |last=Redd |title=Red Giant Stars: Facts, Definition & the Future of the Sun |url=http://www.space.com/22471-red-giant-stars.html |website=space.com |access-date=20 February 2016 |archive-date=9 February 2016 |archive-url=https://web.archive.org/web/20160209042249/http://www.space.com/22471-red-giant-stars.html |url-status=live}}</ref><ref name=schroder>{{Cite journal |last1=Schröder |first1=K.-P. |last2=Connon Smith |first2=R. |doi=10.1111/j.1365-2966.2008.13022.x |title=Distant future of the Sun and Earth revisited |journal=Monthly Notices of the Royal Astronomical Society |volume=386 |issue=1 |pages=155–163 |year=2008 |doi-access=free |arxiv=0801.4031 |bibcode=2008MNRAS.386..155S |s2cid=10073988}}</ref> The heating due to gravitational contraction will also lead to expansion of the Sun and hydrogen fusion in a shell just outside the core, where unfused hydrogen remains, contributing to the increased luminosity, which will eventually reach more than 1,000 times its present luminosity.<ref name="carroll_ostlie" /> When the Sun enters its [[red-giant branch]] (RGB) phase, it will engulf (and very likely destroy) [[Mercury (planet)|Mercury]] and [[Venus]]. According to a 2008 article, Earth's orbit will have initially expanded to at most {{Convert|1.5|AU|e6km e6mi|abbr=unit|sigfig=2}} due to the Sun's loss of mass. However, Earth's orbit will then start shrinking due to [[tidal forces]] (and, eventually, drag from the lower chromosphere) so that it is engulfed by the Sun during the [[tip of the red-giant branch]] phase 7.59&nbsp;billion years from now, 3.8 and 1&nbsp;million years after Mercury and Venus have respectively suffered the same fate.<ref name="schroder" />

By the time the Sun reaches the tip of the red-giant branch, it will be about 256 times larger than it is today, with a radius of {{Convert|1.19|AU|e6km e6mi|abbr=unit}}.<ref name="schroder" /><ref name="sackmann">{{Cite journal |last1=Boothroyd |first1=Arnold I. |last2=Sackmann |first2=I.-Juliana |date=1 January 1999 |orig-date=19 December 1995 |title=The CNO Isotopes: Deep Circulation in Red Giants and First and Second Dredge-up |url=https://iopscience.iop.org/article/10.1086/306546 |journal=The Astrophysical Journal |publisher=The American Astronomical Society (AAS), The Institute of Physics (IOP) |volume=510 |issue=1 |pages=232–250 |arxiv=astro-ph/9512121 |bibcode=1999ApJ...510..232B |doi=10.1086/306546 |s2cid=561413}}</ref> The Sun will spend around a billion years in the RGB and lose around a third of its mass.<ref name="schroder" />

After the red-giant branch, the Sun has approximately 120&nbsp;million years of active life left, but much happens. First, the core (full of [[degenerate matter|degenerate]] helium) ignites violently in the [[helium flash]]; it is estimated that 6% of the core—itself 40% of the Sun's mass—will be converted into carbon within a matter of minutes through the [[triple-alpha process]].<ref>{{Cite web |first=David |last=Taylor |publisher=Northwestern University |url=http://faculty.wcas.northwestern.edu/~infocom/The%20Website/end.html |title=The End of the Sun |access-date=24 May 2015 |archive-date=22 May 2019 |archive-url=https://web.archive.org/web/20190522175414/http://faculty.wcas.northwestern.edu/~infocom/The%20Website/end.html |url-status=live}}</ref> The Sun then shrinks to around 10 times its current size and 50 times the luminosity, with a temperature a little lower than today. It will then have reached the [[red clump]] or [[horizontal branch]], but a star of the Sun's metallicity does not evolve blueward along the horizontal branch. Instead, it just becomes moderately larger and more luminous over about 100&nbsp;million years as it continues to react helium in the core.<ref name=schroder />

When the helium is exhausted, the Sun will repeat the expansion it followed when the hydrogen in the core was exhausted. This time, however, it all happens faster, and the Sun becomes larger and more luminous. This is the [[asymptotic giant branch|asymptotic-giant-branch]] phase, and the Sun is alternately reacting hydrogen in a shell or helium in a deeper shell. After about 20&nbsp;million years on the early asymptotic giant branch, the Sun becomes increasingly unstable, with rapid mass loss and [[thermal pulse]]s that increase the size and luminosity for a few hundred years every 100,000&nbsp;years or so. The thermal pulses become larger each time, with the later pulses pushing the luminosity to as much as 5,000 times the current level. Despite this, the Sun's maximum AGB radius will not be as large as its tip-RGB maximum: 179 {{Solar radius|link=yes}}, or about {{Convert|0.832|AU|e6km e6mi|abbr=unit}}.<ref name="schroder" /><ref name=agb>{{Cite journal |last1=Vassiliadis |first1=E. |last2=Wood |first2=P. R. |doi=10.1086/173033 |title=Evolution of low- and intermediate-mass stars to the end of the asymptotic giant branch with mass loss |journal=The Astrophysical Journal |volume=413 |page=641 |year=1993 |bibcode=1993ApJ...413..641V |doi-access=free}}</ref>

Models vary depending on the rate and timing of mass loss. Models that have higher mass loss on the red-giant branch produce smaller, less luminous stars at the tip of the asymptotic giant branch, perhaps only 2,000 times the luminosity and less than 200 times the radius.<ref name="schroder" /> For the Sun, four thermal pulses are predicted before it completely loses its outer envelope and starts to make a [[planetary nebula]].<ref name=SunIII>{{cite journal |last1=Sackmann |first1=I.-J. |last2=Boothroyd |first2=A. I. |last3=Kraemer |first3=K. E. |date=1993 |title=Our Sun. III. Present and Future |journal=The Astrophysical Journal |volume=418 |pages=457–468 |doi=10.1086/173407 |bibcode=1993ApJ...418..457S}}</ref>

The post-asymptotic-giant-branch evolution is even faster. The luminosity stays approximately constant as the temperature increases, with the ejected half of the Sun's mass becoming ionised into a [[planetary nebula]] as the exposed core reaches {{Convert|30,000|K|F|sigfig=}}, as if it is in a sort of [[blue loop]]. The final naked core, a [[white dwarf]], will have a temperature of over {{Convert|100,000|K|F|sigfig=}} and contain an estimated 54.05% of the Sun's present-day mass.<ref name=schroder /> Simulations indicate that the Sun may be among the least massive stars capable of forming a planetary nebula.<ref>{{cite journal |last1=Gesicki |first1=K. |last2=Zijlstra |first2=A. A. |last3=Miller Bertolami |first3=M. M. |year=2018 |title=The mysterious age invariance of the planetary nebula luminosity function bright cut-off |journal=Nature Astronomy |volume=2 |number=7 |pages=580–584 |doi=10.1038/s41550-018-0453-9 |arxiv=1805.02643 |bibcode=2018NatAs...2..580G}}</ref> The planetary nebula will disperse in about 10,000&nbsp;years, but the white dwarf will survive for trillions of years before fading to a hypothetical super-dense [[black dwarf]].<ref name=bloecker1>{{Cite journal |bibcode=1995A&A...297..727B |title=Stellar evolution of low and intermediate-mass stars. I. Mass loss on the AGB and its consequences for stellar evolution |last=Bloecker |first=T. |journal=Astronomy and Astrophysics |year=1995 |volume=297 |page=727}}</ref><ref name=bloecker2>{{Cite journal |bibcode=1995A&A...299..755B |title=Stellar evolution of low- and intermediate-mass stars. II. Post-AGB evolution |last=Bloecker |first=T. |journal=Astronomy and Astrophysics |year=1995 |volume=299 |page=755}}</ref><ref>{{cite journal |first=Jørgen |last=Christensen-Dalsgaard |title=Solar structure and evolution |journal=Living Reviews in Solar Physics |year=2021 |volume=18 |number=2 |page=2 |doi=10.1007/s41116-020-00028-3 |arxiv=2007.06488 |bibcode=2021LRSP...18....2C}}</ref> As such, it would give off no more energy.<ref name="Johnson-Groh 2020 j255">{{cite web |last=Johnson-Groh |first=Mara |title=The end of the universe may be marked by 'black dwarf supernova' explosions |website=Live Science |date=25 August 2020 |url=https://www.livescience.com/black-dwarf-supernovae-end-universe.html |access-date=24 November 2023 |archive-date=2 June 2023 |archive-url=https://web.archive.org/web/20230602022731/https://www.livescience.com/black-dwarf-supernovae-end-universe.html |url-status=live}}</ref>