<p>The "common theory" you're reading is not about the processes that produce light in stars, it's just intended as a demonstration of the speed of light through space. When it talks about the Sun "shutting down", it's not talking about the nuclear fusion processes stopping, it means that the Sun as a whole stops shining. I'm not a physicist, but I don't think there's anything that would actually cause this to occur at a sudden time, such that we could measure the 8 minute time difference for the last photons to reach Earth.</p>

<p>It's just a thought experiment, using a simplified description of an impossible event, to make a point about some other process. In this model, the Sun either gives off light or it doesn't, and we're measuring the time since it changed that state; we're not concerned with what the photons do before they're emitted from the Sun (any more than measuring time that light travels from a light bulb cares about how the electricity that powered the bulb was produced -- we could measure that all the way back to the dinosaurs dying and eventually turning into fossil fuels that were used in the power plant, or even further back to the solar energy that powered life on earth).</p>

CC BY-SA 4.0
2019-09-30T15:23:17.263Z
2019-09-30T15:23:17.263Z
Barmar
<p>If nuclear fusion were to suddenly stop in the centre of the Sun, then the only clear signature we would have of this is the lack of detectable neutrinos received at Earth, starting about 8 minutes after the reactions ceased. The Sun however would continue to shine for tens of millions of years at roughly its current luminosity.</p>
<p>The power source is <em>not</em> &quot;stored&quot; photons. The Sun itself would simply <em>resume</em> the slow gravitational contraction that was halted about 4.5 billion years ago when nuclear reaction rates at the centre were able to increase sufficiently to supply the radiative losses from the surface of the Sun.</p>
<p>The characteristic (Kelvin-Helmholtz) timescale for the contraction is about
<span class="math-container">$$\tau_{\rm KH} = \frac{GM^2}{RL},$$</span>
which is 30 million years. i.e. The Sun has enough gravitational potential energy to supply its current luminosity for tens of millions of years.</p>
<p>While this is happening, the Sun would approximately maintain its current luminosity, but decrease in radius, meaning that its surface temperature would increase.</p>
<p>Once the Sun had contracted to a few times the size of Jupiter (so about 30% of its current radius), the contraction would begin to slow, because the electrons in the core become degenerate and the pressure increases with density by more than expected for a perfect gas. The slowing contraction decreases the rate of potential energy release and hence the solar luminosity. The contraction continues at a slow rate until the Sun becomes a hot &quot;hydrogen white dwarf&quot; a few times the size of the Earth, which then cools to a glowing cinder, with no further contraction, over billions of years (see <a href="https://astronomy.stackexchange.com/questions/14018/what-would-the-sun-be-like-if-nuclear-reactions-could-not-proceed-via-quantum-tu?rq=1">What would the Sun be like if nuclear reactions could not proceed via quantum tunneling?</a> for some more details).</p>
<p>Even if you were to not allow the Sun to contract, it would take some time to radiate it's thermal energy. This timescale is approximately
<span class="math-container">$$\tau_{\rm therm} \simeq \frac{3k_B T M}{m_H L},$$</span>
which assumes the Sun is a perfect gas of protons plus electrons, with an average temperature <span class="math-container">$T$</span>. If we take <span class="math-container">$T =10^7$</span> K and the current solar luminosity, then <span class="math-container">$\tau_{\rm therm}=$</span> 40 million years.</p>
<p>On the other hand, if your scenario is just that light from the Sun stops being emitted, then of course it goes dark on Earth about 8 minutes later.</p>

CC BY-SA 4.0
2019-09-29T03:06:37.227Z
Even if you were to somehow manage to stop the gravitational contraction as well, the Sun is pretty hot - ~5800K at the "surface" up to !15 million K at the center.  It would take some considerable time to cool.
jamesqf
CC BY-SA 4.0
2019-09-29T09:04:01.723Z
@jamesqf good point. I have added (and calculated) this and the result is surprisingly (to me) similar to the KH timescale.
ProfRob
CC BY-SA 4.0
2019-09-29T12:29:24.450Z
So, if it would take the Sun 40m years to stop glowing at its current temperature, and the gravitational collapse would increase its temperature, how long after the collapse finished would it take for the sun to stop glowing?
nick012000
CC BY-SA 4.0
2019-09-29T12:51:24.963Z
@nick012000 your question is ill defined and the timescales are approximate. i have not said the Sun would take 40m years to stop glowing. I said it would take 40m years to radiate away its current thermal energy at its current luminosty. If the Sun cooled at constant radius, that timescale would go up, because the luminosity would decrease considerably. The Sun would glow in some sense for billions of years. We can use white dwarfs as a model here. The oldest white dwarfs are still hotter than 3000K.
ProfRob
CC BY-SA 4.0
2019-09-29T17:15:31.020Z
This is a mesmerizing read!
Hanky Panky
CC BY-SA 4.0
2019-09-30T04:05:47.503Z
@Rob Jeffries: Then there are brown dwarfs, the lightest of which don't undergo fusion at all, which makes them good examples of objects heated by gravitational contraction: https://en.wikipedia.org/wiki/Brown_dwarf
jamesqf
CC BY-SA 4.0
2019-09-30T05:04:13.737Z
Once again, the universe refuses to let me grok just how big it is.  Wow.
Cort Ammon
CC BY-SA 4.0
2019-09-30T08:44:24.353Z
@jamesqf Yes, a "brown" dwarf is a very low-mass analogy of what happens during the contraction phase, but the Sun would pass through this phase much faster ($\tau_{KH}$ is 30m yr for the Sun vs billions of yrs for a brown dwarf) because of its higher mass. White dwarfs (WDs) are a better analogy for the stage after, where contraction is halted by degeneracy pressure. The only difference is that WDs are made of C and O, whereas our WD Sun would be made (mostly) of H and as a result has a  bigger radius than a C/O WD. Cooling times would be similar. (Too much detail for most people probably).
ProfRob
CC BY-SA 4.0
2019-09-30T11:02:11.660Z
Since "going out" isn't really possible, if someone put an enormous sun shade just in front of the sun, such that no further light (or other radiation) would pass through and reach earth, then from what you say, yes, you'd have 8 minutes of light before the darkness fell on Earth. There's still plenty of physics that makes this scenario improbable though.
Ralph Bolton
CC BY-SA 4.0
2019-09-30T11:16:04.053Z
@RalphBolton Even that is only true until the sunshade heats up. It would have to have a massive heat capacity to absorb all that light and not reradiate it promptly. But perhaps you could make it reflective.
ProfRob
CC BY-SA 4.0
2019-09-30T12:35:13.287Z
Nice story, based on one interpretation of "the Sun shut down."    I fear there are dozens of interpretations, up to the  "Have Space Suit, WIll Travel" version wherein a star is rotated 90degrees out of its 3D space.
Carl Witthoft
CC BY-SA 4.0
2019-09-30T13:16:45.927Z
See "Inconstant Moon" a short story by Larry Niven! See https://en.wikipedia.org/wiki/Inconstant_Moon
MiguelH
CC BY-SA 4.0
2019-09-30T15:17:09.900Z
I find this answer surprisingly reassuring.
stripybadger
CC BY-SA 4.0
2019-09-30T15:32:19.407Z
@stripybadger It's like having a very, very large tank of hot water and thus not having to worry about your supply of heating fuel.
ProfRob
CC BY-SA 4.0
2019-09-30T19:46:31.560Z
How do electrons "become degenerate"?  Do they put on leather jackets, take up smoking, listening to "the Devil's music", and fraternizing with "loose women"?  Or does that mean the electrons would undergo capture, generating degenerate neutron matter?
Monty Harder
CC BY-SA 4.0
2019-09-30T20:11:11.893Z
@MontyHarder the internet is your friend. Google < "electron degeneracy".
ProfRob
CC BY-SA 4.0
2019-09-30T20:24:12.140Z
@RobJeffries I find lots of mentions of things like "electron-degeneracy pressure", and "electron-degenerate matter", but nothing that explains what it means for electrons themselves to be in a state that can be described as "degenerate".
Monty Harder
CC BY-SA 4.0
2019-09-30T21:53:23.573Z
@MontyHarder Every explanation of electron degeneracy pressure must explain that two electrons cannot occupy the same quantum state (the Pauli Exclusion Principle). This means that at low temperatures (or high densities) they cannot all fall to low energy states. Instead, they fully occupy states right up to the Fermi energy (but no higher). This sea of "degenerate electrons" contains huge amounts of kinetic energy and hence exerts a degeneracy pressure.
ProfRob
CC BY-SA 4.0
2019-09-30T23:53:51.273Z
Can you explain why the contraction is slow? I thought the core would catastrophically collapse!
Peter - Reinstate Monica
CC BY-SA 4.0
2019-09-30T23:59:51.520Z
@PeterA.Schneider a "collapse" cannot occur because thermal energy cannot be ignored, it must be radiated away from the surface. "Core collapse", which initiates a supernova can only happen because (I) there is a huge sink of thermal energy (disintegration of iron nuclei and neutronisation) and (II) most energy can escape via neutrino emission. Neither of these is important in the Sun.
ProfRob
CC BY-SA 4.0
2021-08-24T13:03:50.030Z
@ProfRob Is it not currently theoretically uncertain as to whether neutrino emission or hydrodynamical ejection is the dominant source of energy transport in supernovae?
Daddy Kropotkin
CC BY-SA 4.0
2021-08-24T14:16:16.337Z
@DaddyKropotkin 99%+ of the energy of a supernova escapes in the form of neutrinos.
ProfRob
CC BY-SA 4.0
2021-08-24T20:02:14.703Z
Hmmm... then perhaps, am i confusing this with what is the mechanism driving the explosion?
Daddy Kropotkin
CC BY-SA 4.0
2021-08-26T04:44:43.457Z
So the "Trilithium Torpedoes" in the old film _Star Trek: Generations_ are not only magical _once_ - for being able to stop the nuclear fusion in the star, even though the conditions are right for it - but _twice:_ they are also somehow able to magically suddenly dissipate all the star's heat, so as to force it to collapse. :D
The_Sympathizer
CC BY-SA 4.0
2021-08-26T04:48:19.013Z
And also, to put this another way, after the handwavium application in the question the star would essentially revert to the mechanism of heating that Lord Kelvin first theorized 100+ years ago was the powering mechanism before nuclear fusion was discovered, and the timescale would be about what he calculated as being the Sun's age. He wasn't wrong, after all, with his calculation, he just neglected a particular effect - one which we imagine as being shut off in this hypothetical question.
The_Sympathizer
CC BY-SA 4.0
2021-08-26T09:56:33.300Z
@The_Sympathizer yes, I forgot to define $\tau_{\rm KH}$ as the "Kelvin-Helmholtz" timescale.
ProfRob
CC BY-SA 4.0
2019-09-28T11:06:38.707Z
2021-08-26T09:57:10.290Z
2021-08-26T09:57:10.290Z
ProfRob
<p>The common theory is, that if the Sun &quot;shut down&quot;, we would see the light for eight more minutes (the time that it takes the photons to reach the Earth).</p>
<p>However recently I have read that photons need around 100,000 years to reach the Earth, since the reactions are happening at the Sun's core and gamma rays can't leave the Sun without interacting with other particles, unlike neutrinos for example.</p>
<p>Is that theory correct? If the Sun's core &quot;shut down&quot;, would we still receive photons (light) for another 100,000 years, with only neutrinos disappearing immediately?</p>

CC BY-SA 4.0
2019-09-28T11:08:14.080Z
IT would be better to ask *just* if (and why) light is supposed to take so long to reach the surface of the Sun from the core.  Asking using the "what if something impossible happened" approach you have tends to put people off on science based sites like this.
StephenG - Help Ukraine
CC BY-SA 4.0
2019-09-28T19:11:50.047Z
On the other hand, science-minded people tend to like learning the truth about something (and often even educating others).
Ryan
CC BY-SA 4.0
2019-09-29T02:00:46.130Z
*"photons need around 100 000 years to reach the Earth"*. That's a bit misleading. Yes, it takes a long time for energy to travel from the solar core to the photosphere, but no individual photon spends 100,000 years traveling through the Sun. See https://astronomy.stackexchange.com/a/33447/16685
PM 2Ring
CC BY-SA 4.0
2019-09-29T17:00:57.730Z
Also the in the 'theory' the expected behaviour is the Sun instantly doesn't exist anymore, so presumably the photons within it which were trying to escape also don't exist anymore. If this is the case, then it simply would go dark, 8 minutes later on earth. (ignoring the fact the photons don't exactly exist when they are 'trapped')
djsmiley2kStaysInside
CC BY-SA 4.0
2019-09-30T15:26:48.943Z
Please define "went out" and "shut down". If the sun itself was blinked out of existence then yes, we would have 8 minutes of light remaining. If the Sun's core "simply" shut down then it would take very long for that to propagate through to the surface.
MonkeyZeus
CC BY-SA 4.0
2019-10-01T22:06:13.983Z
The photons we see are "created" at the surface of the sun. -- The random walk of photons taking a long time to go from the core to the surface is a poor explanation. Rather, imagine the atoms in the core are giant buckets of water, and each photon is a thimbleful of water. If there's too much water in a bucket, a thimble goes off in some direction, and if a thimble hits a bucket, it adds its water to that bucket. It is no more the "same" energy than if you poured a glass of water in a lake, waited a minute, then drew another glass.
Ghedipunk
<the-sun><stellar-astrophysics><photons>
Would we have more than 8 minutes of light, if the Sun "went out"?
Murg
# Would we have more than 8 minutes of light, if the Sun "went out"?

(Asked by: username_0 on 2019-09-28)

The common theory is, that if the Sun "shut down", we would see the light for eight more minutes (the time that it takes the photons to reach the Earth).

However recently I have read that photons need around 100,000 years to reach the Earth, since the reactions are happening at the Sun's core and gamma rays can't leave the Sun without interacting with other particles, unlike neutrinos for example.

Is that theory correct? If the Sun's core "shut down", would we still receive photons (light) for another 100,000 years, with only neutrinos disappearing immediately?

**Tags:** the-sun, stellar-astrophysics, photons
> **Comments:**
> **username_1**: IT would be better to ask *just* if (and why) light is supposed to take so long to reach the surface of the Sun from the core.  Asking using the "what if something impossible happened" approach you have tends to put people off on science based sites like this.  
> **username_2**: On the other hand, science-minded people tend to like learning the truth about something (and often even educating others).  
> **username_3**: *"photons need around 100 000 years to reach the Earth"*. That's a bit misleading. Yes, it takes a long time for energy to travel from the solar core to the photosphere, but no individual photon spends 100,000 years traveling through the Sun. See https://astronomy.stackexchange.com/a/33447/16685  
> **username_4**: Also the in the 'theory' the expected behaviour is the Sun instantly doesn't exist anymore, so presumably the photons within it which were trying to escape also don't exist anymore. If this is the case, then it simply would go dark, 8 minutes later on earth. (ignoring the fact the photons don't exactly exist when they are 'trapped')  
> **username_5**: Please define "went out" and "shut down". If the sun itself was blinked out of existence then yes, we would have 8 minutes of light remaining. If the Sun's core "simply" shut down then it would take very long for that to propagate through to the surface.  
> **username_6**: The photons we see are "created" at the surface of the sun. -- The random walk of photons taking a long time to go from the core to the surface is a poor explanation. Rather, imagine the atoms in the core are giant buckets of water, and each photon is a thimbleful of water. If there's too much water in a bucket, a thimble goes off in some direction, and if a thimble hits a bucket, it adds its water to that bucket. It is no more the "same" energy than if you poured a glass of water in a lake, waited a minute, then drew another glass.  

---

## 2 answers

### Answer by username_8 on 2019-09-28. Score: 114 | Accepted answer

If nuclear fusion were to suddenly stop in the centre of the Sun, then the only clear signature we would have of this is the lack of detectable neutrinos received at Earth, starting about 8 minutes after the reactions ceased. The Sun however would continue to shine for tens of millions of years at roughly its current luminosity.

The power source is *not* "stored" photons. The Sun itself would simply *resume* the slow gravitational contraction that was halted about 4.5 billion years ago when nuclear reaction rates at the centre were able to increase sufficiently to supply the radiative losses from the surface of the Sun.

The characteristic (Kelvin-Helmholtz) timescale for the contraction is about
$$\tau\_{\rm KH} = \frac{GM^2}{RL},$$
which is 30 million years. i.e. The Sun has enough gravitational potential energy to supply its current luminosity for tens of millions of years.

While this is happening, the Sun would approximately maintain its current luminosity, but decrease in radius, meaning that its surface temperature would increase.

Once the Sun had contracted to a few times the size of Jupiter (so about 30% of its current radius), the contraction would begin to slow, because the electrons in the core become degenerate and the pressure increases with density by more than expected for a perfect gas. The slowing contraction decreases the rate of potential energy release and hence the solar luminosity. The contraction continues at a slow rate until the Sun becomes a hot "hydrogen white dwarf" a few times the size of the Earth, which then cools to a glowing cinder, with no further contraction, over billions of years (see [What would the Sun be like if nuclear reactions could not proceed via quantum tunneling?](https://astronomy.stackexchange.com/questions/14018/what-would-the-sun-be-like-if-nuclear-reactions-could-not-proceed-via-quantum-tu?rq=1) for some more details).

Even if you were to not allow the Sun to contract, it would take some time to radiate it's thermal energy. This timescale is approximately
$$\tau\_{\rm therm} \simeq \frac{3k\_B T M}{m\_H L},$$
which assumes the Sun is a perfect gas of protons plus electrons, with an average temperature $T$. If we take $T =10^7$ K and the current solar luminosity, then $\tau\_{\rm therm}=$ 40 million years.

On the other hand, if your scenario is just that light from the Sun stops being emitted, then of course it goes dark on Earth about 8 minutes later.

> **Comments:**
> **username_9**: Even if you were to somehow manage to stop the gravitational contraction as well, the Sun is pretty hot - ~5800K at the "surface" up to !15 million K at the center.  It would take some considerable time to cool.  
> **username_8**: @jamesqf good point. I have added (and calculated) this and the result is surprisingly (to me) similar to the KH timescale.  
> **username_10**: So, if it would take the Sun 40m years to stop glowing at its current temperature, and the gravitational collapse would increase its temperature, how long after the collapse finished would it take for the sun to stop glowing?  
> **username_8**: @nick012000 your question is ill defined and the timescales are approximate. i have not said the Sun would take 40m years to stop glowing. I said it would take 40m years to radiate away its current thermal energy at its current luminosty. If the Sun cooled at constant radius, that timescale would go up, because the luminosity would decrease considerably. The Sun would glow in some sense for billions of years. We can use white dwarfs as a model here. The oldest white dwarfs are still hotter than 3000K.  
> **username_11**: This is a mesmerizing read!  
> **username_9**: @Rob Jeffries: Then there are brown dwarfs, the lightest of which don't undergo fusion at all, which makes them good examples of objects heated by gravitational contraction: https://en.wikipedia.org/wiki/Brown_dwarf  
> **username_12**: Once again, the universe refuses to let me grok just how big it is.  Wow.  
> **username_8**: @jamesqf Yes, a "brown" dwarf is a very low-mass analogy of what happens during the contraction phase, but the Sun would pass through this phase much faster ($\tau_{KH}$ is 30m yr for the Sun vs billions of yrs for a brown dwarf) because of its higher mass. White dwarfs (WDs) are a better analogy for the stage after, where contraction is halted by degeneracy pressure. The only difference is that WDs are made of C and O, whereas our WD Sun would be made (mostly) of H and as a result has a  bigger radius than a C/O WD. Cooling times would be similar. (Too much detail for most people probably).  
> **username_13**: Since "going out" isn't really possible, if someone put an enormous sun shade just in front of the sun, such that no further light (or other radiation) would pass through and reach earth, then from what you say, yes, you'd have 8 minutes of light before the darkness fell on Earth. There's still plenty of physics that makes this scenario improbable though.  
> **username_8**: @RalphBolton Even that is only true until the sunshade heats up. It would have to have a massive heat capacity to absorb all that light and not reradiate it promptly. But perhaps you could make it reflective.  
> **username_14**: Nice story, based on one interpretation of "the Sun shut down."    I fear there are dozens of interpretations, up to the  "Have Space Suit, WIll Travel" version wherein a star is rotated 90degrees out of its 3D space.  
> **username_15**: See "Inconstant Moon" a short story by Larry Niven! See https://en.wikipedia.org/wiki/Inconstant_Moon  
> **username_16**: I find this answer surprisingly reassuring.  
> **username_8**: @stripybadger It's like having a very, very large tank of hot water and thus not having to worry about your supply of heating fuel.  
> **username_17**: How do electrons "become degenerate"?  Do they put on leather jackets, take up smoking, listening to "the Devil's music", and fraternizing with "loose women"?  Or does that mean the electrons would undergo capture, generating degenerate neutron matter?  
> **username_8**: @MontyHarder the internet is your friend. Google < "electron degeneracy".  
> **username_17**: @RobJeffries I find lots of mentions of things like "electron-degeneracy pressure", and "electron-degenerate matter", but nothing that explains what it means for electrons themselves to be in a state that can be described as "degenerate".  
> **username_8**: @MontyHarder Every explanation of electron degeneracy pressure must explain that two electrons cannot occupy the same quantum state (the Pauli Exclusion Principle). This means that at low temperatures (or high densities) they cannot all fall to low energy states. Instead, they fully occupy states right up to the Fermi energy (but no higher). This sea of "degenerate electrons" contains huge amounts of kinetic energy and hence exerts a degeneracy pressure.  
> **username_18**: Can you explain why the contraction is slow? I thought the core would catastrophically collapse!  
> **username_8**: @PeterA.Schneider a "collapse" cannot occur because thermal energy cannot be ignored, it must be radiated away from the surface. "Core collapse", which initiates a supernova can only happen because (I) there is a huge sink of thermal energy (disintegration of iron nuclei and neutronisation) and (II) most energy can escape via neutrino emission. Neither of these is important in the Sun.  
> **username_19**: @ProfRob Is it not currently theoretically uncertain as to whether neutrino emission or hydrodynamical ejection is the dominant source of energy transport in supernovae?  
> **username_8**: @DaddyKropotkin 99%+ of the energy of a supernova escapes in the form of neutrinos.  
> **username_19**: Hmmm... then perhaps, am i confusing this with what is the mechanism driving the explosion?  
> **username_20**: So the "Trilithium Torpedoes" in the old film _Star Trek: Generations_ are not only magical _once_ - for being able to stop the nuclear fusion in the star, even though the conditions are right for it - but _twice:_ they are also somehow able to magically suddenly dissipate all the star's heat, so as to force it to collapse. :D  
> **username_20**: And also, to put this another way, after the handwavium application in the question the star would essentially revert to the mechanism of heating that Lord Kelvin first theorized 100+ years ago was the powering mechanism before nuclear fusion was discovered, and the timescale would be about what he calculated as being the Sun's age. He wasn't wrong, after all, with his calculation, he just neglected a particular effect - one which we imagine as being shut off in this hypothetical question.  
> **username_8**: @The_Sympathizer yes, I forgot to define $\tau_{\rm KH}$ as the "Kelvin-Helmholtz" timescale.  

---

### Answer by username_7 on 2019-09-30. Score: 11

The "common theory" you're reading is not about the processes that produce light in stars, it's just intended as a demonstration of the speed of light through space. When it talks about the Sun "shutting down", it's not talking about the nuclear fusion processes stopping, it means that the Sun as a whole stops shining. I'm not a physicist, but I don't think there's anything that would actually cause this to occur at a sudden time, such that we could measure the 8 minute time difference for the last photons to reach Earth.

It's just a thought experiment, using a simplified description of an impossible event, to make a point about some other process. In this model, the Sun either gives off light or it doesn't, and we're measuring the time since it changed that state; we're not concerned with what the photons do before they're emitted from the Sun (any more than measuring time that light travels from a light bulb cares about how the electricity that powered the bulb was produced -- we could measure that all the way back to the dinosaurs dying and eventually turning into fossil fuels that were used in the power plant, or even further back to the solar energy that powered life on earth).


---

