<p>We could not-- whatever happens inside a black hole stays inside a black hole.  One way this is often explained is that the time coordinate inside the "event horizon" of a black hole looks to us on the outside like a radial coordinate-- events that occur at sequentially different times inside the event horizon, such as your supernova, seem to us on the outside to happen at sequentially smaller radii, until the last event for your dying star is to reach the center.  The same holds for all the ejecta and even the light and neutrinos emitted-- it all goes to the center in a finite time.  So nothing escapes from the event horizon.</p>

<p>Another way to say this is that "black holes have no hair", meaning there is not information outside the event horizon stemming from anything happening on the inside.  All that happens to the outside stems from what falls into the hole-- how much mass, charge, and angular momentum crosses the event horizon.  What it does while it is in there has no effect on the outside, so could never be detected.</p>

CC BY-SA 4.0
2019-02-23T23:06:28.980Z
You might add that it can't be observed because we can't observe anything cross an event horizon.
ProfRob
CC BY-SA 4.0
2019-02-22T08:46:49.233Z
2019-02-25T14:19:40.163Z
2019-02-25T14:19:40.163Z
Ken G
<p>A black hole swallows a star.</p>

<p>The star remains intact.</p>

<p>Then goes supernova.</p>

<p>What would happen to the star?</p>

<p>Could we detect this event?</p>

CC BY-SA 4.0
2019-02-22T12:38:24.760Z
How would the star remain intact? I don't think this can happen. Therefore the whole scenario is a bit constructed.
AtmosphericPrisonEscape
CC BY-SA 4.0
2019-02-22T12:49:36.233Z
The largest supermassive black holes have a Schwarzchild radius around $6\times 10^{13}$ meters. Tidal forces near that radius would be roughly $10^{-12} g/m$ or a few milli g across the diameter of a not too large star. So it is definitely possible for a star to fall into such a black hole without being immediately destroyed.
Steve Linton
CC BY-SA 4.0
2019-02-22T16:35:46.460Z
I'm voting to close this question as off-topic because the premise is fatally flawed - the situation could never occur.
Carl Witthoft
CC BY-SA 4.0
2019-02-22T16:36:57.793Z
@SteveLinton only if you accept the Schwarzchild R as defining a black hole itself, which I don't think is correct.
Carl Witthoft
CC BY-SA 4.0
2019-02-22T16:37:50.177Z
FWIW I wouldn't be surprised if the required conditions for a supernova cannot occur in a gravitational field of black-hole strength.
Carl Witthoft
CC BY-SA 4.0
2019-02-23T01:58:42.570Z
Interesting scenario. However, this is impossible.
Max0815
CC BY-SA 4.0
2019-02-23T10:31:06.943Z
Related and showing that tidal force might be sustained by a body https://physics.stackexchange.com/questions/218102/is-there-a-point-at-which-spaghettification-is-highest at least mathematically
Alchimista
CC BY-SA 4.0
2019-02-23T11:38:51.367Z
@CarlWitthoft (also Max0815) please explain what makes it impossible. As Steve Linton notes, the event horizon of a large SMBH can easily be 200 AU in radius, and there are a number of plausible situations where a star, white dwarf or neutron star could go supernova at that distance.
Chappo Hasn't Forgotten
CC BY-SA 4.0
2019-02-23T23:04:41.337Z
@Chappo I started to write the same comment, but stopped. It is impossible *to observe*, but not because it can't happen, just not in our reference frame.
ProfRob
CC BY-SA 4.0
2019-02-23T23:56:16.700Z
@Chappo the gravitational pull of the Black Hole will destroy an object before it reaches event horizon.
Max0815
CC BY-SA 4.0
2019-02-24T03:06:33.893Z
@Max0815 Gravitational pull doesn't destroy objects, otherwise Jupiter would have no moons, the Sun would have no planets, and our local black hole at Sagittarius A`*` wouldn't have [stars orbiting it](https://en.wikipedia.org/wiki/Sagittarius_A*#Orbiting_stars). You might be thinking of the [Roche limit](https://en.wikipedia.org/wiki/Roche_limit), but note that this limit scales as the cube root of a BH's mass, whereas the event horizon scales linearly with mass. A BH 1000 times more massive than Sagittarius A* has a Schwarzschild radius 1000x larger but a Roche limit only 10x larger. :-)
Chappo Hasn't Forgotten
CC BY-SA 4.0
2019-02-24T05:08:39.407Z
@Chappo Oh. I see.
Max0815
CC BY-SA 4.0
2019-02-24T16:01:27.550Z
@Chappo right yeah the roche limit of supermassive black hole is inside event horizon.
Max0815
<supernova><supermassive-black-hole>
What would happen if a star went supernova inside a supermassive black hole?
Flood
# What would happen if a star went supernova inside a supermassive black hole?

(Asked by: username_0 on 2019-02-22)

A black hole swallows a star.

The star remains intact.

Then goes supernova.

What would happen to the star?

Could we detect this event?

**Tags:** supernova, supermassive-black-hole

> **Comments:**
> **username_1**: How would the star remain intact? I don't think this can happen. Therefore the whole scenario is a bit constructed.  
> **username_2**: The largest supermassive black holes have a Schwarzchild radius around $6\times 10^{13}$ meters. Tidal forces near that radius would be roughly $10^{-12} g/m$ or a few milli g across the diameter of a not too large star. So it is definitely possible for a star to fall into such a black hole without being immediately destroyed.  
> **username_3**: I'm voting to close this question as off-topic because the premise is fatally flawed - the situation could never occur.  
> **username_3**: @SteveLinton only if you accept the Schwarzchild R as defining a black hole itself, which I don't think is correct.  
> **username_3**: FWIW I wouldn't be surprised if the required conditions for a supernova cannot occur in a gravitational field of black-hole strength.  
> **username_4**: Interesting scenario. However, this is impossible.  
> **username_5**: Related and showing that tidal force might be sustained by a body https://physics.stackexchange.com/questions/218102/is-there-a-point-at-which-spaghettification-is-highest at least mathematically  
> **username_6**: @CarlWitthoft (also username_4) please explain what makes it impossible. As username_2 notes, the event horizon of a large SMBH can easily be 200 AU in radius, and there are a number of plausible situations where a star, white dwarf or neutron star could go supernova at that distance.  
> **username_7**: @Chappo I started to write the same comment, but stopped. It is impossible *to observe*, but not because it can't happen, just not in our reference frame.  
> **username_4**: @Chappo the gravitational pull of the Black Hole will destroy an object before it reaches event horizon.  
> **username_6**: @username_4 Gravitational pull doesn't destroy objects, otherwise Jupiter would have no moons, the Sun would have no planets, and our local black hole at Sagittarius A`*` wouldn't have [stars orbiting it](https://en.wikipedia.org/wiki/Sagittarius_A*#Orbiting_stars). You might be thinking of the [Roche limit](https://en.wikipedia.org/wiki/Roche_limit), but note that this limit scales as the cube root of a BH's mass, whereas the event horizon scales linearly with mass. A BH 1000 times more massive than Sagittarius A* has a Schwarzschild radius 1000x larger but a Roche limit only 10x larger. :-)  
> **username_4**: @Chappo Oh. I see.  
> **username_4**: @Chappo right yeah the roche limit of supermassive black hole is inside event horizon.  

---

## 1 answers

### Answer by username_8 on 2019-02-22. Score: 2

We could not-- whatever happens inside a black hole stays inside a black hole. One way this is often explained is that the time coordinate inside the "event horizon" of a black hole looks to us on the outside like a radial coordinate-- events that occur at sequentially different times inside the event horizon, such as your supernova, seem to us on the outside to happen at sequentially smaller radii, until the last event for your dying star is to reach the center. The same holds for all the ejecta and even the light and neutrinos emitted-- it all goes to the center in a finite time. So nothing escapes from the event horizon.

Another way to say this is that "black holes have no hair", meaning there is not information outside the event horizon stemming from anything happening on the inside. All that happens to the outside stems from what falls into the hole-- how much mass, charge, and angular momentum crosses the event horizon. What it does while it is in there has no effect on the outside, so could never be detected.

> **Comments:**
> **username_7**: You might add that it can't be observed because we can't observe anything cross an event horizon.  

---

