# Thread: How long would the Earth last if the sun became a black hole?

1. ## How long would the Earth last if the sun became a black hole?

From our point of view, time dilation included, how long would we last if the Sun became a black hole?

We'd all be dead from the supernova that comes first, wouldn't we? Is there any way that wouldn't happen?

2. ## Re: How long would the Earth last if the sun became a black hole?

Depends a little on how exactly you assume this happens.

The sun won't go supernova, it doesn't have the mass, so regardless, we're talking about something that's theoretical.

If you assume "All the mass of the sun is suddenly *boop* in a singularity in the current center-of-mass of the sun" then Earth, the planet, would be fine. We would last a few days or weeks before the Earth cooled off enough to become uninhabitable. People might be able to survive for quite a while if they were in insulated, completely-sealed, perfect-recycling buildings with a long-lasting power source.

If you assume a slightly more realistic scenario like "a very small black hole wanders in from outside the solar system, hits the sun perfectly, somehow stops while inside the sun, and starts eating it from the inside out" -- I believe the accretion disk radiation would fry us pretty quick, but I'm not sure how to calculate that.

(Which brings up a point -- if a very small black hole did fall directly towards the sun from interstellar space, would it actually stop? Concepts like friction don't really apply to black holes. I guess the singularity would get more inflowing momentum from its direction of travel, so it would slow down somewhat...)

3. ## Re: How long would the Earth last if the sun became a black hole?

Gravitationally, quite a long time - pretty much the same amount of mass would be anchoring the Earth in orbit.

For other things...

The shortest answer is "approximately 8 minutes", which is the time light would take to reach us from the sun.

If there are any radiation-based effects they will hit at the speed of light (Gamma rays and other photons) and shortly thereafter (for the very small bits that have rest mass).

If we survive that, we are looking at something simillar to the What If Earth got Kicked Out of the Solar System? Rogue Earth scenario described by Kurzgesagt (the relevent stuff starts around 3:58 - the first bit is discussing how the planet would be ejected).

4. ## Re: How long would the Earth last if the sun became a black hole?

The most plausible scenario would be a black hole coming in at initially just over escape speed, and then getting a momentum transfer from the matter it plows into, enough to bring it to just under escape speed. It'd go through the Sun, come out the other side, and head back out into space... but would then eventually slow down, stop, and fall back in, on an orbit around the Sun with its perihelion inside the surface of the Sun. Every time it passed through, it'd lose a little more momentum, and rise to a little lower aphelion, and come back a little sooner, until, after many such passes, it'd eventually be orbiting entirely inside the Sun.

But yeah, the accretion process would release a lot of energy. Maybe not initially, while the hole is still tiny, but at some point, eventually, you'd have a hole that's an appreciable fraction of the mass of the Sun, surrounded by another appreciable portion of a solar mass, and that's going to get hot. I'm not sure we have a name for the resulting phenomenon, but it'd definitely be something in the general ballpark of a supernova.

5. ## Re: How long would the Earth last if the sun became a black hole?

Originally Posted by the_david
From our point of view, time dilation included, how long would we last if the Sun became a black hole?
If the resulting black hole is the same mass as the sun and we're the same distance from it, there won't be any more time dilation than there already is.

6. ## Re: How long would the Earth last if the sun became a black hole?

Really depends on what you mean "the Sun became a black hole".

If the process involves a supernova then we'd last 8 minutes. Because that's how long it takes light to reach the Earth.

If the process does not involve a supernova but simply rapidly squishing the Sun until it becomes a black hole then it'd be maybe a year before the last person dies of cold. This is because all that would be different is that we'd lose all of our precious sunlight, the Earth would cool down, eventually all the way down to about 2-3 Kelvin. We'd be dead long before that though, most would die within a month, a few survivors in underground bunkers could last as long as they have fuel, food and water.
This scenario is the same as just removing the Sun as far as our survival is concerned.

If the process means putting a stellar mass black hole in the sun's place then we'd go into a highly eccentric orbit and potentially the Earth would be torn apart by tidal forces as it approaches periapsis (perihelios? periblackholeius?).

If the process means putting a supermassive black hole in the sun's place then we'd die more or less instantly.

7. ## Re: How long would the Earth last if the sun became a black hole?

Originally Posted by Chronos
The most plausible scenario would be a black hole coming in at initially just over escape speed, and then getting a momentum transfer from the matter it plows into, enough to bring it to just under escape speed. It'd go through the Sun, come out the other side, and head back out into space... but would then eventually slow down, stop, and fall back in, on an orbit around the Sun with its perihelion inside the surface of the Sun. Every time it passed through, it'd lose a little more momentum, and rise to a little lower aphelion, and come back a little sooner, until, after many such passes, it'd eventually be orbiting entirely inside the Sun.

But yeah, the accretion process would release a lot of energy. Maybe not initially, while the hole is still tiny, but at some point, eventually, you'd have a hole that's an appreciable fraction of the mass of the Sun, surrounded by another appreciable portion of a solar mass, and that's going to get hot. I'm not sure we have a name for the resulting phenomenon, but it'd definitely be something in the general ballpark of a supernova.
I'm not entirely convinced that is what would happen. The issue is that the amount of material actually 'eaten' by the black hole would be pretty tiny, so the momentum it would get from that would be quite small. Meanwhile a huge amount of energy would be radiating out, blowing out a bubble of very hot material behind it. Material in front of it would be cooler and higher density than the hotter material behind it, so I think gravity might actually accelerate it! The sun might just spit it back out again.

If it came close to the sun without actually consuming any mass then tidal effects would definitely slow it, but the two effects might not cancel in any sort of stable way. The issue is that the forward force would be strictly prograde, while the tidal forces would be just resisting in the angular direction. I think it would get increasingly eccentric until the black hole was just kicked out of the solar system completely.

The energy available from material infalling into a black hole is absolutely absurd, to the point that solar escape velocity is barely a rounding error. You cannot just assume that friction will behave like you would expect any more than you would expect friction on a bullet to work as expected when it hits dynamite. Friction is intuitive because contact usually reduces energy available, but when a black hole contacts matter you end up with far more energy floating about than you started with. The first instinct might be wrong. Would need proper modelling to be sure though, and I haven't been able to find any (please point me at some if you know of any).

If the sun were suddenly replaced with a stellar mass black hole, the initial impact would be panic, because there would be no other initial impact. Then we would get cold. If material started infalling we might get hit by massive x-ray doses, but largely the solar system would keep on turning. Orbital mechanics doesn't care what the object at the centre is, just that it is near spherical of a certain mass.

8. ## Re: How long would the Earth last if the sun became a black hole?

Originally Posted by Mastikator
If the process means putting a stellar mass black hole in the sun's place then we'd go into a highly eccentric orbit and potentially the Earth would be torn apart by tidal forces as it approaches periapsis (perihelios? periblackholeius?).
When you say "stellar mass" black hole, do you mean a black hole with the mass of the sun? Or one with several to several-dozen times the mass of the sun? My first impression when you say 'stellar mass' is the mass of the sun, but in that case we wouldn't notice anything other than the sun has stopped shining. If you mean "several to several-dozen" solar masses, then yeah, our orbit will rapidly decay and we'll get ripped apart when we hit the black hole's roche limit.

9. ## Re: How long would the Earth last if the sun became a black hole?

Oops! Duplicate Post!

10. ## Re: How long would the Earth last if the sun became a black hole?

Originally Posted by Lord Torath
When you say "stellar mass" black hole, do you mean a black hole with the mass of the sun? Or one with several to several-dozen times the mass of the sun? My first impression when you say 'stellar mass' is the mass of the sun, but in that case we wouldn't notice anything other than the sun has stopped shining. If you mean "several to several-dozen" solar masses, then yeah, our orbit will rapidly decay and we'll get ripped apart when we hit the black hole's roche limit.
I'm just copying NASA's nomenclature to be honest. It's a black hole that formed when a sufficiently large star undergoes supernova. The remaining black hole is at least 3 times more massive than the sun, but probably 5-8 times the mass of the sun.

Plopping down such a black hole on top of the Sun would possibly rip the Earth apart from the sudden gravitational wave, possibly atomize the Earth from the ensuing explosion. If neither of that happens then the resulting higher gravity would pull on the Earth 3-8 times stronger. That would change the orbit, wherever the Earth is would become the new apoapsis, the periapsis would be significantly closer, possibly within the roche limit.

https://www.nasa.gov/image-article/s...ss-black-hole/
https://en.wikipedia.org/wiki/Stellar_black_hole

11. ## Re: How long would the Earth last if the sun became a black hole?

Gotcha. Thanks for the clarification!

12. ## Re: How long would the Earth last if the sun became a black hole?

Originally Posted by Sermil
if a very small black hole did fall directly towards the sun from interstellar space, would it actually stop? Concepts like friction don't really apply to black holes. I guess the singularity would get more inflowing momentum from its direction of travel, so it would slow down somewhat...)
If it's small enough and fast enough it wouldn't stop. It's even been theorized that the Earth has been hit by several black holes.

Originally Posted by Mastikator
That would change the orbit, wherever the Earth is would become the new apoapsis, the periapsis would be significantly closer, possibly within the roche limit
If I calculated it correctly, Earth would not be in the Roche limit. Although Venus and Mercury would enter it.

The Earth-Sun Roche limit is only about 110% of the sun's radius (the sun is not dense). If we octuple the Sun's mass1, we double the Roche limit. Assuming the new periapsis is no less than an 8th of an AU, that still leaves a fair distance.

1 The Roche limit equations from wikipedia use the sun's radius and density, but those exactly cancel out if you keep the mass constant.

Edited for math mistake

14. ## Re: How long would the Earth last if the sun became a black hole?

If I calculated it correctly, Earth would not be in the Roche limit. Although Venus and Mercury would enter it.

The Earth-Sun Roche limit is only about 110% of the sun's radius (the sun is not dense). If we octuple the Sun's mass1, we octuple the Roche limit. Assuming the new periapsis is no less than an 8th of an AU, that still leaves a fair distance.

1 The Roche limit equations from wikipedia use the sun's radius and density, but those exactly cancel out if you keep the mass constant.
I see the equations as giving the Roche limit as in proportion to the cube root of the mass of the primary. So octupling the Sun's mass would double the Roche limit. (But maybe that's what you meant to say, otherwise why use "octuple".)

15. ## Re: How long would the Earth last if the sun became a black hole?

Originally Posted by DavidSh
I see the equations as giving the Roche limit as in proportion to the cube root of the mass of the primary. So octupling the Sun's mass would double the Roche limit. (But maybe that's what you meant to say, otherwise why use "octuple".)
I made a math mistake, the Roche limit should double, as you said. I was octupling the Sun's mass, because that's approximately what would happen if a stellar mass black hole fell into it and stuck.

16. ## Re: How long would the Earth last if the sun became a black hole?

I think we're focusing too hard on the problems this scenario would cause.

Let's highlight some of the positives of this scenario!

17. ## Re: How long would the Earth last if the sun became a black hole?

Originally Posted by Mastikator
If the process does not involve a supernova but simply rapidly squishing the Sun until it becomes a black hole then it'd be maybe a year before the last person dies of cold. This is because all that would be different is that we'd lose all of our precious sunlight, the Earth would cool down, eventually all the way down to about 2-3 Kelvin. We'd be dead long before that though, most would die within a month, a few survivors in underground bunkers could last as long as they have fuel, food and water.
Grow lamps and space heaters exist

The moon would still be orbiting the earth, so geothermal and tidal are still possibilities for generating power, while greenhouse gases would actually be beneficial to keep heat in. Throw in fission and eventually fusion, plus sealed environments, and civilisation could potentially survive for a long time.

Jupiter might be a longer term source of energy - end goal would have to be migrating to at least Alpha Centuri, so you'd need some colonies to start that process.

That's not to say there wouldn't be a lot of casualties from the initial event, and there'd be conflict as nations try and grab what they can to secure what their leaders see as the right way for civilisation to continue (their way and no other), which could well turn into a nuclear exchange.

18. ## Re: How long would the Earth last if the sun became a black hole?

Originally Posted by Storm_Of_Snow
That's not to say there wouldn't be a lot of casualties from the initial event, and there'd be conflict as nations try and grab what they can to secure what their leaders see as the right way for civilisation to continue (their way and no other), which could well turn into a nuclear exchange.
I mean, we must be increasingly on the alert to prevent them taking over other mine shafts space, in order to breed more prodigiously than we do. Thus, knocking us out of these superior numbers when we emerge! Mr. President, we must not allow a mine-shaft gap!

Quite fitting, ain't it?

19. ## Re: How long would the Earth last if the sun became a black hole?

In a thread pulled back from the brink of extinction:
I mean, we must be increasingly on the alert to prevent them taking over other mine shafts space, in order to breed more prodigiously than we do. Thus, knocking us out of these superior numbers when we emerge! Mr. President, we must not allow a mine-shaft gap!

Quite fitting, ain't it?
I was just watching Dr Strangelove a few weeks ago. There is a sub-genre of fiction about societies set underground to avoid the effects of nuclear war.

Some of our deeper mines, that now require active cooling, might end up with habitable temperatures. I don't know whether we would have the time to construct new underground habitations before the surface becomes deadly. Ocean tides would be much harder to use when the top few hundred meters of the seas have frozen solid.

20. ## Re: How long would the Earth last if the sun became a black hole?

Originally Posted by DavidSh
I don't know whether we would have the time to construct new underground habitations before the surface becomes deadly. Ocean tides would be much harder to use when the top few hundred meters of the seas have frozen solid.
Yeah, this is my thought too. I think any proposal that "a small population could survive off geothermal", while technically true, isn't realistic unless there was a huge advance warning. The sun isn't just "a" heat source: it's THE heat source for our planet. If the sun went out suddenly, the surface would quickly become outer-space levels of dangerous. We're not talking about "oh we'll have to supplement with nuclear or geothermal" -- we're talking about everything that isn't meters and meters belowground needing to be its own self-contained life support.

21. ## Re: How long would the Earth last if the sun became a black hole?

Originally Posted by Ionathus
Yeah, this is my thought too. I think any proposal that "a small population could survive off geothermal", while technically true, isn't realistic unless there was a huge advance warning. The sun isn't just "a" heat source: it's THE heat source for our planet. If the sun went out suddenly, the surface would quickly become outer-space levels of dangerous. We're not talking about "oh we'll have to supplement with nuclear or geothermal" -- we're talking about everything that isn't meters and meters belowground needing to be its own self-contained life support.
Frostpunk!

That being said, active mains are not that far away from being viable habitats. Repurposing them would not be that difficult if it was suddenly needed. But... whatever we would manage to scrap together would be a temporary solution. With the whole economy and transport network broken, it would not be possible to maintain the technology long-term. Let alone improve it.

22. ## Re: How long would the Earth last if the sun became a black hole?

Originally Posted by the_david
From our point of view, time dilation included, how long would we last if the Sun became a black hole?

We'd all be dead from the supernova that comes first, wouldn't we? Is there any way that wouldn't happen?
The sun is not going to go supernova, at all, so long as it doesn't ingest twice its current weight in hydrogen (or other mass, but hydrogen is the most common free gas, and ingesting a bigger star would be the bigger star eating the sun).

If the sun was suddenly replaced with a black hole, we'd freeze, very quickly. The air would be frozen out in hours, and the temperature at the surface would drop cllose to the temperature or the cosmic background radiation (2.3 Kelvin?) in days. The core would probably freeze eventually, but since most of that's nuclear, maybe not.

There would be less than a week to organise a heated space, and I'd bet humanity couldn't stop arguing long enough to do it. We would need a population of over a hundred to survive.

23. ## Re: How long would the Earth last if the sun became a black hole?

Originally Posted by halfeye
There would be less than a week to organise a heated space, and I'd bet humanity couldn't stop arguing long enough to do it. We would need a population of over a hundred to survive.
That is the minimum if we have a stable enough environment to live in which will not be the case. Requirements to keep up the technology that would keep humans alive are much stricter - hard to estimate how it could be pulled off in the long-term.

24. ## Re: How long would the Earth last if the sun became a black hole?

Quoth halfeye:

If the sun was suddenly replaced with a black hole, we'd freeze, very quickly. The air would be frozen out in hours, and the temperature at the surface would drop cllose to the temperature or the cosmic background radiation (2.3 Kelvin?) in days.
Technically true, I suppose, in that you could express the time for the atmosphere to freeze out as some number of hours, but it'd be a large number. Consider that it's quite routine for parts of the Earth's surface to be without sunlight for twelve hours or more, and the atmosphere never freezes out overnight. Nor is heat transfer from the sunlit parts of the Earth relevant, because the Earth rotates way too fast for that.

25. ## Re: How long would the Earth last if the sun became a black hole?

Originally Posted by Chronos
Technically true, I suppose, in that you could express the time for the atmosphere to freeze out as some number of hours, but it'd be a large number. Consider that it's quite routine for parts of the Earth's surface to be without sunlight for twelve hours or more, and the atmosphere never freezes out overnight. Nor is heat transfer from the sunlit parts of the Earth relevant, because the Earth rotates way too fast for that.
I'm guessing, but I really do suspect you are mistaken, the thing that would make it quick is that once it starts, it will accelerate. When the temperature drops, the clouds will disappear, and clouds retain some heat, When the pressure drops, air will rush in from elsewhere and that will cause the pressure to drop where the air came from.

The sun pumps a lot of hear into the Earth, currently we're near equilibrium, but there's a lot coming in, and a lot going out. If the in stopped, there'd still be a lot going out, for a short while.

It's not going to happen, so don't worry about it, it's just a discussion on the internet.

26. ## Re: How long would the Earth last if the sun became a black hole?

Originally Posted by Chronos
Technically true, I suppose, in that you could express the time for the atmosphere to freeze out as some number of hours, but it'd be a large number. Consider that it's quite routine for parts of the Earth's surface to be without sunlight for twelve hours or more, and the atmosphere never freezes out overnight. Nor is heat transfer from the sunlit parts of the Earth relevant, because the Earth rotates way too fast for that.
I'd guess it is mostly because of the heat radiated out of Earth surface. There are significant differences in day/night temperatures between, for example, open sea (sometimes there is no difference whatsoever) and middle of a sand desert (typical Sahara: +38 Celcius in the day, -4 Celcius in the night). Differences are significant enough that at a shore you get consistent winds during the day (toward the land) and night (toward the sea).

So it the middle of large landmasses we would probably go into freezing temperatures within a day or at most two. Anywhere closer to the oceans would take some more time but not significantly so as the heat from the depths would not be transferred efficiently enough to the surface.

27. ## Re: How long would the Earth last if the sun became a black hole?

OK, let's use that desert example. In the first 12 hours without sunlight, temperatures drop by over 40º. If that rate holds up, it'd take three days to get down to -200º, for the nitrogen to condense out of the air. And that rate won't hold up, because heat loss by radiation is strongly dependent on temperature, so as the temperature drops, the rate of temperature decrease will slow dramatically. And now consider that, over timescales of multiple days, heat transfer from elsewhere on Earth will be relevant, and most of the Earth's surface is covered with water.

28. ## Re: How long would the Earth last if the sun became a black hole?

Originally Posted by Chronos
OK, let's use that desert example. In the first 12 hours without sunlight, temperatures drop by over 40º. If that rate holds up, it'd take three days to get down to -200º, for the nitrogen to condense out of the air. And that rate won't hold up, because heat loss by radiation is strongly dependent on temperature, so as the temperature drops, the rate of temperature decrease will slow dramatically. And now consider that, over timescales of multiple days, heat transfer from elsewhere on Earth will be relevant, and most of the Earth's surface is covered with water.
The thing is, conditions will stop being livable way before we hit -200º. -20º in areas that are ill prepared for below 0º temperatures will be deadly enough. At any rate, the timescales allowing short-term survival will not be long enough to prepare lasting solutions as the economy network could not be maintained. Even with the currently available technology and infrastructure preparing habitats in mineshafts would take quite likely months. Weeks maybe if we disregard safety procedures and any kind of planning.

29. ## Re: How long would the Earth last if the sun became a black hole?

Originally Posted by Chronos
OK, let's use that desert example. In the first 12 hours without sunlight, temperatures drop by over 40º. If that rate holds up, it'd take three days to get down to -200º, for the nitrogen to condense out of the air. And that rate won't hold up, because heat loss by radiation is strongly dependent on temperature, so as the temperature drops, the rate of temperature decrease will slow dramatically. And now consider that, over timescales of multiple days, heat transfer from elsewhere on Earth will be relevant, and most of the Earth's surface is covered with water.
For a fictional reference, see "A Pail of Air" by Fritz Leiber. Bear in mind, it's "50's sci-fi magazine" science fiction.

30. ## Re: How long would the Earth last if the sun became a black hole?

Originally Posted by Storm_Of_Snow
Grow lamps and space heaters exist

The moon would still be orbiting the earth, so geothermal and tidal are still possibilities for generating power, while greenhouse gases would actually be beneficial to keep heat in. Throw in fission and eventually fusion, plus sealed environments, and civilisation could potentially survive for a long time.

Jupiter might be a longer term source of energy - end goal would have to be migrating to at least Alpha Centuri, so you'd need some colonies to start that process.

That's not to say there wouldn't be a lot of casualties from the initial event, and there'd be conflict as nations try and grab what they can to secure what their leaders see as the right way for civilisation to continue (their way and no other), which could well turn into a nuclear exchange.
Sure, you can do all of that before the sun turns into a black hole. For the unprepared however it's a different story. The difference in temperature between night and day ranges from 4 to 48 degrees Celsius [1]. That is in the span of 12 hours, so every 24 hours will just be double.
Within 2 days the deserts that were previously in the 40-50 temperature range (during the day) are now in the -40 to -50 range. Everyone living there has died from hypothermia, their houses are built to keep the heat out, not in, they have no time to renovate and their space heaters will not keep up.
Near the ocean where the air is humid the it will take 4-5 days until the cold causes the air to lose its humidity, this would then speed up the cooling process. Florida, Cuba, Indonesia are now some of the best places to live (because you get to, for a while).

The funny thing about space heaters is that they require electricity. So as long as the power plants work you can run a space heater. The problem here is that there are 2 factors that are speed-running their quest to kill you. One is that power plants require humans to work, and the humans there are going to freeze to death. The second is the matter of how well insulated your house or apartment is. The outside is getting colder and fast, even at a slowest possible rate of 8 degrees per day and a nice starting temp of 40, that's only 5 days until the humidity turns into snow, then the decline will speed up. Is a space heater really enough when the outside temperature is -60? or -100? Or -270?

Your best bet to survive the longest is to have a fully insulated nuclear powered bunker. Because then you will last until you run out of food, water or nuclear fuel.

Back to the people at the powerplant, can't they use their power to heat up their power plant and live there? Yes! However, again, every day it is getting colder fast, so every day they have to divert more power from the grid into their own space heaters, away from your house and your space heater. But they can probably survive until they run out of water, if they have ample water then they'll either run out of fuel or food. The problem is that they can't get more of either, after the first month stepping outside is a death sentence.

The answer might seem like geothermal power, they won't run out of energy any time soon. Let's suppose they also have limitless supply of water and just so happen to have a hydroponics bay capable of feeding everyone working there. I give them 4-8 months at best. Their problem won't be food, water or heat, it will be air. Or more specifically they suffocate.
Because eventually the air- the nitrogen and oxygen, will start to deposit into frozen nitrogen and oxygen, this will happen slowly and will continuously make the atmosphere thinner and thinner. Soon enough you'll need a space suit just to go outside.

I'd argue that after 8 months the only people still alive are the ones who started off with an underground bunker powered either by nuclear or geothermal. They'll live until they run out of food, water, or something breaks down. They'll never be able to leave and will be completely on their own, they won't be self sufficient indefinitely. I give them a year.

[1] https://en.wikipedia.org/wiki/Diurna...ture_variation

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