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I'm asking this because I'm writing a fictional story where I need a doomsday scenario that means humanity must escape the earth. The timeframe that would fit the plot of the story would be in the order of 100-1000 years.

I'm trying to think of a scenario that is at least plausible according to the laws of physics, given a little bit of handwaving. The handwaving in this case is that some massive energy source is causing the sun's core to speed up (from the things I've read so far, stellar objects actually tend to rotate slower over time because they eject mass and energy in the rotation). So that's where'll you'll have to bear with me: it's a given that the rotation of the sun's core is automatically speeding up. How many years does it take before the earth/humanity is wiped out?

Only recently it was discovered that the core of the sun spins around 4 times faster than the outer layer (source). For longer it has been known that parts of the sun rotate at different speeds due to differential rotation. Basically, because the sun is a hot ball of plasma, there are fluid dynamics at play, causing the sun not to spin as one solid ball of rock like the earth. I'm guessing if the core started speeding up, we would be able to detect pressure and gravity waves on the sun's surface that indicate this?

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    $\begingroup$ Sorry, but this hypothetical not remotely plausible unless you want the Sun god to have become enraged with humans and decided to kill them in a way that showcases his unlimited arbitrary divine power over nature. If you want a plausible impending threat with a well-known time limit, consider putting a rogue brown dwarf on a high velocity path through the solar system that takes it near enough to Earth's orbit that Earth will be kicked into an extreme elliptical orbit. $\endgroup$
    – g s
    Commented Mar 17 at 17:57
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    $\begingroup$ A better question to ask is this, why does it need to be plausible? Why is suspension of disbelief not enough? $\endgroup$
    – JBH
    Commented Mar 17 at 20:46
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    $\begingroup$ @JBH I think an author needs to decide if they're writing sci-fi or fantasy. In either format, a cosmic disaster that's a crucial plot element has a cause even if humans never fully figure it out - and the human reaction is different: If it's sci-fi, the humans start engineering stuff. If it's fantasy, the humans start looking for a divinity to appease or defeat or whatever. (And if it's anime, children start being born with superpowers and a government agency is created to exploit them) $\endgroup$
    – Spike0xff
    Commented Mar 18 at 4:26
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    $\begingroup$ "causing the sun not to spin as one solid ball of rock like the earth" - The earth doesn't either. The inner core of the earth, which is solid, rotates at a different speed and direction than the earth's mantle and crust, because the fluid outer core is in between them and allows independent movement of both. See this wikipedia page for more information. $\endgroup$
    – The_spider
    Commented Mar 18 at 16:48
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    $\begingroup$ -1 The entire premise precludes the possibility of answering this question seriously whilst also sticking to the "astrophysics" tag. The premise begins with throwing physics (conservation of momentum) out the window, and then asks how the physics would work out. $\endgroup$
    – Aron
    Commented Mar 19 at 3:51

5 Answers 5

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We have a problem here: we don't understand the solar core rotation really well. As you said, relatively recently (around 2017) it was discovered that the core rotates at about four times the speed the surface does around the equator, but even this measure might be wrong as it is based on a model - we can't just slice the Sun to figure it out.

So instead, I suggest you focus on something more visible: the solar surface. According to Wikipedia, the sun's surface has a speed of nearly 2 km/s around its equator, and the escape speed there is almost 618 km/s.

If the sun accelerates its surface up to its escape speed, then the whole of the sun will escape itself, with most of the mass becoming a disc of plasma traveling outwards on its equatorial plane at exactly that escape speed. For comparison, coronal mass ejection (CME) usually travels at between 250 to 3,000 km/s, so the plasma disc would reach Earth as fast as a usual CME would.

The Earth's orbit is inclined about 7 degrees relative to the Sun's equator... The Earth is more likely to be missed than hit by the outgoing plasma disc, but might also punch through it as the disc moves out.

Whether the ex-Sun misses the Earth or not on its way out, it will surely be missed itself. The absence of the Sun would quickly turn the Earth into a rogue planet (as opposed to a paladin or wizard planet), and that would be lethal to most lifeforms in the short term - only bacteria that live deep in the crust, and maybe some in the deep ocean might survive the absolute freezing of the entire planet surface.

By the way from 2 to 512 you have eight doublings. The next doubling goes to 11 1,024, which is way greater than 618. So if you double rotation speed every decade, humanity has between 80 and 90 (most likely 81 to 83, I'm too lazy to calculate a log now) years notice of eviction.

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    $\begingroup$ I don't get the 11 but the paladin got me $\endgroup$
    – DonQuiKong
    Commented Mar 17 at 21:50
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    $\begingroup$ @DonQuiKong - en.m.wikipedia.org/wiki/Up_to_eleven $\endgroup$
    – Vilx-
    Commented Mar 17 at 22:12
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    $\begingroup$ Although since the sun isn't even a little bit like a rigid solid sphere it's mostly irrelevant, you may be interested in comparing the gravitational binding energy of a spherical mass distribution and the rotational kinetic energy of a rigid sphere with the same mass distribution. $\endgroup$
    – g s
    Commented Mar 18 at 4:57
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    $\begingroup$ @gs I was thinking that as the Sun loses mass, the remaining mass would have more trouble holding it all together. Smaller gravity, less pressure on the core, increasing angular momentum making the remaining mass spin faster. Also whatever magic is accelerating the sun exponentially would keep unspooling it. On top of that, shouldn't the absurd rotation speed flatten the sun anyway? $\endgroup$
    – The Square-Cube Law
    Commented Mar 18 at 5:15
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    $\begingroup$ Where does the energy come from to increase the rotation? If it's from the ongoing nuclear fusion inside the sun, then once the speed gets high enough and the sun expands that fusion may stop due to the drop in pressure. If that's before escape velocity, then the sun may just get a bit bigger without throwing off any matter. $\endgroup$
    – user4574
    Commented Mar 19 at 2:20
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Perhaps your story would be better served by a disaster with a better understood physical underpinning, such as a wandering star or black hole on a collision course with the sun or a poorly cohesive comet or the remains of a rogue planet (a solid comet or asteroid could be deflected with any plausible escape technology.)

Even just a Jupiter wandering through the gravitational well would probably be a good time to head for stabler shores.

Our sun is too small to go supernova, and the process of turning into a red giant is probably too slow for a story.

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Few years, maybe less than 10.

The important thing is that our fancy star has an enormous heat content and the equilibrium rate for transferring heat to the surface stabilized over quite a while.

Altering the turbulences in the tachocline layer will allow mixing hotter matter radially.

Having 10% hotter Sun (~50% more luminosity) is barely survivable for a year. Maybe.

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Mmmh.. speculating here. But at some point, the centrifugal forces would deform the sun's core, giving it an equatorial bulge. Now this is where it gets interesting, cause this bulge would then tower through the photosphere and basically be able to eject a stream of very hot fusing plasma that spirals outwards. This is not a solar flare.. this is a blowtorch on solar scale. And the concept has been explored previously by Alastair Reynolds in Revelation Space / Absolution Gap of the Inhibitor series.

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    $\begingroup$ Interesting, but how does it answer the question? $\endgroup$
    – Escaped dental patient.
    Commented Mar 17 at 13:36
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    $\begingroup$ Eh, you get phd in physics, calculate the amount of spin necessary to bulge outwards to the point where the pressure vessel of a sun cannot contain the internal pressure and then determinate how long it would take you to get there.. $\endgroup$
    – Pica
    Commented Mar 17 at 13:42
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    $\begingroup$ If you think the question's unanswerable by yourself, then why answer it. (We have people here with PhD's in various subjects - one of our mods is rather onto this sort of subject and might answer presently). $\endgroup$
    – Escaped dental patient.
    Commented Mar 17 at 13:48
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    $\begingroup$ Its a draft.. others can expand on it? Perfect is the enemy of the good? $\endgroup$
    – Pica
    Commented Mar 17 at 13:50
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    $\begingroup$ @Escapeddentalpatient. Perhaps your energy would be better spent trying to improve the answer. $\endgroup$
    – M S
    Commented Mar 17 at 15:49
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Maybe you should define "apocalyptical" more? Do you mean "end of human civilization" or "end of the human race" or "end of life on Earth?"

If you just mean global catastrophe that ends global human civilization and kills billions, but allows some survivors for a desperate post-apo story: then about 15 years.

My take is that the sudden (by celestial standards) speed-up in the rotation of the Sun's core vs the rotation of the rest of the Sun would greatly essentially turn the Sun into a dynamo (more than it already is) and blast the whole Solar System with electromagnetic radiation.

At the very least, the whole human infrastructure would be EMP-ed many times over, instantly taking us back to the 19th century by frying every electronic device that is not extremely well ruggedized or buried. Cancer rates would skyrocket among the survivors (the EM blast would have little to no heavy radiation, but by itself it should be enough to do damage.)

OTOH, there would be pretty Auroras everywhere on the sky, so there is a plus :)

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