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Premise

A large spacecraft is powered by a massive fusion reactor - in essence, a miniature star. This star is surrounded by a dyson sphere at the center of the spacecraft.

The star's radius is about 5 times as small as that of our Moon at 350km, and assuming it would have twice the mean density as that of our Sun at 2820 kg/m³, it would have a mass of about 500 quintillion kg, an infinitesimal fraction of our Sun's mass, and about 0.008% of Earth's mass.

Handwavium technology is used to exert upon the star the enormous pressure it requires to sustain fusion, and it also prevents such silly things as radiation and immense heat from swallowing up the spacecraft and its inhabitants. How this handwavium works and why the builders of the spacecraft even need a fusion reactor with such ridiculously powerful technology at hand is irrelevant to the question.

The question

The handwavium suddenly stops working and the miniature star is now exposed to the harsh realities of the laws of physics. What would happen next?

Would the star simply swallow up the spacecraft and then disperse into a harmless cloud of hydrogen?

Would it do the same thing natural stars do when they can no longer sustain fusion and go nova, ruining the day of anything within a couple AU?

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    $\begingroup$ Boom. Very large Boom. $\endgroup$ Nov 18, 2016 at 14:47

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It will explode, since the externally applied pressure is no longer counterbalancing its internal radiation pressure. It will also stop fusing hydrogen, since it no longer has the pressure and density to be doing that, nor the temperature. This will presumably destroy the spacecraft rather thoroughly.

I would not call this a nova, since the explosion happens for a rather different reason to that of a normal nova.

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  • $\begingroup$ I'd love for someone with better understanding of physics - than I - to calculate the energy output of the explosion and calculate a minimum safe distance. $\endgroup$ Nov 18, 2016 at 13:14
  • $\begingroup$ This is the answer I was looking for. However, it would be nice to know how fast this would all happen and what the radius of the blast would be. $\endgroup$ Nov 19, 2016 at 14:50
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    $\begingroup$ The timescale will be short, but it depends on how quickly and widely the containment fails. If it all stops working instantaneously, the expansion will be very fast -- something like a thousand miles a second. The concept of "blast radius" doesn't really apply in vacuum: if you can explain what kind of object wouldn't be much damaged at the edge of that radius, it may be possible to calculate an answer. $\endgroup$ Nov 19, 2016 at 15:12
  • $\begingroup$ I realize with expansion as rapid as that we might have hydrogen sent off on million-year trips through interstellar space. What I meant by blast radius was how far an earth-like planet would have to be so as not to be burned to a crisp. $\endgroup$ Nov 21, 2016 at 8:50
  • $\begingroup$ I'm not sure it'll really explode. Supernova happen because the star collapses, and under the immense pressure, it can fuse things heavier than helium, so you get a whole new round of fusion. The star in question will just blow its hydrogen away and fade out. $\endgroup$
    – Ryan_L
    Apr 30 at 19:20
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I think it depends on how the star is made up. If it's an entirely artificial construct, I believe it would be a massive explosion similar to John dallman's answer.

It might also simply collapse or expand into a red dwarf, (no, not that one) before boringly becoming a white dwarf after a few trillion years.

Assuming that the starship was negating or counteracting the gravitational pull of the somewhat unconventional power system, please note that this would mean that suddenly there is a huge gravitational well where previously there wasn't one. This would cause huge problems for the star system that this happened in, let alone the poor ship!

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    $\begingroup$ You seem to be assuming it has the mass of a normal star. The OP specified it's only 0.008% of an Earth mass, way below the limit for anything like a star. $\endgroup$ Nov 18, 2016 at 12:54
  • $\begingroup$ ah, well spotted, I hadn't noted that part. I'll leave it as it might be useful should someone else have an actually-nearly-a-star fusion drive idea. $\endgroup$
    – Miller86
    Nov 18, 2016 at 14:33
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Assuming that this handwavium is preventing the massive gravity from having everything collapse in on itself. The first thing that would happen is the starship would collapse in on the fake star, and itself.

This would be immediately countermanded by a rapid expansion in size from the hydrogen cloud, tearing the spaceship apart in a massive combined implosion/explosion.

The debris would then form a small gaseous planet, with a core made of the starship's materials.

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  • $\begingroup$ There isn't enough mass to cause the spaceship to collapse into the false star. The false star is in a state of artificial compression, to raise the temperature to levels required for fusion. When the artificial compression is removed the matter in the false star will rapidly reach a state of equilibrium with it's surroundings, by rapidly expanding and cooling. Ergo, it will explode, violently. $\endgroup$ Nov 18, 2016 at 12:59
  • $\begingroup$ @BinaryWorrier An object some 100-150km radius in size has a large enough mass to form a sphere. Therefor, a mass of 1/5 the size of the moon at 340km radius would easily collapse in itself, and that's only the size of starship's core. $\endgroup$
    – Feyre
    Nov 18, 2016 at 13:18
  • $\begingroup$ I am not an expert I'd have thought that adding the amount of heat/energy necessary to make fusion happen would make it explode. That's an awful lot of highly energized particles that can now spread out to a less compressed volume. It goes against my intuition that the false star would remain in it's condensed state. But all have have to go on is my intuition. I will bow to greater knowledge/experience :) $\endgroup$ Nov 18, 2016 at 13:30
  • $\begingroup$ @BinaryWorrier To be clear, I've had some astronomy classes, but I'm not an astronomer. And you're right that there would be an expansion, and the gas would stay spread out a lot more afterwards (it wouldn't collapse into a star). But the total gravity of the ship (2-3 times the density of common minerals planets are made up [quartz, lime, etc]) would be enough to form a sphere. As far as I understand smaller stars don't undergo a nuclear explosion, and collapse very slowly into white dwarves. $\endgroup$
    – Feyre
    Nov 18, 2016 at 13:57
  • $\begingroup$ So I would expect the fake star to expand solely because the pressure outweighs the gravitational force, meaning you need to ignore analogues to end-stage stars. It is of course possible that the total initial velocity of some of the gas molecules would exceed the ship's escape velocity. Which would actually be quite easy to calculate, but there's not enough info in the question. $\endgroup$
    – Feyre
    Nov 18, 2016 at 14:00

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