What would 2 stars colliding at relativistic speeds do? (1 solar mass, 10% of c)

Submitted by Party_9001 t3_zkuihq in askscience

~background info~

I've found an interesting series of videos that talk about humongous structures in space like the dyson sphere. One of these structures is called the shkadov thruster which is capable of accelerating stars. The actual acceleration is very slow and would take billions of years to get any decent velocity out of it, but the concept is fascinating - taking billions of years, untold resources and a star to make the biggest bomb in history.

So I'm curious, what would it actually do if it hit something? I'm fairly certain everything in the size of the solar system would be basically gone. So would this annihilate some significant portion of the galaxy?

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limacharley t1_j01udkn wrote

There would be huge compression at the point of impact, which would cause fusion of very heavy elements and maybe formation of a neutron star. In a supernova, the compression from the collapsing star leads to a reverse shock which blows apart the outer layers of the star. That might happen in your scenario as well. In any case, you are going to end up with zero stars and lots of debris.

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absrdbrdtrdmagrdIII t1_j0262xy wrote

Could a black hole form?

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boundbylife t1_j02evd2 wrote

Maybe, but unlikely.

2M is considered the low end for the possibility of black hole formation. But for the resultant mass to be exactly 2M, you'd have to have both bodies' motion perfectly cancel each other - a glancing blow by either body would result in some of the mass being ejected from the system. And at a fraction of c, the fractional mass would quickly escape the larger mass's gravitational influence, so it wouldn't return and reform. The odds of two bodies having the exact opposite x, y, and z velocities is just, well, astronomical.

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Connect_Eye_5470 t1_j04q05e wrote

Don't know the math well enough to say definitively, but my gut says nearly impossible. The gravitational 'turbulence' would seem to guarantee you wouldn't get the spherical compression pattern to create a singularity.

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mfb- t1_j026lcb wrote

Modeling a collision process in detail could take a team and a few years, but we can look at the overall energy. Two stars with the mass of the Sun each, colliding head-on at 10% the speed of light, have a total kinetic energy of 1.8*10^45 J. That's about a factor 10 above a type Ia supernova. That's more than fusing all hydrogen and helium to heavier elements could release - we can expect some weird fusion processes to happen but they won't change the released energy by more than a factor 2. It will look like a very bright supernova in some way. Life on nearby stars would have a bad time, but planets would keep orbiting their stars without any large-scale changes. For planets orbiting one of the two colliding stars the outcome could depend on where in the orbit they are - "behind" or "ahead" of the collision.

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Connect_Eye_5470 t1_j04phze wrote

It would vary pretty widely on at least two factors.

1.) Ratio of relevant mass i.e. a white dwarf flung away from a multi-stellar system runs into a massive O-type Giant star (say 150+ stellar masses). Likely it never actually collides. The incredible difference in their gravitational fields would tend to force the incoming dwarf into a decaying orbit rather than a 'whammo'. Then the massive star would just 'eat' the smaller one.

2.) Angle of the collision. A good example of this is Luna (our moon) vs an asteroid belt or ring formation around a planet. If two stars of approximately equal size hit head on? Yeah whether the 'explosion' destroyed that solar syatem or not the gravitic disruptions would almost certainly either fling off the planetary bodies as rogues or would suck them in and vaporize them.

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