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How massive of an object would be be required...

  1. Jan 19, 2016 #1
    How massive would an impactor have to be in order to spectacularly destabilize the sun? Could be either a large mass moving at orbital velocities, or a smaller (initial) mass coming in at relativistic velocity. I know we don't have a lot of statistical data on such events o0), but what would be plausible?
     
  2. jcsd
  3. Jan 19, 2016 #2
    If it's in orbit then it isn't an impactor and if it's a very massive object in orbit then essentially you have a binary system.
    These are commonplace and don't cause spectacular disruption, although there can be situations where one of the binary pair is very small and dense compared too the other, and that can result in the small dense partner robbing matter from the outer part of the bigger but less dense one.
    Actual stellar collisions are highly improbable but not impossible, and as far as I know this would result simply in the objects merging to become one much bigger star, but depending on the size and composition of the two original stars the new combined star might be unstable and could quickly undergo collapse most likely into a neutron star.
     
  4. Jan 19, 2016 #3
    Okay, let me be a little more clear. When I said "orbital speeds", I meant the speeds typical of an object free-falling into the sun. And I am interested only in an object impacting the sun, either at solar escape velocity or greater, or right up to relativistic velocities where the tau would create a significant mass increase. No decaying orbits, no gradual mergers.

    Intuitively, it seems like a large enough mass moving at sufficient velocity would "burst" the sun, producing something like a toroidal splash effect in the sun's mass, and a complete cessation of fusion. Or maybe a smaller mass would create a shockwave through the sun that would create spontaneous fusion in layers that weren't fusing before. Maybe this would cause a nova-like explosion, or at least an immediate growth into red-giant size.

    I'm just looking for what might be plausible, rather than eye-rolling, once you've accepted the premise of a mass coming straight in.
     
  5. Jan 19, 2016 #4
    The density of the impactor matters. A small black hole would likely pass through the sun, causing some vibrations.

    There actually is a sun that has a neutron star inside of it.

    I would think that the collision of two suns would be highly inelastic. I can imagine a collision scattering a sun into pieces, but I've never heard of any object remotely capable of doing that. A huge stony/metal exoplanet traveling at some ungodly speed? It might be possible.

    I imagine a shockwave would induce fusion. It could be significant in such an extreme scenario.
     
  6. Jan 20, 2016 #5
    The sun isn't really all that big, a shockwave would probably add a little extra fusion in the core, but not much. In order for it to explode from such an event, it'd have to be close to some critical limit, which it's not. I have a hard time imagining a planet disrupting the sun much at all, I imagine it could swallow Jupiter whole and barely even hiccup.

    A star hitting another star is probably way more complicated than some type of inelastic collision. Even by the time that they were millions of miles from each other, their tidal gravity on each other would seriously disrupt the star. If they were to merge though, it'd be very violent, but I don't think it would be catastrophic for our star. It'd take a lot more than another star to cause it to crush itself into a neutron star, the sun just isn't very big. A star-star collision thought would almost certainly kill us though, the gravitational disruption alone would do that, and I'm sure a collision of two stars would produce lots of radiation.
     
  7. Jan 20, 2016 #6
    Planet. Not sun, not black hole. A larger one moving at low speed, or a smaller one moving at high speed. "High speed" can include velocities above .99 C where relativistic mass is a factor.

    My question is: (1) What kind of result might such a collision create, where the requirement is that it wipes out all life in the system, and (2) What mass might be required (perhaps as a percentage of the mass of the sun)?

    The two possible effects that I've been able to think of are (A) the sun bursts, like those slow-motion videos of a bullet passing through an apple, and (B) the shockwave induces a spike in fusion within the sun, creating anything from super flares in all directions, through an instant expansion to a red giant, to a small nova-like explosion.
     
  8. Jan 20, 2016 #7
    Like I said, I don't think a large planet moving at low speed would do anything. The sun would rip it apart before it even contacted the surface due to tides and would barely burp.

    A small, super fast moving planet would probably just crash into it and be obliterated. It certainly wouldn't be like a bullet hitting an apple, it'd probably be more like a bullet hitting a cannonball. Even the surface layers of the sun are incredibly dense, nothing would penetrate it very far (except for a neutron star or black hole which is even denser) and the intense gravity would keep the splash mostly in check. It'd probably produce a massive flare and some intense gamma rays during the contact, but most of that momentum would probably end up being transformed into heat. There would be lots of sun quakes, but if you had some sort of sensor at the sun's core and a relativistic planet hit the surface, you probably wouldn't even notice.
     
  9. Jan 20, 2016 #8
    Err, the mean density of the sun is roughly 25% the mean density of Earth, so the surface of the sun is going to be even less dense.
     
  10. Jan 20, 2016 #9
    The problem I'm having here is that reductio ad absurdum says different. Let's take a planet with half the rest mass of the sun, and accelerate it until it has a tau of 20, so that effectively the sun gets rammed by a small object with 10 times its mass. Are you really asserting that this would have no significant effect? If so, then let's increase the tau. How about a planet with 10 times the mass of the sun, with a tau of 1000. How about now? Still no effect? I'm happy to keep increasing the relativistic and rest masses of the planet until it outweighs the galaxy.

    Somewhere between Mercury dropping into the sun at orbital speeds and the ridiculous examples I've given, there's a range of sizes and velocities where there's going to be an effect. I'm just trying to get a handle on what that effect might be, and at what effective mass.
     
  11. Jan 20, 2016 #10
    Well, a "planet" with half the mass of the sun is going to be a red dwarf. Technically the sun is already a red dwarf but it's a largerish red dwarf so I won't be pedantic.

    We really should have started with the most significant factor, which is angular momentum. The more off-center the impact, the higher the net angular momentum will become. When two stars merge, the angular momentum of their co-orbit is converted into rotational angular momentum, causing them to "spin off" significant amounts of their mass into an excretion disk. The star Achernar rotates so fast that its equatorial diameter is nearly twice its polar diameter; much faster, and the centrifugal force at the equator would exceed the gravity at the equator and thus matter would be flung outward into a disk.

    So if the momentum of the impactor is high and its impact is far enough off-center (but not so far off-center that it punches through the outer envelope and escapes), then its "capture" could potentially add enough angular momentum to the star to cause the star to excrete a disk. I'm not sure whether the mass of the excreted disk could conceivably exceed the mass of the impactor.

    At the other extreme: if the impactor hits dead-center, then it's going to sink pretty deep into the core. A few different things can happen at this point. The impact could disrupt nuclear fusion, decreasing the sun's energy output and causing it to contract and then burn brighter for a few million years. Alternately, a more energetic impact could cause shockwaves that trigger accelerated nuclear fusion as they pass through the sun's outer core, heating it up considerably. However, such heating probably wouldn't be enough to significantly disrupt the sun, even though it might make Earth unseasonably warm. At sufficiently high kinetic energies, the impactor could deliver more energy than the gravitational binding energy of the sun...but that's a bit extreme, and I'm not sure that such an impactor wouldn't punch straight through.
     
  12. Jan 20, 2016 #11
    Even punching straight through would be devastating, I think. I keep coming back to the slo-mo of the apple getting shot. Every bit of solar mass that gets shoved aside by the bullet would contribute to a shock wave, and probably create increased fusion along the way. In fact, I wonder if there's enough accumulated ash in the sun to posit some of the heavier-element fusion that you get just before a supernova explosion.
     
  13. Jan 20, 2016 #12
    I don't think that slow-mo thing is the best representation. It would be less like a bullet going through an apple and more like a ripe tomato hitting a wedding cake.

    The sun has very little accumulated ash, but that doesn't really matter. Heavier-element fusion tends to be endothermic and is only possible because of the intense heat of a supernova explosion; it isn't a source of additional energy; the energy of a supernova is due primarily to hydrogen and helium fusion that's accelerated during collapse.

    I'm not certain whether a punch-through scenario would result in the sun getting enough kinetic or added fusion energy to go boom.
     
  14. Jan 22, 2016 #13
    Incorrect analogy: the apple is not held together by gravity. Sun is. It will not split into pieces and fly apart.

    Check out Shoemaker-Levy 9 impact on Jupiter. That's about what would happen if a planet hits the Sun. If you want to seriously disrupt the Sun with a planet falling into it, you need something like a Jupiter mass planet at .99 c
     
  15. Jan 25, 2016 #14
    For reference...

    The sun's gravitational binding energy is on the order of 2.3e41 J. It's actually a good deal more than that, since that's the constant-density approximation and the sun most certainly does not have constant density, but it's a good starting point for what would constitute catastrophic disruption (especially because it would take about 30% less energy to blow it into an spinning gas cloud than it would be to blow it up altogether, but either one qualifies).

    It's actually easier to get to that order of magnitude than I thought. A Jupiter-mass planet would need to be going at only 0.1% of lightspeed, roughly 400 km/s, to hit with this much bang. That's less than half the escape velocity of the Milky Way. For an Earth-mass planet, it's going to be a little steeper -- about 30% of lightspeed -- but that's still less than I expected.

    Jupiter has an average density slightly lower than the Sun, so it wouldn't punch straight through, but Earth's average density is three times that of the Sun so it might. Of course Newton's approximation is wildly inappropriate here, so I don't know for sure.
     
  16. Jan 25, 2016 #15
    Woo hoo! I get to blow up the sun! :woot:

    Er, um, I mean, thanks very much for your help.

    Ahem.
     
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