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Could a star survive a direct hit from a gamma ray burst?

  1. Sep 24, 2011 #1
    What would happen to a star if it took a direct hit from a gamma ray burst?

    For example if a gamma ray burst from a supermassive black hole at the center of a galaxy hit a star, would the star survive? If so, how would the grb effect the star? If not, would the star explode?

    Obviously this all depends on the proximity of the star to the intensity of the grb beam so I'd also be curious how close/far a star would have to be to survive and be destroyed? If applicable. Thank you
     
  2. jcsd
  3. Sep 25, 2011 #2

    Drakkith

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    There might be some upset in the upper atmosphere of the star, but overall it would have little effect to my knowledge.
     
  4. Sep 27, 2011 #3
    Here is how you can solve the problem yourself. I think that the answer is that a GRB won't do anything, but I leave it to you as a home work problem.

    What you need to do is to figure out the gravitational binding energy of the star (i.e. how much energy is keeping the star together). The compare with the amount of energy that gets a added. My guess is that number two is going to be a lot less than number one, but I leave the details for you.
     
  5. Sep 28, 2011 #4
    I guess you do not need to add that much energy to ignite fusion of the remaining nuclear fuel of the star.
     
  6. Sep 28, 2011 #5

    Drakkith

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    That isn't how it works. Not only do you need energy, you need confinement (IE High Density) for fusion to take place in anything but miniscule amounts. I don't think adding a large amount of energy to the outer sections of a star would not induce fusion, but would instead result in an increase in the temperature of the gas which would decrease it's density as it expands outward. I'd expect the vast majority of gamma radiation to be absorbed in the outer layers of the star and never make it to the inner areas where pressure and confinement is much higher. It might be a pretty show, but I doubt it would cause any significant fusion.
     
  7. Sep 29, 2011 #6
    There will be inertial confinement.

    There will be be a compressional wave.

    Do you have an idea how much gas a star contains (even in the outer layer only where absorption takes place)? What do you think how much pressure you need to let it sufficiently expand within seconds (in the case of a short GRB)? And how much pressure you need to start fusion?

    What will happen with this energy after it was absorbed? Reemission? OK, this will happen, but this would take some time. Taken away by the expanding outer gas layer? OK, this will happen too, but it will take even more time. In the meantime the energy will cross the inner areas of the star in the form of a shock wave. If temperature and pressure of this wave are high enough it will start fusion and I still guess the required energy does not need to exceed the potential energy of the star.
     
  8. Sep 30, 2011 #7

    Drakkith

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    I'm not saying that NO fusion will occur, but that whatever amount would be insignificant and nowhere close to the "remaining nuclear fuel".
     
  9. Oct 3, 2011 #8
    Yes. I fact I do.

    The total energy emitted by a GRB assuming isotropic radiation would be 10^54 ergs (it's actually 10^51 ergs along a pencil beam).

    At 10 light years (10^19 cm) that gives you 10^16/ergs / cm^2. Given the total area of the sun (10^10 cm) that gives you a total energy absorption of 10^36 ergs. The mass of the sun is 10^33 g. If you assume that the beam hits the sun and heats up 0.1% of the matter, that gets you 10^6 erg/g.

    That's barely enough to boil water much less cause fusion.

    10^6 ergs/gm is not enough to cause fusion.

    10^6 ergs/gm is not enough to do anything. If you stand out at noon, you are getting hit by 10^12 ergs/second of energy.

    Note that I did this off the top of my head so if I dropped a zero or eight, then sorry, but the numbers are such that even if I made a whopping big mistake, then you still ain't going to get fusion.
     
    Last edited: Oct 3, 2011
  10. Oct 3, 2011 #9

    Drakkith

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    Thanks Twofish! Good to know the Sun isn't going to explode from something like this. Thought I think Earth may have other problems.
     
  11. Oct 3, 2011 #10
    That's not was I was talking about but this calculation is much better than your original approach.
     
  12. Oct 4, 2011 #11
    So what are you talking about? GRB at 10 l.y. is nowhere near powerful enough to cause stellar ignition.

    Now if you a GRB at a few tens of A.U., then something interesting is likely to happen, but I don't think that you are likely to get a self-sustaining fusion reaction. The big problem is that once you start generating energy at the surface, the gas will expand and cool, and quench the reaction. The problem with setting up a shock wave is that the gas will go in the direction of least resistance which will be in the direction of the vacuum rather than setting up a shock wave.

    One other thing that strongly suggests that your scenario wouldn't work is that type II supernova exist. You dump 10^51 ergs of energy in a star, blow it to smithereens and still most of the hydrogen does not burn, because by the time the shock goes into the outer layers of the star, it's far, far too weak to trigger hydrogen fusion. The other problem that you are going to get is the the time scale for H-H fusion is quite long.

    Again, I'm quite willing to be convinced otherwise if you can give me some rough numbers that suggests that you can get a self-sustaining shock.
     
  13. Oct 4, 2011 #12
    About your suggestion to compare the amount of energy that gets a added from a GBR with the gravitational binding energy of the star. I do not think that you need to add the full potential energy of the star to blow it away. That does not mean that the energy of a GRB at 10 l.y. would be sufficient. There are many orders of magnitude between.

    The gas can not escape immediately at any velocity. Is must be accelerated first and that requires lot of pressure. This kind of inertial confinement is used in laser fusion. The pressure in the center of the droplet is retained long enough to start fusion because the outer shell can not escape due to its inertia. This works even better with increasing mass. Of course you also need more energy but a nearby GRB might be a quite powerful heater.

    That's a lot of sophisticated physics and math - even for a rough estimation. If I could do that I would not guess but calculate.
     
  14. Oct 10, 2011 #13
    The cool thing about astrophysics is that you deal with things of different scales so sometimes you can quickly figure out it just won't work. If you work at that the energy absorbed is 0.1 x the potential energy of the star, then maybe you can disrupt things. If you do the numbers and the energy absorbed is 10^-12 x the potential energy, then it just ain't going to work

    The important thing is not the pressure but the *difference* in pressure.

    And that only works because the laser is hitting the target evenly. If you shoot one part of the target but not the other part, then you just end up with a rocket.

    It's not that sophisticated. What you are looking for rough numbers. How much energy is being absorbed, how much energy gets released, how quickly the temperatures increase, how quickly the reaction rates are.

    What I think you'll find if you do that is that you'll end up with nowhere near the numbers to make this work. It's like trying to see if a flea can drink the Atlantic ocean.
     
  15. Oct 17, 2011 #14
    Will not do anything to my knowledge. It might affect the star's atmosphere infinitesimally, but don't take that word seriously. If you do the math you might get an exact value, but I believe nothing will happen to the star.
     
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