Feb. 25th, 2011

demonicgerbil: (Default)
Today's question comes from "Rohius" via what is called "Internet Relay Chat." He asks:

"Hypothetically, what would happen if a black hole swallowed a star of roughly the same size and type the black hole originated from? I'm just wondering if, should I ever achieve omnipotence, if that would be a realistic means of mass extermination of, say, pedophiles and people like that judge, after I teleport them into another galaxy."

Now barring the fact that an omnipotent being could simply smite people of his choice in any number of ways, throwing an equal mass black hole at a star is probably a good way to wipe out the inhabitants of a planet. Now, I don't have a nice hydrodynamics simulation for this situation handy: I suppose I could set the problem up in FLASH, or more likely in GENASIS, but I will speak in the handwaving generalities that Astrophysicists are best at instead.

Assuming a star of large enough mass to form a black hole when it dies, let's go with 20 Solar Masses, the black hole will have some amount of mass that's smaller than the star's initial mass because the star will have blown off some material during its collapse. The black hole will be a compact object, probably on the same order of size as say New York City. Yes I'm avoiding mathematics, because it really doesn't matter too much the exact size of the object.

The black hole flies into the star, sucking in matter as it goes. The stellar matter accretes around the black hole, becoming increasingly energized as it falls down the steep gravitational gradient. Angular momentum will force the disrupted star's matter into an accretion disk, slowly radiating energy and plunging through the event horizon of the black hole. The good news is that this will release lots of highly energetic x-rays which will bathe the planet in life-killing radiation. The bad news is that these same x-rays will cause the star's matter to expand: the black hole does not have a particularly powerful gravitational field once you get some distance away from it. This expansion will slow the rate of accretion down and decrease the amount of radiation being released. It is possible that a non-negligible amount of stellar material will end up in orbit around the black hole.

Hopefully the initial phases of the collision will produce an energy burst of sufficient power to fry the people you're trying to kill. Just as a sort of back of a semi educated guess, you're going to be getting an energy release a few orders of magnitude smaller than if the star underwent its own natural supernova. For a core collapse supernova event, that's about 1045 Joules of energy released in electromagnetic radiation. The Sun releases 1026 Joules per second, just as a number for comparison, and in a year the Sun puts out 1034 Joules per second of power.

I'll be using Phi Orionis (18 solar masses) to make estimates of the habitable zone since I know its absolute magnitude/luminosity (-3.04/20k Sols). The habitable zone will be an orbit at something like 141 AU (this is because of the inverse square law of luminous intensity, the square root of 20,000 is about 141 - since we know 1 AU is habitable for 1 Sol of luminosity). At that distance, 1045 Watts (a Joule per second) is going to absolutely fry the population of the planet.

A search via google scholar for "black hole star merger" gives this article from Zhang and Fryer which discusses the result of a black hole merger with a star with a helium core. Basically, we're looking at, for the low end, a gamma ray burst with energies of 1040 Joules. At the high-end, with a helium-cored star with a mass like Phi Orionis (or our 20 solar mass star I assumed for discussion at the beginning) we'll get something more like 1045 Joules of energy release within 65 seconds of the collision. Which gives us an energy burst only slightly smaller than the supernova energies. 1043 Joules per second for a minute is going to sterilize everything orbiting near the star.

The Earth gets about 1300 Watts per meter2 in power per area from the Sun. Assuming the planet you strand the pedos on is about the same size as the Earth, at that distance the planet is going to be receiving 1019*1300/(141)2=6.53890649 × 1017 Watts per meter2. We are well into "Fry the planet" territory there, I think, given that this is equivalent to 500 trillion times the energy that the earth receives from the sun every second.

Well I hope I didn't ramble too much in finding out your answer, I was working it out as I typed this response. The short version is, if you want to kill something near a star, throwing a black hole at the star is probably a good way to do it!