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Brady Yoon
2004-May-05, 10:46 PM
I have some questions about gamma ray bursts.

1) What is the minimum safe distance?
2) Are any stars likely candidates?

Nereid
2004-May-06, 12:17 AM
1) A: "It depends"
Some quick amplifications:

- since nothing is known about the object(s) which give rise to short duration GRBs, including their distance, little can be said about a 'safe distance' for them

- if long duration GRBs are indeed one kind of supernova ('hypernova' or otherwise), then if we're in the path of one of the two polar jets, a 'safe distance' would be much further than if we're not

- 'safe' for who, or what? If you require that the Earth simply be not vapourised, then a much shorter distance than if you require a viable global economy within 6 months. Survival for extremophile bacteria >20km underground would be somewhere in between

- what is in between us and the GRB? In particular, how much would the 'after-glow' of X-rays to radio be attenuated, e.g. by dust clouds? Ditto for the intense bombardment by EHE cosmic rays? (magnetic fields may be important too).

Here's an interesting exercise: how much energy, in the form of gammas, is required to a) vapourise the Earth, b) strip the Earth of its atmosphere, c) melt the surface of the Earth to a depth of 10 km? In the case of b) (or similar), assuming only the gammas reach the Earth, what would cause life on the 'dark side' of the Earth (away from the GRB when it struck) to die?

Does a long duration GRB (say 1 s) have enough energy in the gammas to vapourise the Earth?

ysandre
2004-May-06, 07:15 PM
2). Eta Carinae. Hopefully the burst won't be pointed at us....

eburacum45
2004-May-08, 05:33 AM
Here's an interesting exercise: how much energy, in the form of gammas, is required to a) vapourise the Earth,
I have it on good authority that this would require approx 10^32 joules;

a supernova gives off around 10^44 joules, so should vapourise the Earth easily if nearby. The problem is the bipolar jets of a hypernova concentrate the energy so you can't use the inverse square law.
I would guess that you wouldn't vapourise the Earth if a hypernova were more that a couple of lightyears away. Probably not even then.
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Chip
2004-May-08, 08:08 AM
I would guess that you wouldn't vapourise the Earth if a hypernova were more that a couple of lightyears away. Probably not even then.

It might make living conditions rough though, if the jet was pointed at us. At least, I recall MIT Astronomer Philip Morrison years ago saying that a "nearby" supernova (and hypothetically a hypernova) would require us to stay inside or even under ground for a few weeks.

Normandy6644
2004-May-08, 02:15 PM
I would guess that you wouldn't vapourise the Earth if a hypernova were more that a couple of lightyears away. Probably not even then.

It might make living conditions rough though, if the jet was pointed at us. At least, I recall MIT Astronomer Philip Morrison years ago saying that a "nearby" supernova (and hypothetically a hypernova) would require us to stay inside or even under ground for a few weeks.

Well if I hypernova were to happen nearby, neglecting the harmful gamma rays at first, I would imagine that the magnitude of the light reaching us would be enough to keep us sheltered. Even though it's only coming in one direction, the size of the jet would mostly likely completely cover the earth. That's not a very nice day at the beach.