How powerful are gamma bursts?

[kheegster]

Hmm. An above link says the source of gamma ray bursts is unknown. Doesn't it occur when two neutron stars collide?

Neutron star collision is one possibility, but hypernovae explosions is another one. Most likely there isn't enough data to determine, or the data analysis to determine between the two possibilities hasn't been completed yet.

[ClaysGhost]

The closest GRB I've heard of had a redshift of about 0.01, which translates to a distance on the order of 10^8 parsecs..

I thought that one was relatively dim, but I may be mistaken.

[Sriad]

What exactly is a hypernova? Unless it's missleadingly named... I can't really see how you kick supernova up a notch from popping-blue-supergiant.

A pair of neutron stars coallescing into a black hole would release a whooole lot of energy though; when matter falls into a black hole up to 45% of its mass can be lost as energy, so you'd be looking at a couple solar masses annihilated in a second.

(actually the brevity of GRBs suggests that it is something to do with neutron stars or other small objects. Anything relating to stars, especially very large ones, takes a little while to unfold since they're light-seconds to light minutes across.)

[ryan8723]

A hypernova is when an extremely large blue supergiant blows up, essentially it is a supernova amped up by a few orders of magnitude.

The size of closest star capable of going hypernova, Eta Carinae, is on the order of 150 solar masses (which is much bigger than what scientists previously thought the maximum size of stars could be).

[Wyrm]

The size of closest star capable of going hypernova, Eta Carinae, is on the order of 150 solar masses (which is much bigger than what scientists previously thought the maximum size of stars could be).

The universe today is too polluted with metals for stars much larger than 100 solar masses to keep that mass. (To an astronomer, any element heavier than helium is a "metal"; I'm not making this up!) If the star is too bright, then the pressure of the radiation alone is enough to blow off material from the star, bringing the mass down. Only in the very oldest stars (which have an extreme poverty of metals) is the process inefficient enough that they can grow bigger than 100 M_sun.

Of course, large stars don't live very long, so most of the hypernova stars go up in the early universe, and indeed you find that all the hypernova candidates so far have had high redshifts. (That is, they happened a long, long time ago in galaxies far, far away!) In addition, Eta Carinae seems to be evolving rapidly to a more stable state, including shedding mass.

We're probably safe, but we ought to keep watch on Eta Carinae anyway.

[ClaysGhost]

(actually the brevity of GRBs suggests that it is something to do with neutron stars or other small objects. Anything relating to stars, especially very large ones, takes a little while to unfold since they're light-seconds to light minutes across.)

There's a summary of the different models(as of 2003) here. It seems that mergers would not release enough energy to account for GRBs. The crossing time is not a very useful argument against the hypernova model; if any crossing time is important (it might not be, because the gamma-ray production mechanism doesn't obviously involve the star's size) then it'd be the travel time across the heavy core at the centre of the supernova. Since this is the bit ends up as a neutron star, you can see that the only potentially important crossing time is similarly small.