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How do you tell the difference between a gamma-ray burst and a star just from a picture of a nebula, in which it cannot flash on and off here and there?

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Well, one way is that it won't be in pictures from the past, and will vanish from pictures in the future. It will also have a different spectrum than a star. GRBs emit mostly gamma rays (hence the name) while stars have very different (roughly thermal) spectra. –  Colin K Aug 26 '12 at 22:16
@William, what do you mean by "from a picture of nebula"... Wikipedia says GRBs are emitted from supernovae, neutron stars, quark stars and black holes. –  Waffle's Crazy Peanut Aug 27 '12 at 1:40
What? How does it vanish from pictures in the future? @CrazyBuddy, I know that, but I heard GRBs go off every .33 seconds in the Crab Nebula, therefore if you see a picture of a Crab or any other nebula, then surely you would think that some of the glowing orbs in the picture aren't stars, and maybe just one is a GRB. It seems logical in my perspective. –  William Aug 27 '12 at 22:36
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GRBs have a spectra far more Red-shifted than that of a star.. In fact, GRBs have such an unique spectra that nothing matches with it. Its profile has one or two high peaks of 1/2 seconds during which most of the energies will be emitted.. An ordinary star would be bright in optical wavelength... But, doesn't have any crazy peaks & hence won't be extremely red-shifted (Otherwise, we won't be able to see them) A GRB's redshift can be measured only with the afterglow when the initial gamma ray flare would be over. Also, Crab nebula doesn't host GRB phenomenon... It only flares up because of some abnormal neutron star spin-down or magnetic disturbances. And we're able to catch the ultra high energy gamma ray photons from Crab only because it's so nearer to us than any GRB you can name of... GRBs are always highly red-shifted extra-galactic explosions.

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