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?
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.