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This question already has an answer here:

I have heard in a talk given at the Fermilab (on Youtube) that we typically detect a neutrino burst from a core-collapse supernova CC SNR explosion 2 hours before we detect its electromagnetic radiation. When the question was raised about whether this violates the principle that nothing can travel faster than light, the answer given by the speaker was that when the collapse is happening, the neutrinos are expelled, whereas light from the supernova comes AFTER the explosion, not during the collapse. But doesn't it take about 1 second for the iron core to collapse completely? Why is there a 2 hours difference between our receiving the neutrino burst and light from the explosion?

Thank you

P.S. I read around and found that the reason is because light has to travel a lot of distance within the star which "slows" it down, however, as mentioned earlier, it takes only 1 second for the entire iron core to collapse, right?

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marked as duplicate by sammy gerbil, Jon Custer, peterh, M. Enns, Community Aug 11 '17 at 1:03

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

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Stars are opaque so we only see the light emitted at the surface of the star.

In a core collapse supernove the core collapses very rapidly but the outer layers of the star are left initially unaffected. The rebound of the core produces a shock wave that travels outwards towards the surface of the star but this shock wave travels much more slowly than the speed of of light so it takes a couple of hours to reach the surface of the star. Only when the shock wave reaches the surface of the star does the light emitted by the star start to brighten into the characteristic SNII light curve.

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