Expert: Courtney Seligman - 6/11/2009

Question
QUESTION: the creation of black holes is formed in the aftermath of supernova so does this happen instantaniously or is there a timescale for this to form,if the star betelgeuse exploded would there be a black hole the following day so to speak?

ANSWER: The formation of the black hole takes only a fraction of a second, but it wouldn't be 'visible' for years, because it takes time for the gases blown away from the supernova to spread out enough to become transparent. However, presuming you could pass through the gases without being vaporized, and that remnants of the star remained close enough to the black hole to reveal its existence, you could tell it was there within a few seconds after the star blew itself to bits.

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QUESTION: given the newly created black holes immense gravitational field would the radiation,gases,light etc from this explosion not be sucked back into this newly formed hole thereby cancelling itself out,cheers

Answer
Any light / gases /etc inside the event horizon (a more or less spherical surface which defines the 'edge' of the 'black hole') would not be able to leave the region. That's why it's called a black hole. However, if there is any material OUTSIDE the event horizon, it could remain outside if it is rotating around or moving away from the 'hole' at sufficient speed. At the event horizon, the velocity required is equal to the speed of light, so nothing can be going fast enough to not fall inward. But outside the horizon, the velocity is less than the speed of light, so it is at least theoretically possible for material to remain outside, although whether it would do so permanently is another matter. In fact, the most common way to 'see' black holes is to observe radiation emitted by material falling toward the event horizon, during the period before it reaches the horizon. You might look up accretion disks around black holes for more information about this.