If a star explodes while being sucked into a black hole will the explosion have any effect on the black hole?

1 Answer
Jan 27, 2016

It will increase the Schwarzchild radius of the black hole.


Lets consider a binary system of a star orbiting a black hole, being slowly sucked in. The star is constantly shedding bits of its outer atmosphere, as stars do, and the black hole just goes on sucking it in, as black holes do. Eventually, the star will reach a point where it doesn't have enough mass to maintain the hydrostatic equilibrium holding it together. This makes our star violently explode.

This is the basis for an absolutely excellent episode of Stargate SG-1, in which a wormhole is created between a star and a black hole in order to siphon the mass of the star, and cause it to explode disposing of the fleet of enemy vessels in the star system; but I digress.

Where were we? Oh right. Fusion pushes the star apart, gravity pulls it together. Not enough mass = not enough gravity = a massive stellar explosion. How fast does this explosion move through space? If there's one thing for sure, it's at or below (my guess is below) the speed of light.

A black hole is defined by it's Schwarzchild radius, which is basically the radius of a sphere a certain amount of mass has to be crammed into before the escape velocity from the surface of the sphere is greater than or equal to the speed of light. Say that in one breath. Since the shell theorem tells us we can consider all the mass to be concentrated as a point particle in the centre, the Schwarzchild radius tells us how far from a black hole the event horizon (the point of no return) will be.

Therefore it's most likely that the remnants of the star will initially become part of the accretion disk orbiting the black hole, and eventually be sucked past the event horizon. Since nothing returns from past this horizon, including information, we don't really know what happens to the structure of the black hole internally. However, we know that from outside the event horizon, the only effect is to increase the mass and therefore the gravity of the black hole, and therefore increase it's Schwarzchild radius.