From: Zardoz ®	04/11/2001 19:33:31
Subject: re: COSMOLOGY FAQ	post id: 484060
How long would the earth survive in the event that the sun turned into a black hole? From: B.C. ® I do know that if our sun should magically become a black hole the earth and the planets would keep on orbiting just the same. So as I said the second time after thinking more the gravity would be stronger within the event horizon, so strong in fact that not even light would escape. So the key to the "paradox" is the density. The sun will not become a black hole. If magically the sun could be compressed into a diameter of 2.5 kms, we would then see a black hole. What would happen to the earth? Absolutely nothing, we would go on orbiting the sun/black hole, as per normal. The only objects that would be affected by our sun becoming a black hole, would be those comets ect that strayed into and beyond the event horizon. I'm guessing here but I would say the event horizon would be the current diameter of the sun, so as I have already said earth and the other planets would continue their orbit. From: James R (Avatar) Not quite, B.C. but close The event horizon radius is given by 2GM/c2. G is the gravitational constant 6.67 × 10-11. c is the speed of light 3 × 108 m/s. M is the sun's mass in this case, which from memory is about 2 ×1030 kg. The event horizon would therefore have a radius of about 3 km (much smaller than the sun's normal radius of about 696 000 km. From: Chris (Avatar) The strongest effects of a black hole are felt closest to it. Further away from the black hole the gravity is the equivalent of a star of the same mass. For example for a star X which collapses to become a black hole, at any point outside the original star's surface the field won't change too much once the star has collapsed to a black hole. The further away you are, the less it will change. If you're on a planet in earth-like orbit about X then you won't notice a difference gravitationally (you might notice a difference in radiation