Why do black holes have stronger gravity than the stars they evolved from?

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Offline thedoc

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Mark Wilson  asked the Naked Scientists:
Naked Astronomy,

Stellar mass black holes are created when massive stars collapse due to their enormous gravity.  Before they collapse the light they generate easily radiates outward just as it does for any other star.  But after they collapse light can no longer escape.

I want to know why this is.

Certainly the mass of the object hasn't changed.  Only the density has changed.  It was my understanding that the gravitational force of an object depends on its mass.  If the mass hasn't changed how can it be that the gravitational force has increased?

Please explain.

Mark Wilson
San Diego, California, U.S.A.

What do you think?
« Last Edit: 11/08/2012 12:30:01 by _system »


Offline CliffordK

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Gravity depends on mass and distance.

A higher density would mean that the surface gravity would be greater.  So, the event horizon of the newly formed black hole would be inside of where the corona was on the star.


Offline Bored chemist

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The simple answer is that, at any given distance from the centre of gravity, the star and the black hole have the same gravitational pull. But you can get much closer to the black hole.
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Online evan_au

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An explanation that doesn't rely on relativity is the idea of an "escape velocity", popularised by Laplace:

Escape Velocity is the minimum velocity of a projectile, fired from the surface of an astronomical object, which could escape from the gravity of an object. The formula is provided here: http://en.wikipedia.org/wiki/Escape_velocity

A star has a certain escape velocity, and assuming it has a "silent"collapse (as distinct from a supernova that blows off lots of matter):
  • The mass "M" of the star will be unchanged
  • The gravitational constant "G" has not changed
  • But the Radius "r"  has shrunk dramatically
  • ...to a white dwarf, a neutron star or a black hole

The escape velocity of the white dwarf or neutron star is much higher than the original star.
The escape velocity of the black hole even exceeds the speed of light, so not even light can escape.
According to Einstein, no projectile can go faster than light, so nothing else can escape either.