Re: Black Hole Question

George wrote:
> What stops a star that will die and become black hole, to be a black
> hole when he is alive , only difference is its size, and mass, and mass
> will be less when he becomes BH, and volume will be point particle, but
> gravity is caused by mass and not volume, so why does BH have more
> gravity and the living actual star


This is a good question, and I'm going to attempt an answer. If my
answer is unclear, or if you want me to use the mathematics of
Newtonian gravity to illustrate things, just post again.

When some stars die, they collapse to form black holes. In one sense
you're right - if the mass of the black hole is the same as the mass
of the collapsing, then gravity outside the star/black hole remains
constant as the (spherical) star collapses (sperically).

Consider astronaut A who uses a rocket to hover above the surface of
the star just before the collapse starts to collapse. As the star
collapses, the astronaut does not need to increase the thrust of his
rocket in order to remain hovering at the same "place" in space. As
the collapse proceeds, the surface of the star receeds from him.

Now consider astronaut B, who choses to proceed differently. Suppose
that this astonaut falls with the surface of collapsing star. Suppose
further that once the radius of the collapsing star reaches half its
original radius, B turns on his rocket, hovers at this place in
"space", and watches the surface of the star receed from him.

There is an important similarity between A's situation and B's
situation: neither A nor B needs to increase the thrust of his rocket
to remain hovering at his chosen place in space.

There is also an important difference between A's situation and B's
situation: even though the mass stays constant, B is closer to the
centre of mass than is A, and thus B experiences (4 times) stronger
gravity than does A. B's rocket must supply more thrust than A's in
order to hover.

You can imagine a third astronaut C who choses to hover at a place
that is even closer to the centre of mass of the collapsing star.
Even greater thust is required. Etc.

Eventually, no rocket can supply enough thrust to hover because the
escape velocity has reached the speed of light. A black hole has
formed.

Regards,
George

.

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