| Topic: |
Science > Physics |
| User: |
"George" |
| Date: |
13 Jun 2005 08:57:16 AM |
| Object: |
Black Hole Question |
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
.
|
|
| User: "Morituri-|-Max" |
|
| Title: Re: Black Hole Question |
13 Jun 2005 12:23:22 PM |
|
|
"George" <georgekinley@hotmail.com> wrote in message
news:xn0e3ge5xk307g6006@news.europe.nokia.com...
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
While it is alive it is exploding constantly due to its fusion that gives
off the light and heat. So it is always trying to throw all its material
into space, but gravity holds it all together. When it runs out of fuel
that it can "burn" gravity takes over and pulls it all together very very
tight depending on how massive it was.
.
|
|
|
|
| User: "Sam Wormley" |
|
| Title: Re: Black Hole Question |
13 Jun 2005 08:59:52 AM |
|
|
All stars are "held up" against the inward pull of gravity
by radiation and pressure due to high temperature from
thermonuclear fusion going on in the star's core. When
all fusion creases, gravity wins
There are four (4) fates for the end of stars depending on their masses
and the masses of their cores:
Red/Brown Dwarfs - less than 0.6 Ms <== Main Sequence 0.01-0.8 Ms
Stars less than about 0.6 solar masses, when nuclear fuel is used up,
gravitational collapse shrinks the star, but no more than the gas
temperature-pressure-volume laws of classical physics allow. We have
not found any white dwarf less massive than 0.6 solar masses. Part of
the answer is that the universe may not be old enough for lower mass
stars to have evolved off the main sequence.
White Dwarfs - 0.6 and 1.4 Ms <== Main Sequence 0.8-10 Ms
Stars with core masses between 0.6 and 1.4 solar masses are
destined to become white dwarfs. White dwarfs are degenerate matter.
Further collapse is halted by electron degeneracy pressure. See pages
456-459 in your textbook. The vast majority of stars are in this mass
range and are destined to become white dwarfs
Neutron Stars - 1.4 and 2.9 Ms <== Main Sequence 10-30 Ms
Core masses between 1.4 and 2.9 solar masses overcome electron
degeneracy pressure and collapse to form neutron stars, a star that is
essentially one gigantic nucleus. Further collapse is halted by neutron
degeneracy pressure.
Black Holes - 3 or more Ms <== Main Sequence > 30 Ms
But for cores with mass of 3 or more solar masses, neutron
degeneracy pressure does not stop the collapse and the star becomes a
black hole with zero physical size, but with all the mass. Gravity
really wins!
In each case, gravity eventually wins, but, to what extent is
determined by the mass and the relative pressures of the quantum
mechanical forces, electron and neutron degeneracy pressure.
See: http://www.astronomynotes.com/evolutn/s1.htm
http://www.eclipse.net/~cmmiller/BH/blkmain.html
.
|
|
|
|
| User: "George Jones" |
|
| Title: Re: Black Hole Question |
13 Jun 2005 09:47:07 AM |
|
|
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
.
|
|
|
| User: "Morituri-|-Max" |
|
| Title: Re: Black Hole Question |
13 Jun 2005 12:25:44 PM |
|
|
"George Jones" <george_llew_jones@yahoo.com> wrote in message
news:1118674027.136392.193590@g43g2000cwa.googlegroups.com...
George wrote:
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.
Interesting answer, but you never did answer him why it can become a black
hole when it was "dead" and not become one to start with.. you jumped right
in to what happens once it has become a black hole.
8 )
.
|
|
|
|
|
|
Related Articles |
|
|
