escape velocity

[madhuparc]

What will happen if the sun's escape velocity gets higher than the speed of light?

[Loosehead]

Then light would not escape. It would probably already be a black hole if that was the case.

[chainfire]

Not sure if this is correct, but this is what i think happens:
FYI, this will not happen to the sun as it is too small, but hypothetically:
The Sun will be a black hole with that amount of gravity. The reasons why black holes are called "black holes", is because light cannot escape the pull of gravity, therefore it is 'black'.

[Loosehead]

Ok must clarify, the sun will not be a black hole because it's not big enough, I was merely stating that for a star to have this property it must have sufficient density, that usually means a black hole. Our sun is destined to become a white dwarf , still pretty dense though!

[Catso]

No, chainfire is right.

If the escape velocity were greater than the speed of light, as the question proposes, then the sun would be a black hole.

Isn't that the definition of a black hole? A body whose gravity is so strong, even light can't escape it.

[Loosehead]

errr.... think I said that Catso!

[Catso]

I think you said "the sun will not be a black hole".

[Loosehead]

Which is what chainfire said!

and catso said "Chainfire was right"

I also said " .... for a star to have this property it must have sufficient density, that usually means a black hole "

[JudgeJ]

It is not just a case of gravity. It is a question of escape velocity. This, as Loosehead correctly points out, is all to do with density. As an object gets closer to the centre of gravity of a body, the velocity needed by the object to overcome the gravitational force of the body increases. The denser a body is, the closer its surface will be to its centre of gravity. Escape velocity for the earth is about 25,000 mph. This is about 0.00004c (c = the speed of light, which is about 670 million mph).

For bodies of �normal� densities the issue of escape velocity approaching c does not arise. However, when super-dense bodies such as collapsed stars are considered, things begin to change.

When a star dies the nuclear reactions which kept it in equilibrium cease. Gravity, now being unopposed, takes over. The star collapses, and its density is raised to unbelievable values. What happens to the star ultimately depends upon its mass. Those of less than 1.3 solar masses shrink to become white dwarfs, and eventually black dwarfs. These burnt out cinders of stars may exhibit densities of hundreds of tons per cubic inch, but because of their relatively small mass, they will not collapse further. Nonetheless, escape velocity from objects such as this could reach 0.3c. This is the likely fate of our Sun.

Stars of between 1.3 and 3.0 solar masses collapse further to form �Neutron Stars�. These may have escape velocities up to 0.5c. Only those above 3 solar masses have the potential to collapse even further. There seems to be no theoretical limit to their shrinkage and it is these objects that can potentially form �Black Holes, where the escape velocity is greater than c.

For the Sun to be a Black Hole it would have to have a diameter of no more than about 4 miles (it is currently 800,000 miles in diameter). For the Earth to be a Black Hole it would have to be about the size of a table-tennis ball.

[Catso]

No, chainfire said "The Sun will be a black hole" (line 4); you said "the sun will not be a black hole" (line 1).

The definition of a black hole is surely 'a body whose gravity field is so strong, even light can't escape' => it WILL be a black hole.

[Loosehead]

Chainfire preceded that with the word "hypothetically", essentially as the judge has kindly pointed out that the required density for the sun to be a black hole can be calculated as it can for the Earth, that is not to say it will be a black hole but for it to have the poperties of an escape velocity to exceed the speed ofg light it would have to fit in a sphere of 4 miles across.

I do hope you are not being deliberately obtuse here catso.

[mibn2cweus]

In terms of our current understanding of physical laws, our Sun does not possess sufficient mass to collapse into the density required to form a black hole. However, if all the material of our Sun did collapse sufficiently to form a black hole, this would not alter it�s mass and so the Earth would continue to orbit as it does now. The difference we would notice most is that we would no longer be warmed by the Sun�s energy as this would no longer be able to escape the intensity of the gravitational field associated with a black hole. The Earth would be a dark frozen planet in orbit around an invisible Sun.