black hole

[dark mark]

Hi i am a bit of a newbie, But have a couple of questions about black holes for anyone who may be able to help. do we have an estimate of the gravatational force require to cause a black hole and when its a fully fledged black hole how much of this force as a percentage is actually exerted back past the event horizion onto our universe.

[acey]

Black hole formation depends on squeezing matter to a certain density - the squeezing is done by the objects's own gravity pulling inwards. How much density you need depends on the total mass: as the mass gets bigger you actually require lower density.

So you can make a black hole out of anything, simply by piling together enough of it, until it collapses under its own gravitational weight. Stars are mostly made of hydrogen and helium, but you could make one out of sand or water if you just had enough of it. Making very tiny black holes requires much higher density so ordinary matter won't work: it might be possible for these to be created by certain elementary particle processes but that's still hypothetical.

The gravity of a black hole is like the gravity of anything else: it extends infinitely. If the Sun were a black hole we'd still be orbiting around it, it's just that we wouldn't see it.

It's only when you get very close to a black hole that things get hairy. The hole is very much smaller than a regular star but has the same graviational influence as one. If you cross a certain critical threshhold then you find you can't turn back, and you're a gonner.

[dark mark]

thats one of the many bits i cant get my head round. the forces of gravity involved are large enough to distort space time and cause the black hole in the first place,why then is such a massive force so localised to the very rimm of the black hole,? why with such a distortion does it not act more violently on the surrounding matter and pull it all in.

sorry for my simple logic.

It's only when you get very close to a black hole that things get hairy. The hole is very much smaller than a regular star but has the same graviational influence as one. If you cross a certain critical threshhold then you find you can't turn back, and you're a gonner.

[brianb]

the forces of gravity involved are large enough to distort space time and cause the black hole in the first place,why then is such a massive force so localised to the very rimm of the black hole,?

It isn't. Gravitational force is inversely proportional to the square of the distance from the singularity, the event horizon is where the escape velocity is equal to the speed of light.

why with such a distortion does it not act more violently on the surrounding matter and pull it all in.

The Sun distorts space/time & the Earth doesn't fall into it. The only difference with a solar mass black hole replacing the sun would be that the event horizon would be approx. 10 Km diameter (the size of a small city), rather than the 1.4x10^6 Km diameter of the sun (measured at the photosphere) (108 times the diameter of the Earth). The distortion becomes increasingly severe as the event horizon is approached until the event horizon itslef is reached, at this point the distortion is enough to tear a hole in the universe, the mass within the event horizon still has an effect but the space/time within the event horizon can be thought of as being outside the universe we inhabit.

[dark mark]

I know the sun distorts space time but not really in the same league the sun doesnt tear the universe and its escape velocity is only 620km/s not the speed of light. how far out from a black hole would you have to go to reach the escape velocity equal to 620km/s.

dont understand how matter around the event horizon can resist these forces. Cant get my head round it, just yesterday i was reading about the asteroid defence theory that to destroy a meteor with explosives would be pointless because gravity would bring it back together. but then matter around a black hole can resist the huge forces they encounter.how ?

does a black hole have a magnetosphere?

I have just started an ou course to answer alot of my simpleton questions.find its all so fascinating. cant believe i didnt used to be that interested in space. how can you not be!

[brianb]

how far out from a black hole would you have to go to reach the escape velocity equal to 620km/s.

If the black hole was one solar mass ... one solar radius.

dont understand how matter around the event horizon can resist these forces

When you get in towards the singularity, even the strong nuclear force (or neutron degeneracy pressure, if you prefer) is overwhelmed by gravity. But, out by the event horizon, the tidal force of a very massive black hole can be quite weak - a less massive black hole has a small event horizon which means that the gradient is very sharp, but with a black hole of millions of solar masses the gradient is quite weak because the event horizon is so much larger. An object falling in from an infinite distance will still acquire the speed of light by the time it arrives at the event horizon.

A black hole will of itself have no magnetic field & therefore no magnetosphere (or rather its residual magnetism will be trapped inside the event horizon). But black holes do not populate empty space. If there is material falling inward, the accretion disk will consist mostly of ionised gas (ionised by the energy of infall) & will therefore generate a current, and consequently a magnetic field. Quite a strong one. It's this magnetic field which causes the polar jets in neutron stars, giving rise to pulsars. The polar jets from very massive black holes in distant galaxies are directly observable with large telescopes.

[dark mark]

Thank you brian, so its all proportional to solar mass. The milky ways blackhole at 3.7 million solar masses would be down to a escape velocity the same as our sun 620kms at 3.7 million solar radius, but for every solar radius closer it goes up by 620 kms.Thats all very neat.

[dark mark]

Just founnd a really good website black holes:gravities relentless pull , on hubblesite.org well worth a look for the newbs.