A question about black holes

[Rum]

This item on the BBC (you probably can't view it in the USA) prompts me to ask a question which has occurred to me a few times. Here's the link: http://news.bbc.co.uk/1/hi/science_and_ ... 555633.stm

The question is this:- I thought that the gravity generated by black holes was supposed to be so overwhelming and powerful that beyond the so called event horizon nothing (energy, light, information) could escape it. And yet this article talks about a black hole emitting and injecting energy into a trapped star.

Actually I think I have answered the question to myself as I have typed this. Probably the energy is created prior to the event horizon by stuff falling into the BH.

Interested none the less in thoughts on this.

[colubridae]

IIRC try hawking radiation.

[Horwood Beer-Master]

It's the matter in the accretion disc, not the actual black hole itself, that is generating these particle jets along it's axis.

[JimC]

Russians say "collapsed stellar object", because to say "black hole" in Russian is very, very naughty...

But on the OP, I'm sure that radiation (eg. X-rays) from the accretion disk is the energy source, as has been pointed out...

Hawking radiation, though real, would be very weak, I suspect...

[The Dawktor]

Hawking radiation, though real, would be very weak, I suspect...

Depends on how much charge he gets into his chair

[Pappa]

Hawking radiation, though real, would be very weak, I suspect...

Hawking Radiation can me responsible for massive amounts of energy being released from black holes, making them eventually burn themselves out of existence.

[JimC]

Hawking radiation, though real, would be very weak, I suspect...

Hawking Radiation can me responsible for massive amounts of energy being released from black holes, making them eventually burn themselves out of existence.

For large black holes formed via stellar collapse the rate of energy release is small; it increases dramatically for smaller black holes.

[Pappa]

Hawking radiation, though real, would be very weak, I suspect...

Hawking Radiation can me responsible for massive amounts of energy being released from black holes, making them eventually burn themselves out of existence.

For large black holes formed via stellar collapse the rate of energy release is small; it increases dramatically for smaller black holes.

I don't know the timescale, but the implication is that all black holes will eventually become super-hot and burn themselves out (especially after they run out of things to eat).

[Clinton Huxley]

Rationalia - discussions about Hawking radiation and fruit cake. It's like Horizon meets Test Match Special.

[Pappa]

Also Rum.... aside from the energy released from black holes via Hawking Radiation (which is a result of quantum effects, pairs of particles popping into existence just inside and outside the event horizon - one gets sucked in, one gets emitted) there is also a massive, swirling, hot flow of matter around all black holes. I imagine it's this that will be feeding the star, not Hawking Radiation.

[Farsight]

Yes, Hawking radiation remains hypothetical, and is inconsequential for anything other than a microscopic black hole. The emitted energy comes out of the infalling matter. See accrretion of matter which says "this process of accretion is one of the most efficient energy producing process known; up to 40% of the rest mass of the accreted material can be emitted in radiation.. I have heard say that only 1% of the infalling material actually ends up in the black hole, and the rest is blown out either as radiation or in polar jets.

[JimC]

Yes, Hawking radiation remains hypothetical, and is inconsequential for anything other than a microscopic black hole. The emitted energy comes out of the infalling matter. See accrretion of matter which says "this process of accretion is one of the most efficient energy producing process known; up to 40% of the rest mass of the accreted material can be emitted in radiation.. I have heard say that only 1% of the infalling material actually ends up in the black hole, and the rest is blown out either as radiation or in polar jets.

Yes, I remember reading something similar; quite different to the popular misconception of the black hole devouring any matter that approaches it.

Pappa wrote:

I don't know the timescale, but the implication is that all black holes will eventually become super-hot and burn themselves out (especially after they run out of things to eat).

As I understand it, it is a question of size. A lone black hole with no infalling matter slowly loses mass via Hawking radiation, very slowly at first, and then, as it gets smaller, at an ever increasing rate until it vanishes in a relatively short and energetic burst at the end. But, for a stellar sized black hole, tyhe earler stage would require billions of years...

[Twiglet]

Also Rum.... aside from the energy released from black holes via Hawking Radiation (which is a result of quantum effects, pairs of particles popping into existence just inside and outside the event horizon - one gets sucked in, one gets emitted) there is also a massive, swirling, hot flow of matter around all black holes. I imagine it's this that will be feeding the star, not Hawking Radiation.

Nice description of Hawking radiation. Btw Hawking radiation does not feed black holes, quite the opposite. The energy to create the particle pair is extracted entirely from within the black hole, and the energy possessed by the escaping particle is lost to the black hole. It is therefore a decay mechanism.

The swirling hot flow of matter is simply stuff that is being drawn into the black hole breaking down into it's constituents, not a property of it.

In terms of capture of matter, there is nothing all that special about a black hole. It has a gravitational mass, much like any other object (star) and objects are attracted to it in the same way as any other mass. Stellar mass Black holes no more suck matter in than the Sun does - the immediate effect is that they draw things into orbits, which like the Earths orbit around the sun - decay very very slowly.

The differences occur only in two respects:

1) Once an orbiting object is drawn within the event horizon it can never get out
2) Supermassive blackholes can exist, which therefore exert a much greater pull than stars.

[Rum]

Given that there is a critical point in terms of mass at which a star, neutron star perhaps (?) collapses and becomes a black hole - i.e. nothing can escape its gravity, are we saying that when that happens they are a standard size and that as more and more matter falls in it grows? I assume what grows in that case is the distance from the 'centre' to the event horizon, i.e. as the mass increases so does the gravity.

[Farsight]

Not quite, Rum. If a star runs out of fuel, it collapses. The result is maybe a white dwarf, or if the Chandrasekhar limit is exceeded, a neutron star, or if the Tolman Oppenheimer Volkoff limit is exceeded, a black hole. Stellar black holes aren't quite a "standard" size, but I believe there is a typical size range. Yes, the mass and gravity will increase with infalling matter, and the event horizon will get bigger, but the distance to the centre is undefined.