If Light cannot escape Stellar-Mass Black Holes because of powerful gravity fields, how can they emit any Gas?

Question

The following paragraph was inside a news article on Yahoo today.
"Like the much larger, supermassive black holes found at galaxy centers, stellar-mass black holes have such powerful gravity fields that not even light can escape them. Astronomers estimate their mass by measuring their gas emissions and the gravitational effect on the stars they orbit."

I understand how they would have a gravitational effect on the stars they orbit and even other bodies.

But how can GAS escape if the gravitational field is so powerfull that not even light can escape?

This does not make any sense at all.

The full article can be found at

http://news.yahoo.com/s/afp/20071031/sc_afp/usastronomy2_071031201428

I believe I have figured it out. The following sentence is the key. I think the writer of the article goofed up earlier at the top in editing or in understanding. But later the facts come out.

Here is the sentence.

Prestwich's team was able to measure the black hole's mass because it has an orbiting companion: a hot, highly evolved star. The star is ejecting gas in the form of a wind.

It is not Black whole that is emitting gas, it is the companion star that is emmitting the gas.

The measure the Strength of the gravitational field of the black hole by measuring wht it is doing to the companion star.

I am sure what happened is that there were two stars orbiting each other and one of them FAILED causing it to become a black hole.

There is no reason that they would Stop orbiting each other just because of the change in gravity. So, the black hole and the star still orbit each other.

I will choose closest answer for the 10 points.

Answer 1

The last sentence of the article explains it: "Some of this material spirals toward the black hole, heats up, and gives off powerful X-rays before crossing the point of no return." So it is x-ray emissions from gas that is falling into the black hole, not gas being emitted by the black hole.

Answer 2

I believe they are referring not to the emission OF gas from the black hole, but the emissions BY gas around the black hole (in the accretion disk). Before material actually falls into the event horizon, it is compressed to such a state that it forms a plasma that emits X-rays at an intensity rather higher than most stars (although not in the visible spectrum). Since these xrays are generated outside the event horizon, the can be detected.

Answer 3

Another thing that was weird about it was that they said the black hole was orbiting the star...shouldn't it be the other way around?

As far as the gas emissions, I believe what they're talking about is something called Hawking Radiation. The idea is that black holes give off a jet of high energy radiation (black body radiation) and eventually, they will dissipate. That is the Hawking Radiation at work (theoretically). So light may not be able to escape, but the jet of radiation (travelling at immense speeds) does.

Of course, it could also be the gas forced outward from the black hole by the energy emitted by falling matter.

But that's just from reading Wikipedia. I may be slightly off here. You should check it out yourself under "Hawking Radiation."

I'd like to see what somebody with a scientific background says about this. Good question.

Answer 4

Light is affected by gravity. Gravity is a warping of space-time, sort of like a rubber sheet would wapr if it were stretched taut and a ball was placed on it. Light follows the curvature of space-time. The path light follows is called a geodesic curve. Now sometimes these curves intersect a blackhole. The curvature around a blackhole is so severe that everything gets pulled toward it. There is radius around the blackhole called the event horizon, which separates spacetime from then actual blackhole. Once you cross the event horizon, you cannot get back to spacetime. So light enters but it can't leave. Another way to view this is watching the light "fall" into the hole. Let's say you put a laser on a spaceship heading into the blackhole and the laser is pointed back at you. As the laser falls into the hole, you'll see the wavelength of the laser light increase (get longer) or red shift, due to the gravitational doppler effect. The photons you receive have less energy than the original laser wavelength photons and one way to explain this is that the light looses energy as it "climbs" out of the gravitational potential. If you go all the way into the blackhole (ignore teh event horizon for a moment), then you'd need an infinite amount of neregy to climb back out. the laser photons only have a finite amount of energy so they can't get out.

Answer 5

Simply because not all the material falls in. The extreme forces on the stuff falling in cause black holes to actually emit a lot of light and blast gas outwards.

Answer 6

It's not that the hole itself emits gas, it's that some gas just doesn't make it into the hole. Just like a comet orbiting the sun, it gets flung around, but if it comes too close, it gets sucked up.

Answer 7

The thing that you must realize is that a black hole has a region called 'The Event Horizon'.
This is the limit where gravity starts to pull matter to pieces. If the resultant bits are inside the limit ,they get sucked in. If they are pushed away from the Event horizon then they can be detected.