# Accretion disk/black hole/gas jets

## Main Question or Discussion Point

I have a few questions regarding these phenomena.

1. I don't quite understand what is going on in the picture. Obviously there is a spin on the black hole but why doesn't the gases get sucked straight towards the black hole. I know this is a simulated picture but I don't see how this would be right. Wouldn't the gases get sucked straight towards the black hole and once they collide with the accretion disk begin to spin?

http://en.wikipedia.org/wiki/Image:Accretion_disk.jpg

2. How exactly are gas jets shot out of the top and bottom?

thanks

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Notice the wide flat disk perpendicular to the jets. That is the gas spiraling into to black hole. The actual swartzchild radius from which nothing can escape is relatively very tiny and at the center of that cloud. When the gas in the disk accelerates toward the black hole it get denser, hotter, and much of that gas collides at high speed. Because this is all occurring outside the event horizon some of this gas gains enough speed to jet away at the axis of rotation. Much more gas gets sucked in. The hot gasses colliding produces light, radio, and x-rays which escapes because it is also outside the event horizon.

My qualitative description may have technical errors but it should make it understandable. The key point is that the only time nothing can escape is if the light and particles gets within a certain distance. The event horizon determines how close light must get before it can't escape. Slower mass particles get trapped much farther away.

So basically the disk of rotating particles extends farther then the picture shows? Because I still don't understand why the particles wouldn't go straight towards the black hole.

Kurdt
Staff Emeritus
Gold Member
It doesn't get sucked straight in because the system is rotating. The problem of how jets are formed is quite complex and involves relativistic magnetohydrodynamics.

tiny-tim
Homework Helper
… black holes have the same gravity as stars …

… why doesn't the gases get sucked straight towards the black hole. I know this is a simulated picture but I don't see how this would be right. Wouldn't the gases get sucked straight towards the black hole and once they collide with the accretion disk begin to spin?
Because the gravity round a black hole is the same as round any ordinary star.

(Except very near the event horizon, where there's some noticeable "frame-dragging" - but that would only whirl the gas round faster, not pull it in.)

The "Schwarzchild metric" for empty space round a black hole is the same as for empty space round the sun - it's just that the sun's surface isn't empty space, so the Schwarzchild metric stops at the sun's surface, and is replaced by something else.

If we couldn't see the sun (if there was a big box round it, for example), then we wouldn't know whether there was a black hole there instead of the sun. Not even observing the paths of satellites and comets would show any difference.

Gas orbiting close to the sun would have no reason to be "sucked" into it - it would just go on orbiting, wouldn't it?

(In fact, it would actually lose energy through collisions etc, so it would very gradually fall in.)

Gas orbiting a black hole would be the same - only gradual loss of energy would make it spiral (not fall) in, rather slowly.

Why, then, is the yellow gas leaving the nearer star and curving off towards the black hole in the picture?

The same reason Mars has lost most of its atmosphere, and Earth has lost most of its atmospheric hydrogen - more-energetic-than-average gas is able to escape Mars or Earth or the yellow star, and once it does so it will be go into orbit round (rather than be attracted to) the next biggest object.

On Earth, only hydrogen is light enough to do that (I think).

The yellow gas gas escapes because it's hot, and it starts to orbit the black hole because … well, because it's there!

The same would happen with any star with the same mass as that black hole.
How exactly are gas jets shot out of the top and bottom?
erm … pass.

(Probably got something to do with magnetic fields.)

Jets shoot out from the poles because those are the paths of least resistance of sort.

So basically the disk of rotating particles extends farther then the picture shows? Because I still don't understand why the particles wouldn't go straight towards the black hole.
No the picture includes everything. The black hole is just a very very tiny dot in the middle that you can't see. They don't all go straight toward the black hole for roughly the same reason the moon doesn't go straight toward the Earth.

Thanks for the posts.

Stupid me, I for some reason I didn't realize that the sun is in orbit around the black hole and not stationary. :)

So matter can't escape in the rotational plane but it does so at the poles, somehow because of magnetism?

Weaksauce. Either matter can or can't escape the event horizon.

Cosmology has gone awry. Cosmologists should just admit we don't know stuff when observable fact falsifies theory instead of doggedly stifling progress, even going so far as piling invented fudge factors on top of each other like a matroshka doll.

Show me gravitational lensing in an otherwise very dark patch of the sky and I'll believe.

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tiny-tim
Homework Helper
So matter can't escape in the rotational plane but it does so at the poles, somehow because of magnetism?

Weaksauce. Either matter can or can't escape the event horizon.
Matter in the polar jets doesn't escape from the event horizon, it's just orbiting matter (outside the event horizon) which in its interaction with other matter and the magnetic field, somehow gets focussed, and expelled at the poles.

(Interactions would also try to expel matter in other directions, including the rotational plane, just as hydrogen is expelled from the Earth, but not fast enough to actually escape.)

Ich
What's the point in not reading a three months old thread but answering by beating up a straw man and then railing against cosmology?

FYI:
my_wan said:
Because this is all occurring outside the event horizon some of this gas gains enough speed to jet away at the axis of rotation.

Matter in the polar jets doesn't escape from the event horizon, it's just orbiting matter (outside the event horizon) which in its interaction with other matter and the magnetic field, somehow gets focussed, and expelled at the poles.
How come that matter is ejected perpendicular to the orbital plane? Isn't matter spiralling in from the accretion disk in the orbital plane? Does it somehow just skirt the event horizon out towards the poles where it is ejected at relativistic speeds perpendicular to the orbital plane?

Another issue I have is I remember reading jets are observed when the BH is feeding (from the accretion disk). Shouldn't they be occurring at times when nothing much in particular is falling in as well?

(Interactions would also try to expel matter in other directions, including the rotational plane, just as hydrogen is expelled from the Earth, but not fast enough to actually escape.)
Let alone at relativistic velocities out to distances far beyond the size of the accretion disk.

And why jets? Why not obtuse cones?

Kurdt
Staff Emeritus
Gold Member
It is still not clear how the jets are formed. It is thought to be due to very complicated magnetic interactions. Jest can't occur if there is no matter to be expelled. So if there is no accretion disc there will be no jet.

Again the jets are necessarily so from the physics of the situation. Unfortunately we still don't know clearly enough how they form.

phyzguy
Show me gravitational lensing in an otherwise very dark patch of the sky and I'll believe.
How about this paper? It shows a large lensing peak (peak 3 in figure 2), which is basically devoid of galaxies.

http://arxiv.org/abs/0706.3048

You've got to know that the two bodies are in rotation round each other. Then the mass from the star moves towards the black hole in this case because the inner lagrangian point( look it up... It's pretty much the point where the two bodies exert equal force on each other) is below the surface of the star. Then due to the conservation of angular momentum (L=mass*angular velocity*radius) the shiz will get faster the closer it gets to the black hole. Since they're rotating round each other there will obviously be a spiral occur....This then becomes a disk because its all compact and is becoming heat energy. It also must be noted this is an artists impression not a photo...

The jets are the confusing bit and there are only a bunch of good hypothesis to explain them. The best is that they're a complex bunch of rotating magnetic fields that are a function of the accretion disk.

If you've got a spare hour...

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The disk is "easy" to explain. What happens is that the gas goes into orbit around the black hole. Now if the gas was in a uniform sphere, the gas would collide with each other, and that turns out to be unstable, so the gas naturally settles into a configuration where you don't have that many collisions, which ends up with a disk. The disk is aligned with the rotation of the black hole because that it is the state with the lowest energy.

The same thing happens with planetary rings and the solar system. You can think of an accretion disk as a "ring" around a black hole.

Now as for jets, we really don't know why those form. One sort of surprising thing is that we know more about black holes than we do about the jets that form around them. Black holes are "easy" theoretically. Jets are hard because you have lots of different physics.

Dotini
Gold Member
Here it is reported remnants of jets from our own galaxy, possibly active within the last million years and possibly originating from a MW "AGN" . The Fermi bubble and jets are said to extend 27,000 light years.
http://www.sci-news.com/astronomy/article00353.html

And here it is reported a VPOS, or vast polar structure of globular clusters, streaks and stars extending 10 to 250 kps, beginning just where the bubble and jets end.
http://arxiv.org/abs/1204.5176

The origin of the VPOS is conjectured to be a collision with another galaxy.

Question: How likely is it that the polar material and jets are related? Could some or all of the polar objects have condensed from the jets? Are the distances reported for the two phenomena necessarily incommensurable?

Respectfully submitted,
Steve

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On formation of the jets, the accretion disc spins hotter and faster towards the center obviously. Now I may have my assumption wrong about the uncertainty principle, as I've only studied it in one dimention, but since we know that the most inward particles in the disc have relativistic velocities in the circumferential direction, wouldn't that mean that we can't be certain of their radial and axial components?

Meaning, with super relativistic velocicties very close to the EH, wouldn't we expect to find particles with large velocities in axial and radial directions; sum over histories and whatnot.

The radial component would only add to the turbulance of the disc, but if the axial component is large enough, the particle would race out towards the poles; either under influence of mag fields or simply momentum. On a large enough scale, couldn't this kind of uncertainty driven exitation in the axial direction.lead to polar jets if high energy material get ejected or collides with other particles and annihilates, giving rise to large amounts of energy at the poles. Any non-massless particles that arrise from the energy would have to be directed nearly perpendicularly away from the BH to escape gravity.

Is this a reasonable suggestion?

I have a some questions that have been bugging me for some time.

1.Can black holes rotate in a clockwise motion in some and counter clockwise in others or do they all rotate in the same direction?

2.If the event horizon of a clockwise rotating black hole collided with the event horizon of a counter clockwise rotating black hole. what would happen? would they cancel each other out or would they rip each other apart sending a gravitational shockwave throughout the universe?

tiny-tim
Homework Helper
Welcome to PF!

Hi TrinityP! Welcome to PF!
1.Can black holes rotate in a clockwise motion in some and counter clockwise in others or do they all rotate in the same direction?
They're like the Earth …

the Earth rotates anti-clockwise as viewed from above the North pole, but clockwise as viewed from above the South pole.
2.If the event horizon of a clockwise rotating black hole collided with the event horizon of a counter clockwise rotating black hole. what would happen? would they cancel each other out or would they rip each other apart sending a gravitational shockwave throughout the universe?
They would merge into a black hole whose radius was the sum of the two radii, and whose angular momentum was the vector sum of the two angular momentums.