Black hole orientation

[Dickie]

Right, I was reading a book the other day which included a section on black holes - I'm fine with the theory behind them, but just curious about one aspect:

Why, does matter get pulled into them in a spiral pattern on a flat plane? Surely the gravitational pull will come from all directions hence matter will too?


Any ideas?

Thanks in advance!

 

[cjp]

I've also had this question. I did some thinking, I don't have a full explanation, but let me share my thoughts:

If there were only a single particle near the black hole, it would probably orbit the black hole in some elliptical orbit. However, there are lots of particles, and they exchange momentum (either through collisions, or through gravity).

In collisions, the particles lose a part of their energy, which is turned into heat, and then radiated away into the universe. As they lose orbital energy, they move closer to the black hole.

They lose energy, but they do not lose angular momentum (except to some rare particles that reach escape velocity after a collision). So, very close to the black hole, they move around it very rapidly.

I think such a particle cloud is probably a kind of self-organizing system, where, if one orbital plane becomes a bit dominant, it soon absorbs all particles. Imagine if the disk already exists a bit, and a single particle moves in a different orbital plane. Each time it moves through the disk, friction pulls it a bit towards the disk's plane. Also, the single particle pulls the disk a little bit towards its own orbital plane. In the end situation, all particles move in the same plane, with the normal vector of the plane perfectly parallel to the original angular momentum of the cloud.

Maybe you can compare the system a bit with other spinning objects, like how some glass artists work:

I think the jets above and below the black hole consist of particles originating from the disk, that have made a hyperbolic orbit around the black hole.

 

[Zatnikitelman]

Well the sun has spherical gravity, but the planets' orbits lie within a handful of degrees of each other and all in the same direction.

 

[Andy44]

Yes, because the sun and the planets all came from the same swirling cloud of dust.

And a black hole used to be a rotating star with its own planetary system, so it still has angular momentum.

But I think cjp's explanation makes some sense, too.

 

[Linguofreak]

Right, I was reading a book the other day which included a section on black holes - I'm fine with the theory behind them, but just curious about one aspect:

Why, does matter get pulled into them in a spiral pattern on a flat plane? Surely the gravitational pull will come from all directions hence matter will too?


Any ideas?

Thanks in advance!

You mean the accretion disk?

This is not just a feature of black holes. Young stars have it too. It's more a matter of interactions between particles of infalling matter than it is the nature of the central body.

Take the example of a young star. It starts out as a huge cloud of gas, that is, for the most part, non rotating. The molecules of gas each have their own orbit around the center of mass of the cloud, and when they hit other molecules, that orbit changes. If two molecules are going in the same direction, their orbits don't change much. If they are going in opposite directions, the orbits change a fair bit, and they are likely to have a fair amount of the kinetic energy from their relative velocities converted into internal vibrations, or shed away as electromagnetic radiation. The effect is the same as both molecules making a strong retrograde burn: They lose orbital energy and start moving towards the center of the cloud.

Now, we've said that the cloud is pretty much non-rotating, but between the randomness of the velocities of the molecules in the cloud and external forces, it will almost certainly have a very small bit of angular momentum. As head-on collisions happen in the cloud, molecules move further in towards the center, and the cloud contracts. In areas where lots of head-on or near head-on collisions happen, more particles move inwards, and in areas where few head-on collisions happen (ie, all the particles are going in the same direction), you don't see as much of a collapse inwards. So the cloud gets flattened into a disk shape, with the poles squished in and the equator bulging out.

With black holes it happens a bit differently, not because of the nature of the black hole, but because of its environment: Instead of a cloud of material falling in on itself, we have another star that is losing mass to the black hole: As a result all the material is coming towards the black hole from one direction, and the fact that the star and the black hole are orbiting each other provides the angular momentum. But with either the protostar or the black hole, the formation of an accretion disk is a matter of centrifugal force and angular momentum in the infalling matter, not a result of any property of the central object itself.

 

[JamesG]

What others said. Our imagies of black holes comes from artists who generally depict the most dramatic part of thier exisitance, when they are "feeding" off another star. But if you had a rogue black hole blunder into say a large nebula it would create a big mess of gases and partilcles infalling and orbiting at random angles. A lot of the matter would be flung off away from it, and its speculated that such gravitational disruptions are what stimulates stellar creation.

I think "Brownian motion" is less the cause of solar rotation/orientation that the coriolis effect from galactic orbital rotation of the nebula. I would bet that most stars in the galaxy rotate in the same direction because of it.

 

[Salamander]

What others said. Our imagies of black holes comes from artists who generally depict the most dramatic part of thier exisitance, when they are "feeding" off another star. But if you had a rogue black hole blunder into say a large nebula it would create a big mess of gases and partilcles infalling and orbiting at random angles. A lot of the matter would be flung off away from it, and its speculated that such gravitational disruptions are what stimulates stellar creation.

I think "Brownian motion" is less the cause of solar rotation/orientation that the coriolis effect from galactic orbital rotation of the nebula. I would bet that most stars in the galaxy rotate in the same direction because of it.

Well, first test: coincides the ecliptic pane with the galactic orbit pane?

Second test: double stars. Double stars that are close enough to have visible orbits should be easy to check out. For longer distances, it would mean variable stars caused by obstruction would have to be more common along the galactic orbital pane than elsewhere.

 

[RisingFury]

Take the example of a young star. It starts out as a huge cloud of gas, that is, for the most part, non rotating.

I read the rest, but I'm gonna cut you off right here.

Every cloud does have a small angular velocity, but because it's usually lightyears across, that comes out to a huge deal of angular momentum.

The mechanism for contracting clouds is not individual particle interactions, but gravity. The cloudcollapses under it's own weight. The same still applies with stars (and gas planets - Jupiter gives off twice as much energy as it recieves... guess where it comes from :thumbup. The reason stars don't collapse, is because they're held back by another force, in this case radiation pressures. Same is not true for a nebula until it contracts so much, that it heats up and stars are born.


I meantioned that a nebula has a great deal of angular momentum... in fact, it has so much of it, that if only one star was born from it, it would have to spin so fast it would tare itself apart. The solution to the problem is that many stars are born and stars form their planets, which also take the angular momentum away (Jupiter has around 99% of angular momentum in our solar system).


Going back to your question, as I understand it: "Why does the stuff form a disk and not a rotating ball of gas?"

Well... this isn't a quick answer. There are many ways an accretion disk forms. Some of them form around dying stars... white dwarwes, neutron stars, black holes... the mechanism here is obvious: These stars didn't have a rotating ball of gas around them. The only answer is a disk.

"But what about accretion disks around proto stars, born inside of a gas cloud?"

I honestly don't know the answer to this one and will ask my professor, but if I had to speculate, it would come down to particle interactions. Collisions between particles would probably force the ball to collapse into a disk.

"So, why does the disk form with an inclination of 0°, relative to the star's rotation?"

My guess would be that the disk formed before the star, thus the star was bound to spin the same way...




It's an interesting question though and I'll ask my prof if there's a better explanation for that.

 

[JamesG]

Well, first test: coincides the ecliptic pane with the galactic orbit pane?

Solar systems don't have to and most likely won't line up with the ecliptic plane due to thier nursery nebula's prevailing angular momentum and their own motion within the galactic arm long after they've formed.

Coriolis would only control the direction in which they rotate, clockwise or anti-clockwise.

I briefly tried to google up the stellar data on a couple of known binary systems and the direction of orbit is never implicitly stated. hummmm...

 

[Loru]

Formation of accretion disc is quite complex process.

Main cause of forming disc is combination of energy and momentum transfer from inner to outer sections of cloud, frame dragging effect, centrifugal force and turbulences in cloud of gas to state only basics.

Maybe you can start here: Accretion discs

 

[RisingFury]

You will have more luck with the reference links on the wiki page, then the wiki page itself. Just be prepared for more reading...

 

[Salamander]

Solar systems don't have to and most likely won't line up with the ecliptic plane due to thier nursery nebula's prevailing angular momentum and their own motion within the galactic arm long after they've formed.

Coriolis would only control the direction in which they rotate, clockwise or anti-clockwise.

I briefly tried to google up the stellar data on a couple of known binary systems and the direction of orbit is never implicitly stated. hummmm...

Cariolis forces, if they have any effect at all, would force the spin axis of a solar system's ecliptic to be roughly paralell to the spin axis of the rotation inducing them. Other effects might induce some sort of precession though.

 

[JamesG]

sigh...

http://www.google.com/search?hl=en&q=Coriolis+effect&aq=f&aql=&aqi=g10&oq=

 

[Linguofreak]

I read the rest, but I'm gonna cut you off right here.

Every cloud does have a small angular velocity, but because it's usually lightyears across, that comes out to a huge deal of angular momentum.

The mechanism for contracting clouds is not individual particle interactions, but gravity. The cloudcollapses under it's own weight.

To be pedantic, yes. But for the cloud to contract beyond a certain point, the particles have to lose orbital energy around the center of the cloud. That energy is shed by collisions, and once it is shed, gravity can draw the cloud further inwards. Particles towards the rotational poles of the cloud, however, lose energy faster than those towards the "equator", because collisions tend to bring their velocities closer to zero, whereas collisions along the equator tend to bring velocities closer to the average rotational velocity.