Why do black holes have rings around them, rather than spheres?


I get the explanation that they have gravity and things are attracted, but why is it in a ring rather than a sphere? why do things from one AXIS get attracted but not from other?

In: 49

That’s not how it works. The light in the ring is only visible from one place, where we are. In other locations you’d have to observe the BH at other times when it was lensing light from a different star. Things are attracted (space is curved) in all directions, but we only care about light we can observe from here.

Do you mean the ring of light/dark or the accretion disk?

The ring of light/dark happens because the hole itself absorbs light but around it light is lensed. This means that whatever is behind it, the light from that gets bent around the hole and you see it on all sides. This produces a ring, or usually several rings, around it. [Example image.](http://www.nasa.gov/sites/default/files/thumbnails/image/simulated_bh.jpg) Because it is an optical phenomenon, the rings appear to move as the observer does, so it always appears as a perfect ring.

As for the accretion disk, consider how orbits work. Matter always orbits in a flat plane. Around a bhole, there’s a bunch of matter. At first, it’s moving every which way. All of those orbits produce a sphere. However, all of those orbits must cross eachother to make a sphere, so they eventually collide.

All of these collisions serve to average out the velocity of the matter until it ‘agrees on’ a direction of spin. Once it is all spinning the same direction, it must all orbit on the same plane, and so you’re left with a disk.

Centrifigul force.

Hold a piece of string in your hand and start spinning your hand around. Does it move in a sphere around your hand or does it spin in a circle.

Or think of a playground merry-go-round.. when it spins the kids on the ride get pushed outward from the center of rotation the faster it goes.

Astronomical bodies like planets, stars, black holes, etc are usually spinning (for various reasons). Some spin fairly quickly and drag the debris in space around them like a merry go round or your hand spinning a string… the debris tends to flatten out and collect like a ring around the object. Like Saturn has rings of rocks and ice.

While it is very possible for objects to spin around an orbit that is not on the central axis of the main body… it is less likely. Pluto is slightly off axis. On Earth.. many satellites are artificially orbiting off axis.

Please check this video: [https://www.youtube.com/watch?v=Q1bSDnuIPbo](https://www.youtube.com/watch?v=Q1bSDnuIPbo) . At around 10:40 it starts explaining why it looks like this. This is combination of fact that all matter will tend to fall onto black hole in circular pattern, and rest is gravity so strong, that you can see back of black hole from even when looking directly from the front.

There’s only a couple of comments so far that actually attempt to answer your question (not only have some people answered the wrong question, they even got the answer to the wrong question wrong), so I’ll weigh in with a short answer:

Angular momentum.

When you’ve lots of stuff spinning around in a big cloud, you can look at the momentum of each particle and sum it all up, you end up with a direction for this total angular momentum. As particles in the cloud collide, their velocities perpendicular to this plane of total angular momentum tend to get cancelled out. The total angular momentum must be conserved though, and so you end up with a spinning disk.

This is the same reason why the planets are roughly on the same plane, why planetary disks (eg Jupiter’s rings) are a ring, and why galaxies tend to be flat. https://youtu.be/tmNXKqeUtJM

It’s complicated. But Veritasium did a really good video a couple days ago explaining it: [https://www.youtube.com/watch?v=Q1bSDnuIPbo](https://www.youtube.com/watch?v=Q1bSDnuIPbo)

I won’t attempt to explain it fully since I’m sure I’ll screw it up, but basically it has to do with the way the black hole captures the light and how some light escapes in the direction you’re looking.

Every orbit is centered around the center of mass of the system. So you can’t have more than one orbital plane without them crossing each other. So when you have a bunch of material swirling in from various angles, it all collides and averages out into an orbital plane that is the average of everything falling in, and you end up with an accretion disk.

Because Black Holes spin.

In a reverse method, take a koosh ball, soak it in water, and then spin it really fast… You notice that the water droplets flying outwards dont fly outwards in sphere, but as a ring, perpendicular to the axis of spin.

Same concept, but flowing inwards, not outwards.

If something falls towards the black hole, chances are, it will not hit the black hole directly. It will likely miss, because it has some sideways velocity to begin with, and gravity alone will not counteract it. Basically, the object has an initial angular momentum around the black hole, and angular momentum is conserved.

So what happens to all the stuff that fall towards the black hole and miss? They collide with each other. This tends to slow down the object and capture it in an orbit around the black hole. Now you have all this stuff orbiting the black hole. But they are all in random directions, so they still collide with each other. Eventually they shave off velocity from each other and coalesce.

But, all the angular momentum from all the falling stuff is still conserved. If they average out precisely, then they cancel out completely. But chances are, they add up to a non-zero value. Which means a flat disk.

Usually debris around any large body (not just black holes) does start out going every which way, but eventually it settles into a ring.

Why? Simple, because objects not traveling parallel to each other are more likely to collide. When they hit another piece of debris, their velocities will average out, causing the particle to either slow down enough to de-orbit, change direction, or if one piece is significantly larger than the other it will just change the direction of the smaller debris to match it’s own.

Do this billions of times over the course of billions of years and eventually everything that’s left is going the same direction at roughly the same speed. Also the pull of gravity due to the rotation of the body can have an effect, speeding up a certain plane enough that it helps that direction dominate.

You will notice that all bodies in space tend to form ring systems rather than spheres or clouds. For instance planets with rings or moons, solar systems with orbital planes of planets, and galaxies with galactic disks.

The truth is that when the system first forms, you do get a disorderly cloud or sphere orbiting the central mass. The thing is, this is an unstable configuration, so as the galaxy, solar system, planet, or black hole ages, it naturally settles into a more stable ring configuration.

The reason for the instability is collisions. If you have bodies orbiting in all orientations, then collisions will happen frequently between bodies in differently oriented orbits. If you have only a single orbital plane, then you can have lots of bodies orbiting in that plane without colliding. So in the early phase when you have the sphere, most of the bodies will collide and either fall into the gravity well, bounce out of orbit completely, or join into a common orbital plane. Eventually only objects in the single orbital plane are left.