Black Holes

Jillian's Guide to Black Holes: Forming - Types - Outside - Inside - Finding - References - Websites

The funny thing about black holes is that you don't directly peer at the event horizon. There's usually all this stuff surrounding 'em. Where does that stuff come from?

So, my neighbor turned into a black hole and wants to borrow my lawn mower?

Stars can form in binary systems, where the two stars orbit around each other. If these are massive stars, there's a chance that one (or both) could leave a black hole when they die. This leaves a binary with a black hole and a massive star. The black hole can tidally distort the star to the point where it can steal material from the star's surface. This stolen gas falls toward the black hole, forming an incredibly hot disk of fast-moving, swirling plasma around the black hole.

I've drawn what this might look like, but the accretion disk should really be extremely bright and very white instead of shading from reddish to blueish. I colored the shading to give you an idea tha the temperature of the gas in the disk increases as it "falls" to orbit closer to the black hole.

Accretion disks

Why should it go so fast, and why shouldn't it just tumble in instead of forming yon great mucking disk? There's this thing in physics called gravitational potential energy. The higher up in a gravitational field, the more gravitational potential energy something has. Take a bit of gas way above a black hole. It's got a lot of potential gravitational energy because it is very far from the black hole. Say that the piece of gas starts to fall toward the black hole. It is now lower in a gravitational field, so it must have less gravitational potential energy.

Where did that energy go? Well, some of the energy changed into kinetic energy (energy of motion), and the bit of gas sped up, which means it follows some kind of shrinking, erratic (possibly precessing), elliptical orbit. Some of the energy also went into heat. Hot objects radiate light, from a hot stove emitting infrared to a light bulb filament emitting visible light. The friction between the bit of gas and the other bits of gas around the black hole translates into more heat and more light being emitted. After a while the bit of gas starts to visibly glow. It falls some more, and soon it gets so hot that it emits X-rays. Astronomers have telescopes that scan the sky for X-ray sources and have found quite a few black holes that way.

So, that's why it goes so fast. But, what about the disk shape? Wouldn't it make sense for stuff to form a sphere around the black hole? There's one main reason why it forms a disk, and a secondary reason closer in to the black hole.

The main reason that disk forms is because of angular momentum. Those are big words to throw around, so, instead, think "things that are spinning like to stay spinning". Material falling onto a black hole has some angular momentum (it's really tough not to have any), so it won't head straight for the event horizon. Instead, it orbits around the black hole. As it orbits, the material interacts with itself, bumping and jostling like a large crowd of people. This jostling redistributes the angular momentum, so suddenly some of the material is spinning more and some less. The material that gained momentum moves into a wider orbit, while the losers fall into small orbits. This spreads the material into a disk.

What's the second reason close in? Remember when I talked about rotating black holes and mentioned the ergosphere? Material that gets close enough to the black hole to enter the ergosphere finds itself moving with the "rotation" of the black hole. The ergosphere billows out from the outer event horizon, and it billows the most at the equator and the least at the poles. Material at the equator (that is, in the accretion disk) will get an additional spinning boost and keep from falling into the black hole longer. Material at the poles wouldn't get this boost.

What are those jets coming out of the disk?

A false-color image of the twin radio jets from the black hole X-ray binary SS 433. Image credit Blundell & Bowler, NRAO/AUI/NSF.

There is a curious property of accretion disk of all sizes, from proto-stellar to X-ray binary to supermassive black hole sizes, where somehow the inner part of the accretion disk launches a jet of material perpendicular to the disk. Only the inner part of the disk? Yeah, like very near the (for instance) black hole. Astronomers know that it's not the whole disk because they measure how quickly the jet flickers: the rate of flickering gives a clue about the size of the region causing the light. A large region, like the whole accretion disk, has a maximum flickering rate, otherwise to make it flicker any faster you would have to have parts of the outer accretion disk moving faster than the speed of light. The variability of the jet tells us that the part of the accretion disk that makes it is very small, indeed.

How does a disk of swirling, jostling, super-hot plasma launch a tight pencil beam jet of particles? Good question. Astronomers know it involves how magnetic fields are generated and wrapped up in the accretion disk, but the precise details are still being worked out. Moving groups of charged particles can create magnetic fields, and those magnetic fields would be dragged along with the plasma like a straw in a fast-moving river. Jostling magnetic fields, however, is different from jostling charged particles because squishing magnetic field lines releases a ton of energy. Somehow, this process creates an overall powerful field at the center of the accretion disk, pointing straight out of the disk.

The material jostling and swirling around the accretion disk is very hot, as I mentioned. It is so hot that electrons are knocked off atoms, leaving a soup of charged particles called a plasma. Charged particles also like to swirl around magnetic field lines like a bead on a string. Somehow (again, that word!), particles are launched from the inner part of the accretion disk to shoot out in a tight beam. Astronomers see the light emitted by the particles in the jet, such as in the twin precessing jets of the black hole X-ray binary SS 433 (to the right). This is a false-color image taken in radio light by the VLA. The binary itself is too small to see in the image, but it's at the center of the two jets in that reddish area.

Just like how each element emits a certain pattern of light, allowing astronomers to identify them, jets emit a certain pattern of light. Jet emission has a variety of names, so you might see it called "continuum" emission, meaning it's not spikey emission and absorption lines, "non-thermal" or "synchrotron" emission, which refers to the hot fast-moving gas that causes this light. Whatever name you give it, this kind of light is only caused by very hot gas or plasma.

Okay, a jet of particles; that's cool. Wait --- I haven't told you exactly how powerful these jets are, and I assure you that it is AWESOME! Accretion disk jets can launch material at near-light speeds! For instance, SS 433's jets move at 24~28% the speed of light. The fastest jet that has ever been observed is 99.98% the speed of light, and this was observed in the X-ray binary Cir X-1. Bizarrely, this system doesn't have a black hole at the center --- it's got a neutron star (which is slightly less dense and lacks an event horizon). It's unknown why a less massive, less dense object would have a disk that launches a faster jet. As I said, astronomers are still working out the details of how disks launch jets.

The energy from the accretion disk and the twin jets aren't the only thing a black hole radiates. There's also ...

Hawking radiation

You can make a black hole out of anything, really, it would just take tremendous pressure. That's why the sun could never become one---it couldn't compress past the pressure of the electrons not wanting to be close to one another and then past the pressure of the neutrons not wanting to decompose. However, if you could compress the sun that much (IF!), it would shrink down to its Schwarzschild radius and become a small black hole. Small black holes such as this theoretical one have very strong tidal forces, recall. Famed astrophysicist Stephen Hawking knew these facts and more. He knew that space wasn't really empty: it was full of virtual particle and antiparticle pairs. He realized that the tidal forces of small black holes were strong enough to tear apart the pair, producing enough energy to make the virtual particle materialize in real space. According to this theory, black holes give off Hawking radiation of particles and antiparticles. What happens when the particles "form" inside the event horizon, aren't they trapped there? Well, not really. Due to quantum mechanics there is a certain probability that the particles can tunnel through to the outside universe.

Since black holes radiate particles, physicists can give 'em a temperature (albeit a very small one). This doesn't really matter for the large black holes (those with a mass of the Earth2), for they do not radiate many particles and therefore have a temperature that is so frustratingly close to absolute zero that it's inconsequential3. Small black holes radiate more particles by comparison. Should Mercury (the planet, not the element and not Sailor Mercury) *pwoof* into a black hole, it would have a temperature of about 10 Kelvin.

Hawking radiation is the key to the black hole's mortality. As a black hole radiates particles and antiparticles, it loses mass. Eh? Well, the energy it took to materialize the virtual particle come from somewhere. The smaller the black hole gets, the more it radiates. The more it radiates, the smaller it gets. People like to call this process evaporation. Eventually, it will get so small that it releases massive amounts of energy and loses mass at an incredible rate. In the last second of the black hole's existence, it releases the same energy as billion one megaton hydrogen bombs would2. After this I've been told that either the black hole ceases to exist or it leaves a Planck particle, something with Planck mass (1019 g) and Plack size (1.62 × 10−35 m) --- very small and very massive.

Energy from black holes

Don't laugh, it works much better than anything we've got now! Okay, there are two ways to generate energy with a black hole: dumping stuff into them and throwing stuff near them. Actually, you can only do the second with a rotating black hole; the first is applicable to any type of black hole. I consider dumping stuff into a black hole dangerous, for it only makes the black hole larger. Eventually, you would have to move your dumping station, or it would be swallowed by the black hole.

I should explain how the system works before I criticize it, I imagine. I won't go into painful detail about the workings of relativity, so you're going to have to accept my word on this (or go read up on the topic yourself to see if I'm right!). The formula for the energy released by an object that falls into a black hole is as follows:

Ereleased = (mass)*(speed of light)2*[1 - (1 - Rs/radius)1/2]

It looks complicated enough, but it's kinda simple. I'm sure you've heard Einstein's famous equation E = mc2. It just means that the energy of anything is just the mass times the square of the speed of light. The speed of light is a rather large number, 299,792,458 m/s (670,616,629 mph). That means there's a lot of energy locked up in matter. Anyway, the energy unlocked by that matter as it falls in the black hole is mc2 times a certain factor describing how close to the black hole the matter is.

It has to do with gravitational energy, which I explained when I was describing an accretion disk. So, the closer an object gets to a black hole, the more energy it releases. How much? Well, it can't release it all. How much energy it releases depends on the Rs/radius factor. When the object is far away from the black hole, that factor is less than one. That means the energy released is only a certain percentage of mc2. The closer an object gets to the black hole, the closer it gets to 100%.

Very nice, very mathematical, but how does that generate energy?? Imagine a really big conveyor belt attached at one end to a generator station (a station with big ol' rockets to keep it from moving closer to the black hole). At the other end the conveyor belt goes real close to the black hole (how close depends on how much energy you want to generate). Attached to this belt is an electric motor. What will we be dumping into the black hole? Trash. Nuclear waste. Those little tabs from aluminum soda cans. It doesn't matter, we just dump what we want to get rid of. Each time some trash falls into the black hole, the conveyor belt gives a mighty jerk that turns the generator and generates a staggering amount of energy. This process is much more efficient than using nuclear fission or even fusion and utterly dwarfs the energy generated by using fossil fuels. Very clean, very safe energy.

What if we don't want to make a black hole bigger by throwing stuff in it? Easy enough, that's the second way to get energy from a black hole. Imagine a rotating black hole. As I explained up above, there's this area outside of the event horizon called the ergosphere. Stuff in the ergosphere has to move with the rotation of the black hole. If you put a delicate glass statue in the ergosphere, it would start moving. Perchance, since it is glass after all, the statue breaks in half. Half falls closer towards the black hole and half flies out of the ergosphere with a more energy than when it came in. Where did the extra energy come from? What really happened was that the part that flew out stole rotational energy, kinda like kinetic energy but a little more complicated. That made the black hole spin a little slower, though. At least the event horizon didn't get any bigger.

You don't have to do this process with matter; it also works for light. Throw a light ray in the ergosphere with a certain energy (say, a visible light ray), and it comes out of the ergosphere with more energy (say, an X-ray ray). The extra energy came from the black hole --- it rotated a little slower after this energy-theft, but it didn't slow down as much as it would have with the glass statue. At the price of a little rotational energy, you can amplify light rays. This process is called superradiance.


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