And also, stuff falls into them, just like stuff falls into our atmosphere and creates shooting stars.
|So much birthing.|
Supermassive black holes [the ones that are at the centers of (most) galaxies], on the other hand, can be millions-billions of times the mass of the sun and are thought to have formed when many ~stellar-mass black holes coalesced; a stellar-mass black hole accreted tons of other material; an unstable, heavy kind of star that only existed earlier in the universe collapsed into a crazy-large black hole that accreted tons of material and/or coalesced with other black holes. As you can see, the theory is still a bit up in the air, and likely involves multiple explanations.
However, back to business.
Spacetime: it's what the universe is built on. But it's not a firm foundation. It is bent and warped by mass. The more massive something is, the more it affects spacetime, and the bending of spacetime is what causes a little thing I like to call "orbit."
Not to use a tired analogy, but I'm going to use a tired analogy, slightly modified, because it's tired from being the best. If you take bowling ball and set it on a trampoline, the trampoline dips toward the ground, forming a little depression around the ball. Other objects that encounter the depressed bowling ball will obviously be affected by the effect it has on the trampoline.
For instance, if you took a marble and sent it slowly toward the outer edge of the depression, it would probably fall in and hit the bowling ball. However, if you gave it some oomph (read: velocity), its path would curve as it encountered the depression, but it would probably escape and continue in its new direction. Somewhere in the middle, the marble might circle around (orbit!) the bowling ball along the curve of the trampoline. So the faster something goes, the less likely it is fall into the depression and collide with the bowling ball.
However, imagine that the bowling ball was so massive that it caused the trampoline to bend all the way to the ground (or, more accurately, to the center of the earth, or down forever). If your marble encountered the the depression, it would fall toward the bowling ball pretty much no matter how fast you'd flicked it. The velocity an object must have to escape the gravity of another object is, sensibly, the escape velocity.
|Relative spacetime / trampoline curvatures.|
But! If you flicked it a few inches away from the edge, it would continue on its regular path, unaffected. The marble is only affected by the bowling ball if it gets close enough. Otherwise, the bowling ball might as well not even be there.
The same is true of black holes. Every black hole has a different Schwarschild radius. This is the distance from the "center" of the black hole at which the escape velocity is the speed of light. This means that even photons traveling at the cosmic speed limit can't emerge from their encounter with the black hole. The circle that describes the Schwarschild radius is called the event horizon, because information about anything that happens inside that radius cannot be known by someone outside the radius (because information can only travel as fast as light and thus will not be able to escape).
Slightly outside the Schwarschild radius, the escape velocity is slightly lower; far outside that radius, the escape velocity is the speed at which you power-walk. You suck.
Objects far enough outside the event horizon, or closer to the event horizon and with larger velocities, orbit black holes, spinning around the most powerful nothing around.
We also do see light from them--it's just not optical light. We see X-rays, gamma rays, radio waves, etc. These are all light--they're all photons, just at different energies (/wavelengths/frequencies). So how do they escape the black hole? They're not going any faster than optical light. Them's the rules.
|The rainbow is so small.|
Credit: UC Regents.
However, stuff doesn't just fall straight into a black hole. It spirals inward, forming an accretion disk, or a disk of swirling material. As this material swirls closer and closer to the event horizon, it moves faster and faster (accelerating) and is also subject to high amounts of friction, since it's accelerating right alongside a bunch of other particles. Imagine sudsy water going down your bathroom drain.
But, unlike your dirty water, this accretion disk's friction causes the material to get so hot that it emits high-energy photons, like X-rays (in astronomy, the hotter something gets, the more energetic the photons it can emit).
Black holes also have jets of material that spew out from above and below the black hole itself, at speeds approaching the speed of light. While the machinations of this phenomenon are still a matter of debate, the general consensus is that magnetic field interactions within the accretion disk cause material to become "collimated", like this:
|Look how much bigger the jet is than the galaxy.|
And think about how large a galaxy is.
Recently, astronomers caught a star in the process of being eaten by a black hole. The event was seen in X-rays, gamma rays, and radio waves, and made it into Nature as two separate papers and a letter in the same issue. Now that's massive.
Lazzati, D. (2011). Astrophysics: The awakening of a cosmic monster Nature, 476 (7361), 405-406 DOI: 10.1038/476405a
Burrows, D., et al. (2011). Relativistic jet activity from the tidal disruption of a star by a massive black hole Nature, 476 (7361), 421-424 DOI: 10.1038/nature10374
Zauderer, B., et al. (2011). Birth of a relativistic outflow in the unusual γ-ray transient Swift J164449.3+573451 Nature, 476 (7361), 425-428 DOI: 10.1038/nature10366