Black Holes aren't so Black

Who wouldn't be amazed by an object with such a high mass density that everything nearby, even light, is sucked in, never to escape? Black holes are one of the most fascinating astrophysical objects we've discovered. They've played a role in countless science fiction shows and books. People can't help but wonder what happens inside.

Even physicists are mystified. Well, I'm sure some of them would deny it, but they're only lying to themselves. The laws we think we know say that everything inside is compressed to a point, or as we call it a singularity. Of course, from our daily experience we're all pretty sure that such a thing isn't possible. What does that mean? New laws of physics! There are theories out there with cool names like quantum gravity to try to figure out what actually happens at the center of a black hole. We may never know the truth, but it's fun to think about (at least for a bit—eventually everyone needs a break to put their overblown minds back together).

I'm not even going to attempt to go into all of the physics of black holes here, but I wanted to point out something which, in my opinion, makes black holes even cooler. To get to that we all must first admit that, once we're past the mystery, black holes don't really look like much. I mean, they are supposedly totally black. Just picture it:

Wow?

You might think you're staring straight at a black hole when an astronomer taps you on the shoulder and asks, “Why are you looking at that empty patch of space?”

Ever snappy on your feet, you would say, “Uh...I was just pondering electro-quantum-chromo, um, light-particle space physics. Um, Yeah!” Mm hmm, very convincing.

Evidence (and theory) now suggests that black holes may, in some cases, put out quite a bit of light. We still won't see any light emitted from within the Schwarzschild radius, true, but outside the black hole a lot can be going on. First, there can be accretion which is just a fancy word for stuff falling into the black hole. Since it is a massive object, matter in space is drawn toward it. I also want to point out that black holes are spinning (just like Earth). Through some fancy physics, it turns out that this spin preferentially pulls all that loose stuff into a disk around the black hole. There that stuff bumps around and gives off a ton of light, at least until it's finally sucked beyond the horizon.

Second, not all of this stuff is actually pulled into the black hole. Some of it is accelerated to relativistic speeds (which just means it's going close to the speed of light—super fast). This relativistic matter is expelled in two enormous relativistic jets, shooting out in opposite directions (perpendicular to the disk for those who care). These jets emit very high energy light, including X-rays, which we've actually seen with a special telescope in space, the Chandra satellite.

My super-crude picture:

Now, this can only happen if there is a lot of matter near the black hole, so many aren't so exciting. However, in the center of a galaxy there tends to be a lot of stuff close together, often orbiting a supermassive black hole. When these gather disks and shoot out their particle jets, we call them active galactic nuclei (AGN).

So next time you think of a black hole, picture one of these—it's a lot more fun than staring at empty space!