Black holes are what happens when the universe divides by zero and eats anything that tries to notice. They're cosmological grizzly bears: an inevitable result of nature that is majestic and terrifying to every species intelligent enough to comprehend them.
Quick! Choke it with your Nobel Prize!
Black holes happen when reality has an overflow error: you put too much stuff in one place, and it breaks both the stuff and the place with gravity. Gravity is usually the responsible older sibling of the universe, always pulling things together. Black holes are where gravity goes full Al Capone, calls a meeting of all the fundamental forces of existence and makes a big showy spectacle of crushing them. It doesn't just crush matter; it crushes the quantum laws that define matter, stomps them all into a compacted nugget until matter stops existing so much. It simply overrides reality.
Electromagnetism just cried "HAX THIS IS BULLSHIT."
And yet, most people treat them like cosmic vacuum cleaners. Science fiction characters are worse at understanding black holes than they are at aiming laser weapons, and the coverage they get in most schools only encourage students to kill themselves with trampolines and bowling balls. That's a shame, since black holes are literally the ultimate everything, so we're looking at how cool they are.
#6. They're the Brightest Things in the Sky
"Black hole" is as simple and descriptive a title as "Pied Piper of Hamelin," and equally misleading. The one thing everybody knows about black holes is that not even light can escape, meaning they're pictured as the interstellar equivalent of open manhole covers: pitch-black doom awaiting the unwary. But black holes are often the brightest points in the sky.
A black hole.
What people forget is that while there is an "event horizon" boundary inside of which light can't escape, there's also an "entire rest of the universe" where it can, often in galaxy-blinding quantities. When a rotating black hole consumes a cloud of interstellar gas, the material is drawn into a spiral, like fluid swirling down the plughole of existence ... which is actually what's cosmologically happening.
NASA, ESA, the Hubble Heritage Team (STScI/AURA), and R. Gendler (for the Hubble Heritage Team) Acknowledgment: J. GaBany
The hair around the universe's plughole is ENTIRE GALAXIES.
Meteors light up because a thin layer of gas is being compressed by plummeting space rock and further heated by friction. When gas clouds fall into a black hole, the whole thing is being compressed, plummeting, and being heated by friction. The consumed cloud is its own meteor and atmosphere, and both are burning with cosmic fire. They get so hot, they don't just glow white, they glow X-ray, converting 10 percent of their total mass into pure energy. For comparison, fusion warheads only convert 0.5 percent of their mass into energy. Understand: Black holes create a place where dropping something releases 20 times more energy than thermonuclear detonation. And our galaxy's central black hole, Sagittarius A*, will be doing that this year.
Black holes can glow so brightly that they defeat their own gravity. Supermassive black holes can reach the Eddington limit, where continuum radiation force defeats the otherwise irresistible gravitational attraction. (That sentence contains more band names and anime series subtitles than anything else I've ever written.) The radiation becomes so intense that it blows away the incoming material. And this isn't radiation as in "nuclear"; this is radiation as in "light." As in "move toward the light, except in the real heavens, the light can be so intense that it shoves you back."
#5. Galactic Explosions (in Our Galaxy)
Black holes create the most powerful gravitational accelerations in existence, and they're also the heaviest things in existence. Dropping one into another is the sort of thing you do when you're trying to crash God's computer, and we think it's already happened, creating the biggest explosion in the galaxy.
Yes, Michael Bay, there is a heaven.
Those are two cosmic explosions, vast expanding bubbles of high-energy particles ballooning out through space. How large are they? That thin smear of dirt across the middle is our galaxy. Those bubbles are each 25,000 light-years across. Here it is again with scales.
Ladies and gentlemen, we have located Thor's balls.
These bubbles cover half the visible sky, day or night, and have probably been expanding for millions of years. We've always known that humanity is but a blip in the universe. But now we know that we're only a blip in one of the universe's special effects.
The images are rendered in false colors, because gamma rays are to visible light what the Hulk is to arm-wrestling competitions. Scientists recently theorized that this cosmic kaboom was caused by a dwarf galaxy dive-bombing our Milky Way, like an intergalactic Gimli waving a singularity instead of an ax. At that point, its central black hole played Katamari to the death with ours and lost. (Oh yeah, pretty much all galaxies are being eaten by a central black hole forever.) The incoming black hole would have spiraled through our galaxy, making a bull in a china shop look like Jackie Chan in a china shop, before being eaten in the highest stakes sumo match possible. The team is now hunting for the stars this gravitational death match probably flung out of the galactic core, because when you do forensics on a black hole murder, you look for stars instead of semen.
#4. There Are At Least Millions of Them
The bright black holes above are also known as active galactic nuclei or quasars. They aren't just the brightest objects in their home galaxies -- they can be thousands of times brighter than everything else in their galaxies put together, by both eating and becoming everything else in their galaxies put together. They're so bright that, despite being halfway across existence, we used to think they were nearby stars. They're lights so bright, God would say, "Woah, I didn't mean that much."
And on the eighth day, God created the best and most underrated Star Trek.
For small black holes, the mass supply can be the companion star, locked in the most destructive relationship possible.
"It just makes sense that we move in together until I find a job and finish eating you."
Lighthouses tell you where you are and why you should stay away from them. That has never been more true. These luminous black holes concentrate huge energy outputs in small sections of sky, making them extremely useful in building maps of the universe. People forget that building a 3D map of the universe is an act of unparalleled genius. The sky looks like someone sneezed on a monitor, and without seriously impressive science, that's about as good a map as you get. People studied the stars for millennia, and the best most could come up with was a family of pervert gods who pretended to be adulterous swans, or the best time for Virgos to meet a man.
Hint: not during swan mating season.
We can't move a bit to the side to see what's there because we don't have a starship Enterprise yet. But the emissions from black holes are accurate markers in space, useful for making maps. Viewed as the Big Bad Boss of astrophysics, people imagine that a black hole is fairly rare. One: Almost every galaxy has a monster black hole at its center. 2.5 million: the number of actively feeding supermassive black holes the WISE survey found. They have us surrounded. And just behold this glory.
Not pictured: NOTHING. EVER.
LOOK AT IT! You see those black empty black bits in the upper right?
WRONG! Every pixel of this image contains millions of times more stuff than our entire planet.
If people understood how amazing our maps of the universe are, the only reality shows would be people talking about how awesome reality is, thereby creating an opposite universe where "science" channels hosting reality shows wasn't a blatant contradiction.
We're going to need the word "discovery" back now. Your new name is the Don't Watch This Channel.