Ever hear the one "Black holes are out of sight"? Or how about, "Two protons walk into a black hole" (end of joke)? How about the definition, "Black holes are what you get in black socks"?
All joking aside (though I may not hold myself to that), the black hole has been deeply entrenched in human imagination as well as popular culture for about as long as it has been an idea, theory, or studied object in science. The first scientific ideas on regions of gravity so strong that light cannot escape were kicked about in the 18th Century. In 1916, Karl Schwarzschild brought the idea into the realm of mathematics, and almost 50 years after that the first observational evidence for the existence of a black hole was discovered.
In 1964, a rocket-born probe peaking at space from just outside of Earth's atmosphere detected intense X-ray emissions coming from a spot in the constellation Cygnus. Designated Cygnus X-1, this location earned the distinction of being the first probable detection a black hole. The X-rays were explained as coming from the gases of a nearby companion star being pulled off and swallowed up, heating up to a searing X-ray glow before disappearing into the black hole.
This indirect detection of the black hole's theorized presence set the character for future detections of black holes. Since light cannot escape from such a beast, we cannot see the black hole directly but must infer its presence through the effects on its surroundings.
Someone once likened a black hole to the Cheshire Cat from Alice's Adventures in Wonderland: the original star has vanished from sight, and all that is left is its grinning gravity.
As the theory went, a black hole like Cygnus X-1 is formed when a massive star runs out of nuclear fuel, the core collapsing under its own powerful gravity and the outer shell blown away as a supernova explosion. The collapsing core crushes itself to a mere point in space that contains all of its mass—the ultimate in trash compactors. Infinitely dense and with such intense gravity that nothing, not even light, can escape from the region surrounding it, the "singularity" formed by the collapse becomes one of the most mind-warping and thought-twisting things in existence.
Since then, many other black hole candidates have been detected. In the case of Cygnus X-1, observations have shown that it has a mass 14.8 times that of our Sun, and has a "light trapping" reach of about 16 miles—the black hole's "event horizon." To all things--light, matter, and a heroic astronaut caught up in an epic science fiction adventure—the event horizon is the ultimate point of no return.
The equations that showed how an object like the Cygnus X-1 black hole could, or even should, exist in nature also told scientists something that really opened up the imagination: there should be no limit on the size, or mass, of a black hole. As difficult as it may be to imagine the mass of ten, twenty, or thirty Suns crushed to a point in space, the math promised that black holes with millions or even billions of solar masses could exist, which pointed scientists to the cores of galaxies, regions known to contain a lot of material from which such supermassive black holes might form.
It is believed that the core of every major galaxy probably contains a supermassive black hole. Our own Milky Way is believed to contain a 4.3 million solar mass black hole at its core—and that's thought to be at the lower end in size for galactic supermassive black holes. The Milky Way's "dark heart" has been inferred from the motions of a number of stars in the galactic core, which have trajectories indicating that they are orbiting something quite massive (like, 4.3 million solar masses), yet unseen.
Today, observations are showing us that black holes may be quite common in the Universe. Data from satellite observatories like NASA's WISE spacecraft have shown the probable presence of millions of candidates. And soon we should be seeing the highest resolution X-ray results yet from NASA's recently launched NuSTAR spacecraft.
And recently, evidence for the existence of the long-sought "Intermediate Mass Black Hole" (the Jan Bradys of black holes, maybe?)—objects with masses in the range of thousands or tens of thousands of solar masses--has been acquired, shedding even more light on this family of objects—the Black Hole Bunch.
One thing we know with probable certainty: What happens in a black hole