If you’ve ever thrown a baseball into the air, you probably noticed it quickly returned to the Earth. The harder you throw it, the faster it will travel when it leaves your hand, and the higher and farther it can go before returning to the ground. Unless you really suck at throwing in which case it just plops down harder right in front of you. If you throw proper a few times you may notice that it traces the shape of a parabolic arch as it travels, like the big one in St Louis, or the medium ones at McDonald’s.
In theory if you could throw a ball hard enough the curve of the arch would be greater than the curve of the Earth itself, then it would leave the Earth’s surface and go into low orbit, or even escape the Earth’s grip altogether. The gravity of the Earth will not be enough to return it to the surface if it travels fast enough, because the shape of its fall will insure that its path and the surface of the Earth do not cross each other. This speed that the ball must have in order to achieve that path is known as the escape velocity, which for the Earth is about 11 kilometers per second, or 24,000 miles per hour. Be sure to account for air friction, at least until it gets up above the atmosphere. Maybe wrap it in some sort of heat resistant material.
As a mass is compressed into a smaller and smaller volume, the gravitational attraction at its surface becomes more severe, as though it is being concentrated. This means the escape velocity at the surface is higher. Objects have to travel faster in order to get that ballistic arch to exceed the curve that allows escape into space. Eventually, if the mass is concentrated enough, a point is reached when not even light can get a flat enough curve to escape. This extreme condition, by definition, is a black hole. In spite of what you may have heard, it doesn’t suck. Stuff falls and falling has a shape. To a fair degree gravity and falling is all about geometry.
Gravity exerts crushing pressure on a star because of all the gas falling inward. Meanwhile, as long as its fuel lasts, the core of a star generates outward radiation through nuclear fusion. This outward pressure balances against the inward pressure keeping the star dense but suitably fluffy and not too stuffy. As the star’s fuel is exhausted, gravity begins to win the fight which compresses the star stuff inward. If the mass is high enough the star will eventually collapse. High mass stars eventually become black holes because they put too much of their stuff in too small of a place.
In the crowded cores of galaxies it is believed that millions of stars have combined to form supermassive black holes. Some supermassive blackholes even have billions of solar masses worth of stars, dust, gas, letters notifying me of jury duty, and so forth. This is the simplest explanation for the high velocities we measure for stars and other piles of crap orbiting near galactic cores. It also neatly explains certain bright bursts of energy from deep space where black holes are probably gobbling up stars like popcorn. And I mean the way popcorn was meant to be eaten, cramming it into your face by the handfuls with bits of it flying everywhere. Not all of it makes it in the hole.
Our own galaxy, the Milky Way, is theorized to contain a black hole in the core on the order of 3 to 4 million solar masses. Compared to others that’s nothing to brag about but it’s respectable.
So what happens as you approach the so-called event horizon, the point of no return where even light has no chance of escape? Where “return to sender” has no practical meaning? First, the spaghettification. Before you even get close the tidal forces begin to stretch you. Gravity decreases sharply over distance so the side of you nearest the black hole is pulled much harder than your backside. At first it feels kind of good, like a massage. But eventually you stretch until you turn into a thin noodle several kilometers long. As the leading edge of the pasta falls, you spiral down, still trying to create a parabolic path. You still following the same basic rules that apply to a baseball headed to home plate. Only there is no hitting it out of the park and the umpire’s head appears to be stretching around the Universe touching his feet.
Time gets all stretched out of shape as you fall down that well too. According to Einstein that’s to be expected because time and space are like some weird polyester blend and someone left the iron on high.
So what is at the center? I don’t know, something super dense that’s for sure. Mathematical models suggest ludicrous density, infinity + 1 density. And whatever is at the center is small, like somewhere right around size zero. They call that hard little bad boy a singularity. For now he’s likely to remain theoretical so you don’t have to think about him too much if you really don’t want to. But you have to take into account the gravity he’s creating because it’s making a serious warp in the fabric of space-time, it’s like my skinny jeans on the day after Christmas.
Is there a worm hole down there? A cosmic subway express train to stations somewhere out in the burbs? Probably not but we’ll pretend that there is because we need some sort of literary device to turn our galaxy into a grand stage for a futuristic Shakespeare. Boy meets girl. Boy pisses girl off. Girl throws boy’s stuff in a black hole. Boy goes traipsing around the Universe collecting his scattered stuff and never does find his Creedence albums.