Stars are incredibly massive collections of mostly hydrogen atoms. They collapsed from a giant gas cloud under their own gravity. At their core, nuclear fusion crushes hydrogen atoms into helium, releasing a tremendous amount of energy. The energy, in the form of radiation, fights against gravity. This helps maintain a delicate balance between radiation and gravity. As long as fusion is happening at its core, a star remains stable enough. But for stars with way more mass than our sun, the heat and pressure at their core allow them to fuse heavier elements. Until… they reach iron.
Unlike all the elements that went before, iron is created in a process that doesn’t generate energy. Iron builds up at the centre of the star and reaches a critical amount. This breaks the balance between radiation and gravity. The core collapses. Within a fraction of a second, the star implodes. The star feeds mass to its core, moving at about a quarter of the speed of light. It’s at this very moment that all the heavier elements of the universe are created. As the star dies in a supernova, it forms either a Neutron Star or a black hole.
If you look at a black hole, what you’d really be seeing is the event horizon. Anything that crosses the event horizon has to be travelling at more than the speed of light to escape. In other words, it’s impossible. So we just see a black circle that reflects nothing. But if the event horizon is the black part, what’s the ‘hole’ part? It’s the singularity. We’re not sure what it is exactly. A singularity can be infinitely dense, meaning its mass is connected to one single vortex in space, without any surface or volume. Or it could be something completely different. Right now, we just don’t know. It’s like a ‘dividing by zero’ error. By the way, black holes don’t suck things in like a vacuum cleaner. If the sun was replaced by an equal mass black hole, nothing much would change on Earth. Except that it would freeze to death, of course.
So what would happen to you if you fell into a black hole? The experience of time around a black hole is different. Time seems to slow down as you approach the event horizon. At some point, to the outside, you would seem to freeze in time, slowly turn red and disappear. In your perspective, you can watch the universe in fast-forward, kind of like seeing into the future. Right now, we don’t know what would happen next, but it could be two things.
One: you die a quick death. Black holes curve space so much that once you cross the event horizon, there’s only one direction left: forward. You can think of it like this: once you pass the event horizon, it’s like you’re walking through a very tight alley that closes behind you after each step. The mass of a black hole is so concentrated that, at some point, even a tiny distance would mean that gravity acts with millions of times more force on different parts of your body. Your cells get torn apart and your body stretches more and more until you’re a hot stream of plasma, one atom wide. Not pleasant.
Two: you die a very quick death. Very soon after you cross the event horizon, you would hit a firewall and be terminated in an instant. Not pleasant at all. How soon you’d die depends on the mass of the black hole. A smaller black hole will kill you even before you enter its event horizon. You could probably travel inside a supermassive black hole for quite a while.