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| image source: NASA |
Stars are an incredibly massive collection of hydrogen atoms that collapsed from enormous gas clouds under their own gravity. In their core, nuclear fission crushes hydrogen atoms into helium atoms releasing a tremendous amount of energy. This energy in the form of radiation pushes against gravity maintaining a delicate balance between two forces. As long as the fusion in the core the star remains stable enough. But for stars with way more mass than our own sun, the heat and pressure at the core allow them to fuse heavier elements until they reach iron. Unlike all the elements that went before, the fusion process that creates iron doesn't generate any energy. Iron builds up at the center of the star until it reaches a critical amount and the balance between radiation and gravity is suddenly broken. The core collapses within a fraction of a second, the star implodes. Moving at about a quarter of the speed of light, feeding even more mass into the core. It's at this very movement that all heaver elements in the universe are created, as the star dies, in the supernova explosion. This produces either a neutron star or if the star is massive enough, the entire mass of the core collapses into a black hole.
If you looked at a black hole what you'd really be seeing is the event horizon. Anything that crosses the event horizon needs to be traveling faster than the speed of light to escape. In other words, it's impossible. So we just see a black sphere reflecting nothing.
But if the event horizon is the black part, what is the whole part of the black hole?
The singularity. we are not sure what it is exactly. A singularity may be infinitely dense, meaning all its mass is concentrated into a single point of space, with no surface or volume or something completely different. Right now we just don't know it's like dividing by zero error. By the way, black holes do not suck things like a vacuum cleaner. If we swap the sun for the equally massive black hole, nothing changes to us except we would freeze.
what would happen to you if you fell into a black hole?
The experience of time is different around the black holes, from outside you seem to slow down as you approach the event horizon, so time passes slower for you. At some point, you would appear to freeze in time, slowly turned red, and disappear. While your perspective you will see the universe fast forward, kind of seeing into the future. Right now we don't know what happens next, but we think it could be one of two things: one you die a quick death. A black hole curves a space so much, that once you cross the event horizon there is only one possible direction. You can take this literally inside the event horizon, you can only go in one direction. The massive black hole is so concentrated at some point even a tiny distance of a few centimeters would mean that the gravity acts with millions of times more force on different parts of your body. Your cells get torn apart as your body stretches more and more until you are a hot stream of plasma one atom wide. Two you die a quick death. Very soon after you cross the event horizon you would hit a firewall and be terminated in an instant. Neither of these two options is particularly pleasant.
currently, the largest black hole is known as s50014+81 which is 40 billion mass of our sun.