Karl Schwarzschild crafted the first exact solution to Einstein’s equations of general relativity. He found that as gravity increased around an object, there must be a point where even light could not escape. He theorized black holes.
Stars are in a balance between gravity trying to collapse it inward, and energy of fusion in its core which pushes outward. But when large stars run out of fuel, gravity causes it to collapse. If the star is massive enough, this results in a supernova. A black hole remains in the center of the debris, if the collapsed core has a mass of 2 to 3 times the mass of our sun.
In a Black Hole, General relativity says all its mass is collapsed into an infinitesimally small volume, called a singularity. A singularity has all its mass in zero volume of space, thus it has infinite density. But infinities usually mean errors in math, so singularities may not be real.
The singularity is enclosed by a boundary, the event horizon, within which the curvature of spacetime is so strong that light cannot escape. The radius of this sphere is called the Schwarzschild radius. Since no light can escape from the event horizon, anything inside, including the singularity, can’t be directly seen. Anything that crosses into this horizon is swallowed forever. For this reason, black holes are considered the of edge of space, a one-way exit from our universe.
The size of a black hole is defined by its event horizon, and is very small. If the sun was a black hole, it would be a sphere 6km or 4 miles wide, and earth would be the size of a ping pong ball.
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As you get closer to the event horizon, the flow of time slows, compared to flow of time from a point far away from it. From the perspective of an observer far from the black hole, time stops completely at the event horizon. General Relativity still works inside it, but not at the singularity.
According to Relativity, time and space trade places inside the black hole. Relativity predicts that time gets destroyed at the singularity. So a black hole is like the “reverse of creation.” Whatever is inside the event horizon is causally disconnected from us. It remains forever in the future. Whatever is inside hasn’t happened yet from our perspective.
Since no light can escape, we can only “see” black holes indirectly because of the way their gravity affects stars and pulls matter into orbit. As gas flows around some black holes, it heats up, paradoxically making them some of the brightest things in the universe.
But most black holes don’t have accretion discs, so they are not easily seen. Other methods have to be used to find them. The supermassive black hole near the center of our Milky way galaxy called Saggitarious A* was found because of the tight orbits of stars we COULD see orbiting it.
Black holes are rather common. Scientists estimate that a new black hole is formed in our universe, every second. There are an estimated 100 million black holes floating around in the Milky Way. So for every 1000 stars you can see in the sky, there is a black hole among them that you can’t see.
The gravitational gradient around a black hole is so steep that it allows for MILLIONS of potential orbiting planets around them, whereas a regular stars can only support a fraction of that.
The maximum theoretical number of Earth-like planets that could exist around our Sun in the habitable zone is six. Replace our Sun with a black hole a million times its mass but about the same size and 550 Earth-size planets could orbit in the same region without bumping into each other. A black hole’s gravity is so dominant, it negates how planets disrupt each other, and that allows for far more stable orbits to exist. It is possible that a planet around a black hole could support life.
Black holes last much longer than our sun, 10^84 years vs 10^10 years. They are going to be around for a long time, after the all the stars have died out, and the universe goes dark.
