When matter is compressed beyond a certain density, a black hole is created. It's called black because no light can escape from it. Some black holes are the tombstones of what were once massive stars. An enormous black hole is thought to lurk at the center of the Milky Way galaxy.
Black holes were once stars.
As nuclear energy starts running out, gravity starts to overcome pressure, increasing core density and gravitational force.
Nuclear reaction stops and the star explodes as a supernova, expelling outer parts of the star into space.
The core collapses under its own weight into single point of infinite desity.
Gravitational forces at this stage are so strong light cannot escape from its pull.
Black holes, because of their very massive mass and intense gravity, will cause light to bend around them, which will cause the appearance of background objects (like stars) to be distorted. See how that looks like in the next section...
The below simulation shows how black holes affect the trajectory of light.
While this visualization below doesn't specifically show a black hole, it does help intuit how objects would have to behave to stay in orbit around an object with increasing mass (black hole). We also assume that the masses of the orbiting objects are negligible compared to the center mass and that the orbits are perfectly circular"