A black hole is a region of space time where gravity
prevents anything, including light, from escaping. The theory of general
relativity predicts that a sufficiently compact mass will deform space time to
form a black hole.
Around a black hole
there is a mathematically defined surface called an event horizon that marks
the point of no return. It is called "black" because it absorbs all
the light that hits the horizon, reflecting nothing, just like a perfect black
body in thermodynamics.
When a gigantic star reaches the final stage of its life and
is about to go supernova (which normally takes billions of years), it spends
all the nuclear fuel by then. So it stops burning and heating up and cannot
create the nuclear energy required to feed the star and let it make a pivotal
balance to support its own gravitational draw against the intense pressures
brewing inside.
Therefore its stability cracks under its own gravity. The
radius of the star shrinks to a critical size, called the Schwarzschild radius
and it starts to devour anything and everything that comes a bit too close,
including light. Gravity does its job and the core of the star caves in and
implodes.
The outer shells of the star explode into the space. They
may even fall into the already dense black hole making it even heavier and
denser. And that’s how you get a stellar mass black hole.
