It took a long journey for scientists to prove that black
holes exist. John Mitchell was the first person to come up with the idea that a
black hole could exists in 1783, he developed the theory of black holes when he
accepted Newton's theory that light consists of small material particles,
called photons. He wondered how the movement of these light particles is
impacted by the gravitational pull of the star they are escaping. He referred to
them as "dark stars", however he doubted that such objects could
exist and after publishing his information, he abandoned further research on
the subject.
Mitchell’s research was further enhanced by Pierre Simon
Laplace in 1795. Using Newton's Theory of gravity,
Laplace calculated that if an object was compressed into a small enough
radius, then the escape velocity of that object would be faster than the
speed of light.
Then all this research took a turn of events as Einstein himself wrongly thought that black holes
would not form, because he held that the angular momentum of collapsing
particles would stabilize their motion at some radius. This led the general
relativity community to dismiss all results to the contrary for many years, and
black holes were considered nothing more than abstract mathematical concepts.
But in 1915, Einstein's
theory of general relativity predicted the existence of black holes, which
ended up proving his previous statement wrong. And then in 1967 John Wheeler,
an American theoretical physicist, applied the term "black hole" to
these collapsed objects.
Now I'll get to the point and explain how black holes are created. To keep it as
simple and straightforward as possible, we can say that black holes are created
when a massive star dies.But I would like to
go into a bit more detail because a one line definition isn't enough!
A common type of
black hole is produced by certain dying stars. A star which has a mass greater
than approximately 20 times the mass of our Sun may produce a black hole at the
end of its life.
In the normal life
of a star there is a constant tug of war between gravity pulling in and
pressure pushing out. Nuclear reactions in the core of the star produce enough
energy and pressure to push outward. For most of a star’s life, gravity and
pressure balance each other exactly, and so the star is stable. However, when a
star runs out of nuclear fuel, gravity gets the upper hand and the material in
the core is compressed even further. The bigger the core of the star causes the
greater the force of gravity that compresses the material, making it collapse under
its own weight.
For small stars,
when the nuclear fuel is exhausted and there are no more nuclear reactions to
fight gravity, the repulsive forces among electrons within the star eventually
create enough pressure to halt further gravitational collapse. The star then
cools and dies peacefully. This type of star is called a "white
dwarf."
When a very massive
star exhausts its nuclear fuel it explodes as a supernova. The outer parts of
the star are expelled violently into space, while the core completely collapses
under its own weight. If the core
remaining after the supernova is very massive (more than 2.5 times the mass of
the Sun), no known repulsive force inside a star can push back hard enough to
prevent gravity from completely collapsing the core into a black hole.