I'm Fraser Cain, the publisher of Universe
Today.
Black holes are the most exotic and awe inspiring
objects in the Universe.
Take the mass of an entire star.
Compress it down into an object so compact
that the force of gravity defies comprehension.
Nothing, not even light, can escape the pull
of gravity from a black hole.
The idea was first conceived in the 18th century
by the geologist John Mitchell.
He realized that if you could compress the
Sun down by several orders of magnitude, it
would have gravity so strong that you'd need
to be going faster than the speed of light
to escape it.
Initially, black holes were considered nothing
more than abstract mathematical concepts;
even Einsten assumed they didn't actually
exist.
But in 1931, the astronomer Chandrasekhar
calculated that certain high mass stars might
be able to collapse into black holes after
all.
They turned out to be real, and over the next
few decades, astronomers found many examples
out in the Universe.
Stars are held in perfect balance by two opposing
forces.
There's the inward pressure of gravity, attempting
to collapse the star, counteracted by the
outward pressure of the emitted radiation.
At the core, millions of tonnes of hydrogen
are being converted into helium every second,
releasing gamma radiation.
This fusion process is an exothermic reaction,
meaning it releases more energy than it requires.
As the star consumes the last of its hydrogen,
it switches to the stockpiles of helium that
it has built up.
After it runs out of helium, it switches to
carbon, and then oxygen.
Since the star continues to pump out radiation,
it balances out the gravitational forces try
to compress it.
Stars with the mass of our Sun pretty much
stop there.
Not massive enough to continue the fusion
reaction, beyond oxygen, they become a white
dwarf and cool down.
But for stars with about 5 times the mass
of our Sun, the fusion process continues further
up the periodic table to silicon, aluminum,
potassium, and so on, all the way to iron.
No energy can be produced by fusing iron atoms
together.
It's the stellar equivalent of ash.
And so, in a fraction of a second, the radiation
from the star turns off.
Without that outward pressure from the radiation,
gravity wins out and the star implodes.
An entire star's mass collapses down into
a smaller and smaller volume of space.
The velocity you would need to escape from
the star goes up, until not even light is
going fast enough to escape.
And this is how you form a black hole.
Well, that's the main way.
You can also get black holes when dense objects,
like neutron stars, collide with one another.
And then there are the supermassive black
holes at the heart of every galaxy.
And to be honest, astronomers still don't
know how those monsters formed.
Thanks for watching.
