Away out there in space there's huge clouds of dust and gas
and if one of those clouds of dust and gas
is massive enough, it's own gravity causes it to start to collapse
so it folds in on itself
towards the center of that cloud
it gets denser and denser
and hotter and hotter
and eventually, the particles of that the gas and the dust are made of
are brought so close together
that they start to stick together
they start to fuse
that's the energy source of a star.
The star switches on and begins to shine
Inside every newborn star hydrogen atoms are fused together to make helium
This process is called fusion, and it creates the energy of every star.
What happens to a star during the rest of its life depends on how massive it is at its birth.
A star like the sun is in a delicate balance
between gravity which wants to make the star collapse in on itself
and the pressure that pushes outward
that comes from the energy produced in these fusion reactions at its core
At some point in the future, the hydrogen runs out.
At that point, the core of the star will start to collapse in on itself
under its own weight
it gets denser, it gets hotter
until the point where you can actually start to use the helium atoms
themselves as the fuel for the fusion
pushing helium atoms together, making carbon and oxygen
the next heavier elements on the periodic table
As the star begins to fuse helium, it creates more energy
and that causes the outer layers of the star to expand
One day, our sun will grow so large
it will swallow up the inner planets of the solar system
out as far as the Earth!
It will become a red giant.
For the sun, this will be the beginning of the end
What happens is that the outer layers of the star get farther and farther from the middle
The force of gravity that they feel is getting weaker.
And, actually, the star loses hold of its outer atmosphere.
Its outer atmosphere drifts off out into space.
It expands to become a planetary nebula.
And they're some of the most beautiful objects in the universe.
Once the outer layers have drifted away,
all that is left of the star is its core
A white dwarf star is the dead, remnant
core of a star like the sun at the end of its life
It's something that might weight as much as 1/2 the mass of the sun
but it's only about the size of the Earth, so it's an incredibly dense object.
It's dead, there's no nuclear fusion going on any more, it's incredibly hot
but then over millions of years, it will gradually cool down
to become a black dwarf
Some stars, however, are much more massive than the sun
and they lead very different lives.
They are able to fuse heavier and heavier elements inside their core
the star gets bigger and bigger
some grow up to 1000 times the size of our sun
until it has fused elements all the way up to iron
and once we've formed an iron core,
there's no more energy that can be got from fusion
that core collapses
the rest of the star starts to collapse in after it
but then it bounces off. There's a huge shock wave
And in just a second:  BANG!
The outer parts of the star are blasted off in to space
in a huge super-nova explosion!
These super-nova explosions are so powerful
that when one of these stars explodes
it can actually outshine the whole galaxy
of which is part (a galaxy of maybe a 100,000 million stars).
For these super giant stars
all that is left is a super dense core
known as a neutron star
an object that can have a mass greater than our sun
but be less than 20 kilometers across
but for the most massive stars of all
we think, that when the core collapses
the gravity is so strong, it becomes a black hole
from which not even light can escape
so stars are actually the places in the universe where the elements are created
after the Big Bang, our universe contained only hydrogen and helium.
All the other, heavier elements
were therefore fused inside stars.
The amazing thing is that virtually everything you see around you
was made inside a star
billions of years ago before the sun and planets were formed
and when that star died
and blasted its guts out into space
that formed the raw materials from which our sun
the planet earth and indeed ourselves were made
And, ultimately, that's one of the major reasons I think understanding stars
is crucial, because its actually telling us
where we came from
Subtitles by the Amara.org community
