“Space, “ Douglas Adams said, “is big. Really big. You just
won't believe how vastly, hugely, mind-bogglingly big it is. I mean, you may
think it's a long way down the road to the chemist, but that's just peanuts to space.”
And it is. The diameter of the observable universe is estimated at 93 billion
light-years, with a light-year being just under 10 trillion kilometres. This is
so large (it has 26 zeroes at the end) that it is difficult to comprehend.
Perhaps just as amazing, is the fact that of all the matter
in this universe, 74% is hydrogen, the simplest element, and 24% Helium, the
second least complex element. They occur in large molecular clouds across the
universe, with these clouds able to stretch almost 100 light-years across and
having a mass of up to 6 million times that of the sun. A mass that big has a
large gravitational field, and so it is as well with these clouds.
When these clouds collide, they start to form gravity
hotspots, spot where the gravity is stronger than the surrounding area, and they
begin to break up. Matter begins to collapse into these hotspots, strengthening
them and increasing its gravitation pull. The density of matter at the center
of these hotspots increase, and as a result, starts heating up. When there’s
enough mass (about 8% the mass of the sun), the temperature can rise up to 10
million degrees Celsius, and at this temperature and in this incredibly dense
cloud of matter, something new occurs. Nuclear fusion.
Hydrogen atoms collide and ‘fuse’ together to form helium.
This releases a momentous amount of energy, enough to offset the force of
gravity and preventing the cloud from collapsing further inward. This energy
release coincidentally also releases photons, or light particles. The cloud of
gas lights up, and becomes a star.
But this process consumes the hydrogen, meaning that
eventually, it will run out of fuel. Less massive clouds have less
gravitational pressure, and fusion occurs more slowly, which means
paradoxically that they’ll shine longer. Unfortunately, these red dwarf stars
(with a mass of less half that of the sun) also don’t have the gravitational
energy to continue fusion after their hydrogen runs out, and after six to
twelve trillion years, they die to become white dwarfs – still quite hot, but
no longer releasing massive amounts of energy.
Stars with a mass more than half that of the sun to about
ten times its mass burns through its hydrogen more quickly comparatively (from
only half a million years for stars ten times as massive as the sun, to 100
million years for stars half as massive), but as more and more of the star
turns into helium, the helium sinks to the core. With helium’s greater mass and
density, the gravity becomes more intense, speeding up the hydrogen fusion
process, and expanding it to a greater and greater area around the helium core.
The outer layers of the star then expands as fusion moves
closer to the surface, and the amount of light it emits increases massively as
well, becoming a thousand to ten thousand times brighter. The star becomes a
red giant. Eventually, the helium core becomes massive enough and hot enough
that it reaches its next stage, helium fusion. At its core, the star fuses
helium into carbon and oxygen, and this even greater energy release causes the
core to expand, dissipating its outer hydrogen fusion layers, and reducing its
energy output. Slowly the helium is used up in fusion, but even these stars are
not large enough to induce the next stage of fusion, and they, too, die to
become white dwarfs.
But when a star is more than 10 times as massive as the sun,
it does not expand into a red giant, as the force of gravity continually causes
nuclear fusion. Hydrogen to helium, helium to oxygen and carbon, then those
into neon and sulphur – with fusion continually occurring, matter is continually
used as fuel, and the star shines as a bright blue supergiant for only a
hundred thousand years, until the core starts fusing into iron. This results in
the gravitational force becoming so strong that the atoms can no longer stay separate,
and the entire core fuses together. Lighter stars have the core fuse together
into a neutron star – a single massive atom of billions and billions of
neutrons. But in heavier stars the gravity overcomes even light, and the star
collapses into a black hole, from which no light can escape. Either way, as a
result of this gravitational collapse all fusion energy is released at once,
exploding the outer layers of the star into space in what is known as a
supernova.
A supernova is so bright it is briefly brighter than an
entire galaxy. The outer layers explode in a shockwave travelling at up to 10%
of the speed of light. So much energy is released that nuclear fusion occurs in
the shockwave – but unbounded by gravity, it happens more haphazardly that it
does in the core, fusing in many different combinations. It is thus in these
supernovae shockwaves that all the elements heavier than iron are formed. As
they impact other hydrogen clouds, they trigger the formation of new stars –
but these new stars now have heavier elements that don’t all collapse to the
core, but rather orbit the core. We call them planets. And on some, these
heavier elements start combining. Start self-replicating. Start the beginnings
of life itself…
Carl Sagan famously said “We are all made of star stuff.”
Every atom in your body, and the bodies of everyone around you, was once part
of an exploding star, scattered across the cosmos. At this time of year, I
cannot think of any other fact that can imprint such a profound connection
between ourselves and the universe, and our loved ones, than this.
