Cooking the elements

Everything around us is made up of atoms. These come naturally
in ninety-two different varieties, called the elements. They range
from the very lightest gases, such as hydrogen and helium,
then carbon, oxygen, nitrogen and heavier elements such as
aluminium, nickel, iron, gold and onto the big boys like lead and
uranium. Have you ever wondered how these different atoms
came to be made in the first place? The process is known as
nucleosynthesis—try saying that three times quickly. Less than
a minute after the birth of the Universe conditions were such that
the lightest two elements could be synthesized and the Universe
thereafter contained roughly 75% hydrogen and 25% helium, with
a seasoning of the next few elements in the periodic table such as
lithium and beryllium. This concoction is the raw material of stars.
When clouds of this interstellar gas form, they begin to contract
under the influence of their own gravitational attraction. As the
gas becomes denser, it heats up and, slowly, a new-born star is
formed at the centre. When this temperature reaches a scorching
few million degrees, conditions become hot enough for the star to
switch on.
Stars shine due to the process of thermonuclear fusion. This is
when the nuclei of two hydrogen atoms fuse together to form the
nucleus of a helium atom, releasing in the process a vast amount of
energy. Scientists have been trying, unsuccessfully so far, to mimic
this process on Earth in a controlled way to produce an unlimited,
clean (in the sense of not being radioactive) energy supply. The
problem is, of course, that we cannot stop the extremely high
temperature plasmas in our fusion ovens from escaping. Stars,
on the other hand, continue to burn and shine brightly all the time
the fusion reactions are going on inside them because their gravity
keeps them together. At the same time, this process provides an outward pressure that keeps at bay the crushing inward pressure
of the star’s gravity.
This has been going on inside the Sun for the past five billion
years since it was born (along with its nine planets) from a cloud
of gas and dust. The Sun will continue to shine happily like this
for a further five billion years. So it is roughly half way through
its life at the moment. As far as stars go, this is an impressively
long lifespan, for which it has its small mass to thank. The more
massive a star is, the stronger its gravitational pressure will be,
and so the denser and hotter its interior becomes, and the faster it
burns its nuclear fuel. The very largest stars, a million times the
mass of the Sun, will live for just a few million years.
Five billion years from now the Sun will begin to run out of
its hydrogen fuel and will gradually move into a new phase of its
life. It will become something called a red giant star. When it uses
up all the hydrogen in its core it will begin to collapse under its
own weight and all the matter in the core will become compressed
and so heat up again. At this point, two very different things
happen. First the heat in the core is such that helium atoms are
now forced together to make heavier elements. At the same time
the outer layers of the Sun expand and swell up to such a size that
the closest planet to it, Mercury, will be swallowed up. The Sun
will now be many times brighter than it was before, and will fill
up half the sky as seen from Earth. Unfortunately, we will not be
able to witness this event since the surface of the Sun would now
be so close it would vaporize the Earth. In any case, if humans are
still around five billion years from now they will, hopefully, have
long since found a new home.
After a further billion years the Sun will enter the final phase
of its life by shedding some fraction of its contents out into space.
This forms a rather pretty disc of gas called a planetary nebula,
at the heart of which will sit the Sun’s dying core: a white dwarf
star. Such an object forms when the bulk of the Sun’s mass has
collapsed in on itself due to its own gravity when the processes
of thermonuclear fusion finally cease. It will comprise mainly
of crystallized carbon and oxygen and will resemble a massive
spherical diamond the size of the Earth. Gradually, this white dwarf will cool and become dimmer and colder until it finally
goes out completely. Such an object is extremely dense and just a
pea-sized fragment of it would weigh about a ton.
Thus will our Sun end its days rather unremarkably, even
ignominiously, when compared with many bigger stars which can
lay on an impressive fireworks display.