Stars shine because of the nuclear fusion reactions going on
in their core. The most common reaction converts hydrogen to helium and the energy
produced is radiated out into space. A star like our Sun has a power of about 450 million
million million MW!
There is one rather strange thing – a star is so vast that light
generated near its centre may take thousands of years to diffuse up to the surface and then
travel off into space.
The nuclear fusion reactions in a star convert helium into
hydrogen and then some heavier elements. However in a normal star the temperature is not
hot enough for elements heavier than iron to be formed. So – where have all the heavy
elements like lead, silver, platinum, gold and uranium come from?
The answer to
this lies in the history of some of the stars.
Sometimes the nuclear fusion furnace
that powers a star gets out of control. When this happens the star blows up. The exploding
star is so bright that it was named a nova – or new star.
This was because the original star was too faint to be seen from the Earth but the explosion
released huge amounts of energy with a massive increase in the brightness of the
star.
In the explosion giving a
nova – vast quantities of the outer layers of the star are blasted off into space by the
enormous forces within the star – the gravitational pull is not big enough to hold them back.
In 2005 a star in the constellations exploded giving out as much energy in one day as the
Sun does in several centuries and emitted several times the mass of the Earth at speed of
thousands of km per second.
Sometimes the star undergoes an even more violent
fate – the enormous explosion formed by this is called a supernova. The photograph shows a supernova that was
discovered in the distant galaxy M51 in 2005 – 37 light years away. Very simply a supernova occurs when a
large old star – such as a red giant collapses and as the matter falls inwards it heats up the
core of the star that then gives a huge increase in nuclear fusion.
In a supernova
explosion a star may emit as much energy in a few months as the Sun would in over a billion
years. This is what happened in 1054 when the Chinese saw what they thought was a new
star, it was really one that had got much brighter because it had blown up. This explosion
was so vast that we can still see the remains of it today.
The huge cloud of gas that
it produced is still expanding and it is called the Crab Nebula.
The Crab Nebula in
shown in the second photograph remnants of another supernova explosion – vast clouds of
glowing gas swirling through space.
Don't expect to see one in the sky, they don't happen very often, and
sometimes the explosion is not big enough for us to see from the Earth.
So what
about the heavy elements we mentioned earlier on? In a supernova explosion the
temperature rises to enormous values so that elements heavier than iron can be made from
nuclear fusion. These new atoms are then blasted out into space – seeding the galaxy in
which the supernova with heavy elements. The gold fillings in your teeth or in a gold ring may
well have come from a supernova explosion somewhere in the galaxy many millions of years
ago!
I am very grateful to Vivek Hira and Marcos Mataratzis for permission to reproduce the
photographs in this section.