Spark image

Exploding Stars

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.


Heavy elements and a supernova

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.

 

A VERSION IN WORD IS AVAILABLE ON THE SCHOOLPHYSICS USB
 
 
 
© Keith Gibbs 2020