P3bL5  Eternal Cycles

Key Words

Black dwarf - the cold remains of a small star that has long since stopped shining

Black hole - the remains of a large supernova which has collapsed in on itself.

Gravity - an attractive force between large objects.

Hydrogen - a colourless gas that has very low density.

Neutron Star - remains of a smaller supernova.

Nuclear fusion - light nuclei fuse to form helium, giving out lots of energy

Pulsar - the remains of a supernova that spins rapidly and gives out radiation in the form of pulses.

Radiation pressure - the outward force of the nuclear explosion going on in the middles of a star.

Red giant - an old star that has swollen up and has a relatively cool surface.

Red supergiant - an old massive star that has swollen up and has a relatively cool surface.

Singularity - a region (like a black hole) where the laws of Physics don't apply.

Supernova - a titanic explosion caused by an old star collapsing in on itself.

White dwarf - the remains of a star that was once a red giant.

Grade E

Stars don't last for ever.  The fusion reactions make helium causing the star to swell.  Also mass is ejected in the solar wind, so the total mass goes down.  The grip of gravity gets less, so the star swells even more.  It becomes a red giant.     

This is what will happen to the Sun.  It will end up frying the Earth and most of the planets in the Solar System.  Then its temperature will fall and it will collapse under its own gravity, to end up as first of all a white dwarf.  It will cool down to be a brown dwarf, then finally a black dwarf.

For a much bigger star, its fate ends up like this:

The large star (about 10 times the mass of the sun) grows into a red supergiant.  It then explodes as a supernova, before following the path to become a black hole. 

While each of these cycles takes hundreds of thousands of million years it is thought that much of our galaxy is on its third cycle.  The explosions of supernovae have provided us with an astonishing array of heavy elements.       

Grade C

As the fuel is used up, the mass of a star gets less, so the radiation pressure becomes the dominant factor.  The star swells up massively.  It stays like that until all the hydrogen at the core has been used up.  Meanwhile at the outer shell of the star, fusion produces light elements like carbon, iron, and nickel.

As the core cools, the radiation pressure reduces, and the star contracts under its own gravity.  The compression of the material makes the small star very hot, a white dwarf.  Then it cools off to form a brown dwarf, then finally a black dwarf.

When the star is stable, the radiation pressure (outwards force due to the explosion in the centre) is balanced by gravity holding everything together.  The stable state lasts several thousand million years.

The explosion comes from the nuclear fusion of deuterium and tritium, both isotopes of Hydrogen.

Fusion gives out massive amounts of energy.

Grade A

When the Big Bang occurred the heaviest element was hydrogen.  As stars formed, light elements like carbon, oxygen, and silicon were formed by fusion processes.  The heaviest element that can be formed by fusion is iron.

Every other element bigger than iron has been formed in the extreme conditions of a supernova explosion, in which a red supergiant collapses in on itself.  The process takes less than a minute, and huge amounts of material and energy are thrown out.

All heavy elements that are around today, like gold and lead, came from the material thrown out by supernova explosions.