P1bL13 Life and Death of a Star
Key Words
Black dwarf - a cold dead star
Black Hole - the remains of a star where gravity is so intense that even light can't escape.
Force of gravity - attractive force between objects.
Neutron star - the remains of a star where the material is very dense indeed.
Pulsar - a dead star that gives out pulses of radio energy.
Radiation Pressure - the outward force of the fusion explosion which is balanced by the gravity.
Red Giant - a huge old star with a fairly cool surface.
Red supergiant - one that's even bigger.
Supernova - a titanic explosion caused by the collapse of a red giant star.
White Dwarf - the remains of 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.
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.
Large stars burn their fuels quickly. The star swells as before to form a red supergiant, which is unstable.
Then suddenly the radiation pressure falls away and the whole thing collapses within the space of about 60 seconds. The immense shock wave and the raising of the core temperature to several million Kelvin make a titanic explosion which strips off the outer layers. The star is now a supernova, and the brightest can even be seen in daylight.
Heavy elements are formed at this stage.
The core collapses to make a neutron star, which is very dense. 1 cm3 of the material has a mass of several million tonnes. The Earth at that density would be about 300 m across, and would fit onto our school.
If there is sufficient mass, the neutron star collapses further into a tiny space giving off intense energy as it does so. The energy is highly directional, like the beam of a lighthouse. The star is now a pulsar, because the beam comes as pulses.
At the end of the process, there is a black hole, where the gravity is so great even light can't escape. If you went to one, you would see:
It would be the last thing you saw, as gravity would attract you to it. You would end up stretching out like spaghetti, before you spent the rest of eternity there.
Grade A
Helium itself can be used as a fusion fuel, to form elements like carbon and nickel. The ultimate product is iron. To produce elements heavier than iron, we need the extreme conditions found in a supernova. Elements like gold and copper are the results of supernova explosions.