Stars are created from the huge volumes of gas that exist in space (the Interstellar Gas). One theory is that the shock waves of other exploding stars cause regions of instability so the volumes of gas separate and begin to contract due to the force of gravity. As the cloud shrinks, the gas molecules move faster and the temperature of the cloud rises. To form a star like the Sun may take 10 million years, after which the core temperature is a few million C.

Once burning, the star will continue like this for about 10 thousand million years (our Sun is about 5 thousand million years old). All stars burn by destroying mass; when mass m is destroyed, energy E is created in the form of electromagnetic radiation according to the equation

E = mc

where c is the velocity of light (300 million metres/second). Main sequence stars burn by the process of nuclear fusion. The process is complicated using nucleii like oxygen in an analogous way to catalysis in chemistry the results of the process are that four protons or hydrogen nuclei are brought together to become a helium nucleus with two of them changing into neutrons and emitting a positive electron and an anti-neutrino.

For each kg of hydrogen converted into helium, 660 million million joules of energy are radiated. The Sun is still mostly hydrogen and this burning takes place in the core.

During this 10 thousand million years, the star is stable but the burning of hydrogen causes the core to become enriched with helium. As less and less hydrogen remains to be burned, energy generation dies away within the core. This burning moves into an expanding shell surrounding the core and the star becomes a Red Giant at the end of its stable, main sequence phase of its life.

After the Red Giant phase has ended, when all the hydrogen in the outer layers has been converted into helium as well, the star may become:

a. a White Dwarf in which there is no more nuclear fuel. White Dwarfs form about 3% of all Stars in the galaxy, because they are slowly cooling, they are being used to measure the age of the Galaxy.

b. unstable and explode. This is called a Nova or a Supernova.


c. variable in luminosity, with the star pulsating like an enormous heart.


1. Assume you weigh 50 kg and that electricity costs 10 pence per 10 million joules. Calculate how much you are worth as pure energy?

2. The Sun generates 400 million billion billion joules of radiant energy per second. How much mass is it losing per second?


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