The Energy that Powers the Sun



As you know, the Sun is a big ball of gas in the sky that shines. It obviously produces light and heat, but how.....?



Kelvin and the Sun

 The source of energy for the Sun was a major concern for scientists last century. Lord Kelvin (born William Thomson, picture on the right) thought that the Sun was powered by the conversion of gravitational potential energy into heat and light. Given the size and mass of the Sun he calculated that the Sun was about 50 million years old.

Now this was a problem since the Earth was calculated to be at least 400 million years old! It hardly seemed possible that the Earth could be older than the Sun. Incidentally, both the Sun and the Earth are now thought to be around 4.5 billion years old. 		Picture of Lord Kelvin


Picture of Albert Einstein

Einsten to the rescue!

 Clearly there was a problem! There wasn't a source of energy that could power the Sun, and yet it still shone. This problem remained until Albert Einstein published his famous formula

E=mc2


The formula is very simple - it basically says that matter (everything you see around you) is a congealed form of energy! It also tells us that a very small amount of mass can generate an awful lot of energy. In the formula, `E' is energy, `m' is the mass of something and `c' is the velocity of light (c2 just means c x c)


For instance, suppose you had a litre of water (1 litre of water has a mass of 1 kilogram) and you converted it all into energy using Einstein's equation. The amount of energy released is

9 x 1016 Joules


You could power 1 million 100 watt light bulbs for about 30 years with the energy from one litre of water! 		One lightbulb




The process that powers the Sun is called fusion and it works by turning mass into energy in accordance with Einstein's equation above. The centre of the Sun is extremely hot (around 15,000,000 degrees), so hot that all the atoms lose their electrons and become ions. It is also hot enough that the ions can overcome their mutual electric repulsion and come close enough to join together.

Fusing particles


If you measure the mass of the atoms before they come together and compare it to the mass after they fuse then you find that there is less mass after than before. The mass difference has been converted into energy according to Einstein's equation. This energy is carried away by photons. and neutrinos. It is these photons that can eventually shine on Earth.


To find out what happens to the photons, click on `Radiative Zone' below....



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