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Author Topic: Why doesn't the sun burn all its hydrogen at once?  (Read 11017 times)

Des

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Des  asked the Naked Scientists:
   
Hi

If the sun burns hydrogen as it's fuel, and hydrogen is so flammable, why doesn't all the hydrogen ignite in one go?

Cheers
Des Enright

What do you think?
« Last Edit: 29/04/2011 16:30:02 by _system »


 

Offline Bored chemist

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Why doesn't the sun burn all its hydrogen at once?
« Reply #1 on: 29/04/2011 18:34:22 »
The sun doesn't burn hydrogen in the conventional sense. There's no air.
 

Offline JP

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Why doesn't the sun burn all its hydrogen at once?
« Reply #2 on: 29/04/2011 22:30:49 »
Hi Des,

Bored Chemist is right.  The sun doesn't use up all it's hydrogen because it's not burning it in the way you could burn it with a match here on earth.  The usual way of "burning" hydrogen is to get it to combine with oxygen to make water.  This takes a little bit of energy, but releases a lot of energy.  If you have a lot of hydrogen and oxygen mixed together, then the energy released when it starts to "burn" will cause a chain reaction by providing the initial energy to get more hydrogen to react with more oxygen and burn until the hydrogen or oxygen is used up.

On the sun, the process of "burning" is completely different.  There is no oxygen, as BC says, but the sun is so hot and so big that gravity and the heat energy of the sun can actually force hydrogen atoms to fuse together into helium, releasing a lot of energy in the process.  The reason this isn't a chain reaction is that if the sun were to heat up, it would expand and this would actually slow down the fusion rate.  If the sun were to shrink, it would become denser and this would speed up the fusion rate.  So the sun is at the perfect point where the heat released by fusion is counteracted perfectly by gravity to keep it going at a nearly steady rate. 
 

Offline yor_on

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Why doesn't the sun burn all its hydrogen at once?
« Reply #3 on: 04/05/2011 18:16:51 »
So you could say that the gravity is what defines its 'burning' right :)
Another nice one.
 

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Why doesn't the sun burn all its hydrogen at once?
« Reply #3 on: 04/05/2011 18:16:51 »

 

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