MadSci Network: Astronomy

Re: If there is no oxygen in outer space, how does the sun burn?

Date: Tue Nov 10 18:37:09 1998
Posted By: Everett Rubel, Degree in Physics
Area of science: Astronomy
ID: 910712010.As


Thank you for the question.  To answer it I will need to clarify what the 
term "burn" means.

For your average Earth-bound human, burning means a substance combines with 
the oxygen in the atmosphere and releases heat, light, and various fumes in 
the process.  The rate of combination is important, since very slow 
combination of fuel with air, like with a glowing ember of charcoal, is 
usually not called burning.   Also, very fast combining of fuel and oxygen 
would be called an explosion, not burning.

We usually associate oxygen together with burning because there is so much 
of it around where we live.  However, when you look at what is available in 
the Solar System, you see that oxygen only makes up a small part of the 
materials that form the Sun and planets.  The main elements that make up 
the Sun are hydrogen and helium.  Helium does not burn, but hydrogen will 
burn very nicely if sufficient oxygen is present.  A problem is that there 
is not nearly enough oxygen to burn with the hydrogen in the Sun.  Another 
problem is that the Sun is hotter than a normal flame, so that even if 
hydrogen and oxygen tried to combine, the high temperatures would just 
break them apart again.

The process that powers the Sun is the fusion of hydrogen.  Fusion is a 
nuclear process, which is much more energetic than a chemical process like 
burning hydrogen with oxygen.  If you combined a gram of hydrogen with 8 
grams of oxygen, that is burn the hydrogen, you would release enough energy 
 to start boiling a cup of water.  If you could take that same gram of 
hydrogen and make it completely fuse into helium, you would release enough 
energy to bring a large swimming pool of water to a boil.  Fusion literally 
can release about a million times as much energy as normal chemical 

Fusion of hydrogen does not need any other elements around like oxygen.  
Basically all that fusion does need is very high pressure and temperature 
to squeeze hydrogen nuclei together to make helium.  The pressure and 
temperature needed is so high that fusion can not occur in most parts of 
the Sun.  The only place that it can occur in the Sun is near the center, 
where the pressure and temperature is the highest. In a sense, the Sun 
"burns" from the inside out.  The heat generated in the center of the Sun 
slowly makes its way out to surface of the Sun, which is much cooler than 
the center.   When we examine the surface of the Sun, we are not observing 
where fusion happens, we are only seeing material that is heated by 
fusion. The fusion at the center of the Sun is like a huge hot plate that 
heats the rest of the Sun, which is then like a huge pot of water set on 
the hot plate.  When we look at the surface of the hot water in the pot, we 
are not seeing what is really doing the heating.

There is a debate going on now among scientists about how much fusion is 
going on inside the Sun.  Some scientists have measured a byproduct of the 
fusion reaction called neutrinoes, and they have not found nearly enough to 
explain the energy given off by the Sun. It looks like there is not much 
fusion going on within the Sun at present.  It is like having a pot of hot 
water on the hot plate and then noticing that the hot plate is turned off. 
Much work is being done to figure out what is really going on with the Sun.

Here are some links about how the Sun gets its energy and also one on 
trying to burn candles in space.


Everett Rubel

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