|MadSci Network: Astronomy|
Christian, 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 burning. 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. http://www.madsci.org/posts/archives/mar98/889980585.As.r.html http://www.madsci.org/posts/archives/mar97/853714295.Ph.r.html http://www.madsci.org/posts/archives/dec96/847580323.Ch.r.html http://zeta.lerc.nasa.gov/expr/candle.htm Regards, Everett Rubel
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