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Name: Rise P.
Status: educator
Age: 40s
Location: N/A
Country: N/A
Date: 2001


Question:
How does the sun keep burning when there is no oxygen? My student asked me this. I talked with him about nuclear reactions. He looked up fire in the World Book. He found that fire needs oxygen, fuel and heat. Can you explain this? Thank you. Rise P., teacher, Bloomington, IN


Replies:
The definition that you were looking at is, narrowly, chemical burning in air. Fire, more formally, is the phenomenon of atomic (molecular) recombination that is manifest by a new product and electromagnetic radiation. Chemical combustion just rearranges the atoms from parent material(s) to product material(s). The radiation given off is often light or infra-red (heat). Note that this does not require oxygen. Oxidation reactions are remarkably common, but by no means the only way to do this.

In nuclear burning, the atoms themselves are combined to create new and different atoms. Nuclear burning does not require oxygen. The main nuclear burning that is happening on the sun is a hydrogen-hydrogen process, forming helium. There are other processes, too, but this is the main one.

Some good sources:

A good conceptual physics source that is easy to read and will help you is: Paul Hewitt "Conceptual Physics" Addison-Wesley

George Abell "Exploration of the Universe"

Nathan A. Unterman


The nuclear reaction is fusion. The light atoms on the sun, such as Hydrogen and Helium, combine and expend energy in the form of light and heat. This is the same principle utilized in the hydrogen bomb. More peaceful uses are fusion reactors, which are still not yet developed despite more than 40 years of scientific research.

Dr. Harold W. Myron


The sun really does not burn in the "fire" oxidation sense. The sun is a big fusion reactor, like a hydrogen bomb going off continuously. If he likes looking things up in encyclopedias, try "nuclear fusion" or "hydrogen bomb" or things along those lines.

Nuclear reactions do not require oxygen to run, just fuel and energy.

Don Yee


Fire...which we commonly call burning consumes oxygen by combining oxygen in the air with the carbon in the wood releasing chemical bond energy stored in the cellulose (mostly carbon and hydrogen ). Stars do not "burn" in the same sense, since they do not use oxygen nor do they need it. The reaction is completely different. It is driven by the action of immense gravity on the mass of the young star...if there is enough mass, the gravity will crush the atoms together and with enough mass the atoms are crushed together enough to fuse them...every 4 hydrogen atoms are fuse to create 1 helium atom and since 4 hydrogen atoms weigh a little less than 1 helium atom the balance of mass is made up by the energy released. We compute the difference in mass converted to energy by the famous equation E=MC2

Peter Faletra Ph.D.
Assistant Director
Science Education
Office of Science
Department of Energy


Hello Rise -

Heat (infrared rays) is only one form of energy released by the sun. Light is another. In fact energy covering much of the electromagnetic spectrum comes from the sun. None of these radiations are formed by combustion... or, as your student calls it fire.

He or she is right. Combustion is an oxidation reaction. Heat is released when oxygen combines with a fuel... usually containing hydrogen or carbon and creating water, carbon monoxide, and/or carbon dioxide.

The energy coming from the sun is released from a nuclear reaction in which hydrogen is changed into helium (and some other heavier elements) by nuclear fusion - the combining of nuclei. To combine the nuclei of atoms requires a tremendous amount of energy, but when it is accomplished, it releases a great deal more.

A metaphor for this could be a car sitting close to the crest of a hill. With just a little push it goes over the hill and moves rapidly down the back side. A little push in the right circumstance creates a great deal of energy.

The real answer is that energy can be created by processes other than oxidation. The sun is an major example of that fact.

Larry Kengel


The energy from the Sun and certain other classes of stars comes from the conversion of hydrogen into helium. There are several steps involved, but the net effect is that a certain amount of the change in mass is converted into energy according to Einstein's famous equation: E = m*c^2. This fusion is far more energetic than any chemical combustion reaction that involves the oxidation of some fuel by oxygen. The term "fire" refers to the latter combustion reaction; the term "fusion" refers to this conversion of mass into energy. There are many sites that explain the solar fusion process at almost any level of sophistication you desire. One that I found to have a good balance of detail and clarity is the site:

http://starchild.gsfc.nasa.gov/docs/StarChild/shadow/questions/ question13.html

Vince Calder


You were correct to point your student to nuclear reactions. The hydrogen atoms of the sun undergo nuclear fusion, forming helium and releasing lots of energy in the form of heat and light. It is not combustion. You might remind your student that the filament in a typical light bulb gives off heat and light, in a bulb filled with stable gas (not oxygen). Combustion is clearly not the only reaction that can produce heat and light.

Paul Mahoney, Ph.D.



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