Chemical vs Nuclear Burning
Name: Rise P.
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
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
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.
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.
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
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.
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:
You were correct to point your student to nuclear reactions. The hydrogen
atoms of the sun undergo nuclear fusion, forming helium and releasing lots
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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Update: June 2012