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
I still couldn't absorb your education on chemical reactions obeying E = mc^2.
Mass and Energy
Michael Fowler
University of Virginia
Physics 252 Home Page
Link to Previous Lecture.
Rest Energy
The fact that feeding energy into a body increases its mass suggests that the mass m0 of a body at rest, multiplied by c2, can be considered as a quantity of energy. The truth of this is best seen in interactions between elementary particles. For example, there is a particle called a positron which is exactly like an electron except that it has positive charge. If a positron and an electron collide at low speed (so there is very little kinetic energy) they both disappear in a flash of electromagnetic radiation. This can be detected and its energy measured. It turns out to be 2m0c2 where m0 is the mass of the electron (and the positron).
Thus particles can "vaporize" into pure energy, that is, electromagnetic radiation. The energy m0c2 of a particle at rest is called its "rest energy". Note, however, that an electron can only be vaporized by meeting with a positron, and there are very few positrons around normally, for obvious reasons-they just don't get far. (Although occasionally it has been suggested that some galaxies may be antimatter!)
Einstein's Box
An amusing "experiment" on the equivalence of mass and energy is the following: consider a closed box with a flashlight at one end and light-absorbing material at the other end. Imagine the box to be far out in space away from gravitational fields or any disturbances. Suppose the light flashes once, the flash travels down the box and is absorbed at the other end.
Now it is known from Maxwell's theory of electromagnetic waves that a flash of light carrying energy E also carries momentum p = E/c. Thus, as the flash leaves the bulb and goes down the tube, the box recoils, like a gun, to conserve overall momentum. Suppose the whole apparatus has mass M and recoils at velocity v. Of course, v << c.
Then from conservation of momentum:
.
After a time t = L/C the light hits the far end of the tube, is absorbed, and the whole thing comes to rest again. (We are assuming that the distance moved by the box is tiny compared to its length.)
How far did the box move?
It moved at speed v for time t, so it moved distance vt = vL/c.
From the conservation of momentum equation above, we see that v = E/Mc, so the distance d the box moved over before stopping is given by:
.
Now, the important thing is that there are no external forces acting on this system, so the center of mass cannot have moved!
The only way this makes sense is to say that to counterbalance the mass M moving d backwards, the light energy must have transferred a small mass m, say, the length L of the tube so that
and balance is maintained. From our formula for d above, we can figure out the necessary value of m,
so
.
We have therefore established that transfer of energy implies transfer of the equivalent mass. All we had to assume was that the center of mass of an isolated system, initially at rest, remains at test if no external forces act, and that electromagnetic radiation carries momentum E/c, as predicted by Maxwell's equations and experimentally established.
But how is this mass transfer physically realized? Is the front end of the tube really heavier after it absorbs the light? The answer is yes, because it's a bit hotter, which means its atoms are vibrating slightly faster -- and faster moving objects have higher mass.
Mass and Potential Energy
Suppose now at the far end of the tube we have a hydrogen atom at rest. As we shall discuss later, this can be thought of as a proton with an electron bound to it by electrostatic attraction, and it is known that a flash of light having total energy 13.6eV is just enough to tear the electron away, so in the end the proton and electron are at rest far away from each other. The energy of the light was used up dragging the proton and electron apart -- that is, it went into potential energy. Yet the light is absorbed by this process, so from our argument above the right hand end of the tube must become heavier. That is to say, a proton at rest plus a (distant) electron at rest weigh more than a hydrogen atom by E/c2, with E equal to 13.6eV. Thus, Einstein's box forces us to conclude that increased potential energy in a system also entails the appropriate increase in mass.
It is interesting to consider the hydrogen atom dissociation in reverse -- if a slow moving electron encounters an isolated proton, they may combine to form a hydrogen atom, emitting 13.6eV of electromagnetic radiation energy as they do so. Clearly, then, the hydrogen atom remaining has that much less energy than the initial proton + electron. The actual mass difference for hydrogen atoms is about one part in 108. This is typical of the energy radiated away in a violent chemical reaction -- in fact, since most atoms are an order of magnitude or more heavier than hydrogen, a part in 109 or 1010 is more usual. However, things are very different in nuclear physics, where the forces are stronger so the binding is tighter. We shall discuss this later, but briefly mention an example-a hydrogen nucleus can combine with a lithium nucleus to give two helium nuclei, and the mass shed is 1/500 of the original. This reaction has been observed, and all the masses involved are measurable. The actual energy emitted is 17 MeV. This is the type of reaction that occurs in hydrogen bombs. Notice that the energy released is at least a million times more than the most violent chemical reaction.
Final example -- give a ballpark estimate of the change in mass of a million tons of TNT on exploding. The TNT molecule is about a hundred times heavier than the hydrogen atom, and gives off a few eV on burning. So the change in weight is of order 10-10 x106 tons, about a hundred grams. In a hydrogen bomb, this same mass to energy conversion would take about fifty kilograms of fuel.
http://musr.physics.ubc.ca/~jess/p200/emc2/node4.html
BELIEVE ME NOT! - - A SKEPTICs GUIDE