So if gravity can't power the sun, how about
processes inside an atom? We have two choices here:
1. Chemical reactions
(ie reactions dealing with atomic bonds between atoms and
2. Nuclear reactions (reactions between atomic nuclei)
- Typical energy per atom of around 10
(the amount of energy stored in the atomic levels of hydrogen
- Not enough energy to power the Sun for very
So time for a little NUCLEAR PHYSICS (woo-hoo!)
- Typical energies are
of times larger than for chemical reactions.
- Lots of energy
Inside the Atom
Let's look at Hydrogen and Helium (98% of the Sun)
At the center of the Sun, the temperature is high
enough that Hydrogen and Helium (and just about everything else)
is ionized -- the electrons
are no longer bound
(attached) to the atoms. We just deal with bare nuclei.
Let's do a few definitions to make life easier.
1 AMU = 1.660540x10-27
Einstein into the picture. Einstein realized that E=mc2. Mass
and energy are equivalent, related by the speed of light, c.
First define the atomic
as being 1/12 of the mass of the carbon-12 atom:
So we can also talk about mass in terms of
1 AMU =~ 931.5
(where 1 eV = 1.6x10-19 Joules)
Okay, now how does ionization work? If we add
the atom, we break apart the electron from the proton. Let's
this from a mass perspective.
Since a hydrogen atom is simply a proton plus and
electron, the mass of the hydrogen atom should simply be equal
to the mass if the proton
plus the mass of the electron, right?
M(hydrogen) - M(proton) - M(electron) = -13.6 eV
What? What is this energy difference? It is the binding energy of the hydrogen
In other words the correct (schematic) equation
= proton + electron
+ energy = proton + electron
What about nuclear
Instead of ionizing atoms, let's look at fusing
A hydrogen nucleus is simply a proton.
A helium nucleus is two protons and two neutrons.
We can make helium by fusing together 4 hydrogen
atoms. But look:
4 x M(hydrogen) - M(helium) = 26.71 MeV.
Compare this to what happened when we ionized
So this time our schematic
- The energy difference is MeVs, not eVs.
- The energy left over is positive: energy is produced.
hydrogen + hydrogen
+ hydrogen + hydrogen = helium +
So every time you fuse 4
atoms together to make helium, 26.7 MeV is released. This is
about 0.7% of the mass of the 4 original hydrogen atoms.
But is it enough to
Let's estimate how long the
shine by fusing hydrogen into helium.
How much energy is that?
Oooh, now we're cooking...
- The Sun was originally
- The Sun can only
inner 10% into helium.
Nothing is free -- what's the problem
There are two forces acting inside atoms:
(what are the other two
- The electromagnetic
- Long range
- Coulomb repulsion/attraction
- The strong nuclear
- Very short range
- Holds the nucleons (protons + neutrons)
Protons have positive charge. Like charges repel
-- the electromagnetic force.
We need to overcome this repulsion to have
How do we do this? Energetic
How do we make energetic nuclei?