Thermonuclear Reactions
in the Sun
No Need for Confinement!
The enormous self-gravity of the sun holds
it together. There is no way the fuel can
escape (or the sun can blow itself apart).
So the nuclear reactions just keep ‘ticking
away’ in the hot central regions, slowly
and steadily releasing the energy that
keeps the sun in equilibrium.
A ‘Solar Model’
Turning Hydrogen into Helium

Atomic masses: Hydrogen 1, Helium 4

The Hydrogen nucleus is a proton

Helium contains two protons (positively
charged) and two neutrons (uncharged)
The Fundamental Effect
4 H  He + energy
(from E = mc2)
One Obvious Problem
How can 4 protons (4 H) merge to produce a
Helium with 2 protons and 2 neutrons?
The masses add up pretty well – although a little
bit vanishes -- but where do the extra positive
charges go?
(Remember that charge is conserved – it cannot
simply vanish!)
A Second Problem
What are the chances of four protons
running into each other all at the same
time to form a Helium nucleus?
Answer: slim to none.
A Frequent Occurence
Much Less Frequent…
Exploring The Analogy
Two-car accidents are much more common
than multiple-car collisions.
So too with the atomic nuclei in the stars.
A proton has a good chance of running into
another single proton, but not three at
once!
Instead: A Chain of Events
Another Visualisation
…and yet again:
The Proton-proton (pp) Chain
Important Things to Note
Net effect:
4 H  1 He + energy
(Note that only a fraction of the original mass is
converted to energy via E = mc2. Less than 1% of it
vanishes, with a big lump of He being left over.)
Positrons are created
Neutrinos are released
(more on this later!)
Meet The Positron:`Antimatter’
For every type of particle, there is an antiparticle.
When they meet, they annihilate completely and
are converted to energy.
They have equal masses. If they are charged,
they must have opposite charges (conservation
law).
The positron (+) is the anti-electron (-).
What Use Is a Positron?
It can’t be contained: it will annihilate with an
electron the instant they meet. We can’t build
vacuum chambers empty enough to prevent this
happening quickly!
But we use them every day in hospitals, in PET
scans (Positron Emission Tomography)
They meet electrons and create gamma rays.
PET in Diagnostics
But how do we get positrons into the body?
Not Just From Nuclear Reactions!
Positrons are also emitted by some naturally
radioactive isotopes. These can be included in
organic molecules and ingested, then go to the
tumour.
PET Scans
Radioactive elements
produce the positrons
right where we need
them!