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 ProblemHow 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 AnalogyTwo-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 NoteNet 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 DiagnosticsBut 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 ScansRadioactive elementsproduce the positrons right where we needthem!