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’
Astrophysics is Highly Mathematical
Let’s Turn Hydrogen into Helium
4 H à He + energy (from E = mc2)
Protons are consumed, but a big lump of He is the result.
(Less than 1% of the mass actually vanishes completely! )
n 
Atomic masses: Hydrogen 1, Helium 4
n 
The Hydrogen nucleus is a proton
n 
Helium contains two protons (positively charged) and two
neutrons (uncharged)
One Obvious Problem
How can 4 protons (4 H) merge to produce a
Helium with 2 protons and 2 neutrons?
Where do the extra positive charges go?
(Charge is conserved – it cannot simply vanish!)
A Second (Less Obvious) Problem
What are the chances of four protons
running into each other all at the same
time to form a Helium nucleus?
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 Sequence of Events
Called the pp (proton-proton) Chain
(No need to memorize these steps! But let’s count the
protons in the figure to make sure that 4H à He)
Meet Two New Players
In the first step, two protons
merge as shown.
A positive charge is carried off by a positron
(symbolized here as β+). One proton turns into a neutron!
The resulting lump (1 proton + 1 neutron) is called a
deuteron. It is an isotope of ‘heavy Hydrogen’
A particle called a neutrino (ν) is also emitted. We will
learn much more about this in a later presentation.
After That…
The new deuteron merges with another proton to form a
“light” isotope of Helium (2 protons, 1 neutron) and
releasing a gamma ray (an energetic photon of light)
Later still, two isotopes of light Helium merge, yielding
‘regular’ Helium (and releasing two protons).
The net effect, as promised: 4 H à He + energy!!
Once Again
Another Visualisation
…and Yet Again:
The Critical Points
about the ‘p-p’ Cycle
Net effect:
4 H à 1 He + energy
(Again, 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!)
More on Positrons:`Antimatter’
For every type of particle, there is a corresponding 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 (-). When a positron
meets an electron, they annihilate and produce a pair of
gamma rays (radiant energy = light of very high energy).
Antimatter was Actually Predicted
before its Discovery
Antimatter
in Fiction
Remember E = mc2!
Question: could we ever
make, save, transport and
use antimatter -- say, as
an ultimate weapon?
The Practical Reality
Physicists at CERN managed to capture a small number (some
dozens) of antimatter particles for ~1000 seconds (about 16
minutes) in a special ‘magnetic trap’
So: 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 that are empty enough to
prevent this happening quickly!
But we use positrons every day in hospitals, in
PET scans (Positron Emission Tomography).
How??
Positrons are Formed
Not Only in Thermonuclear Reactions!
Positrons are also emitted by some naturally radioactive
isotopes.
These isotopes can be included in organic molecules (food
or drink) and ingested or injected, after which they
accumulate in (say) a tumour in the body.
PET Scans
As they decay, the
radioactive isotopes
then produce positrons
right where we need
them!
Positrons meet electrons,
and the tumour ‘lights up,’
glowing in gamma rays that
we can detect.
PET in Diagnostics
A Lesson For Us All
Positrons (‘anti-matter’) were predicted by the pure
theoretical mathematics of quantum mechanics, many
decades ago
There was no obvious ‘marketability’
Yet they now provide a life-saving medical methodology:
a totally unexpected spinoff of pure physics research
You just never know…