Layers of the sun
¾Core
¾Rad zone
¾Convection
zone
¾Photosphere
¾Chromosphere
¾Corona
¾Solar wind
Proton-Proton chain
¾Main
source of
fusion in core
¾Two protons
come together
¾One proton
changes to
neutron,
forming
deuterium
¾Note that a
positron and
neutrino are
ejected
Proton-Proton chain-continued
¾New
proton
strikes
deuterium—
result is tritium
and a gamma
ray
¾Tritium
nuclei
collide forming
helium and two
free protons
¾Check
out: http://burro.cwru.edu/Academics/Astr221/StarPhys/ppchain.html
Proton-Proton chain
¾Net
result:
¾Four protons
produce
¾Two positrons
¾Two neutrinos
¾Two gamma
rays
¾One helium
nucleus!
If you weigh the helium nucleus and the
positrons and electrons, they weigh about
one percent (.007) as much as the four
protons.
The difference in mass (Δm) is radiated
away as energy via E = Δmc2.
The energy is in the form of kinetic
energy (i.e. heat energy) and
electromagnetic energy (gamma rays).
How much mass is radiated into space?
¾The mass difference
between the four hydrogen
and one helium nucleus is
about 0.7% the mass of four
hydrogen atoms
¾To provide the energy
output of the sun, this
means that approximately
four million tons of mass
disappear each second in
the sun
¾or about ten billion metric
tons per hour of hydrogen
disappear, becoming
energy. This is about the
mass of Mount Shasta!
•The core of the sun, however, has enough mass
to sustain this rate of fusion for at least another
five billion years…
•The helium created, however, will prove to be the
sun’s undoing before the hydrogen runs out!
Proton-Proton chain—as a reaction
¾Note that the first step
above takes about one
billion years per proton to
occur
(positron)
(hydrogen)
(Deuterium)
(neutrino)
¾Since there are well over a
billion,billion,billion (1027)
protons in the core of the
sun compressed to nearly a
million, billion atmospheres
of pressure, 600 million tons
of hydrogen fuse each
second!
¾In the lab, this is done by
starting with deuterium and
Tritium, separated from
“normal” isotopes of
hydrogen and helium at
great cost.
(Tritium)
(gamma ray photon)
(Helium)
•In lab, on earth, we can’t achieve these densities
and pressures, so this first step is virtually
impossible to achieve.
•Even with this “shortcut” Much more energy is put
(in the form of microwaves for heating, and
magnetic fields for confinement) than comes out
in in the form of heat. Fusion as a power source
on earth is far from becoming a reality
Of course, we can let the sun perform
¾As a result of the
fusion for us!
fusion process, about
1,400 Joules per
second (1400 watts)
strike each square
meter of the earth’s
atmosphere.
¾About half of this is
reflected or absorbed
by the atmosphere,
leaving about 800
watts per square
meter to strike the
ground.
¾Solar
panels have
about 15% efficiency, so
between 60 and 120
watts of power can be
generated by each
square meter—
depending on the angle
to the sun .
A typical three bedroom home has an
roof area of about 200 square meters.
If only half covered with Solar panels,
this would generate about 10,000
watts of power while the sun was
shining, or 5 kilowatts averaged day
and night—about twice the average
energy used by the occupants!
Back in side the sun, what happens to the
particles created?
¾Gamma
rays
random walk
out of core and
rad zone,
loosing energy
¾Process takes
about one
million years!
http://www.lactamme.polytechnique.fr/Mosaic/image
s/SOLE.31.0512.D/display.html
This calculation implies that the
photons you see and feel today were
created about one million years
ago…
If the sun had been slowing down its
rate of fusion for the last one million
years, would we know about it before
it was too late?
http://outreach.atnf.csiro.au/
What about the neutrinos?
Since Neutrinos have no charge, they
don’t “feel” the presence of the
electrons and protons in the sun, and
pass out into space within seconds of
their creation. Traveling at, or very
near the speed of light, they reach the
earth eight minutes later.
A Neutrino Telescope would allow
astronomers to peer into the core of the
sun and see what reactions are taking
place today.
Such detectors exist today, though they
are in their early stages. The interior of
the “Super K” detector is shown here
being cleaned. For more information
on Neutrinos, check out: What is a
Neutrino
Early experiments with neutrino
detectors found 1/3 as many coming
from the sun as expected. Two
theories exist--either neutrinos
oscillate between three different
“flavors” on their way from the sun, or
the sun’s rate of fusion has decreased
over the last million years. Which do
you think it is?