Layers of the sun

1. Layers of the sun Core Rad zone Convection zone Photosphere Chromosphere Corona Solar wind

2. Proton-Proton chain Main source of fusion in core Two protons come together One proton changes to neutron, forming deuterium Note that a positron and neutrino are ejected

3. Proton-Proton chain-continued New proton strikes deuterium—result is tritium and a gamma ray Tritium nuclei collide forming helium and two free protons • Check out: http://burro.cwru.edu/Academics/Astr221/StarPhys/ppchain.html

4. Proton-Proton chain Net result: Four protons produce Two positrons Two neutrinos Two gamma rays One helium nucleus! The difference in mass (Dm) is radiated away as energy via E = Dmc2. The energy is in the form of kinetic energy (i.e. heat energy) and electromagnetic energy (gamma rays). If you weigh the helium nucleus and the positrons and electrons, they weigh about one percent (.007) as much as the four protons.

5. How much mass is radiated into space? The mass difference between the four hydrogen and one helium nucleus is about 0.7% the mass of four hydrogen atoms To provide the energy output of the sun, this means that approximately four million tons of mass disappear each second in the sun or 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!

6. (positron) Proton-Proton chain—as a reaction (hydrogen) (Deuterium) (neutrino) (gamma ray photon) (Tritium) Note that the first step above takes about one billion years per proton to occur Since 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! In the lab, this is done by starting with deuterium and Tritium, separated from “normal” isotopes of hydrogen and helium at great cost. (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

7. Of course, we can let the sun perform fusion for us! As a result of the fusion process, about 1,400 Joules per second (1400 watts) strike each square meter of the earth’s atmosphere. About half of this is reflected or absorbed by the atmosphere, leaving about 800 watts per square meter to strike the ground. Solar 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!

8. Back in side the sun, what happens to the particles created? http://www.lactamme.polytechnique.fr/Mosaic/images/SOLE.31.0512.D/display.html Gamma rays random walk out of core and rad zone, loosing energy Process takes about one million years! 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/

9. 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?