• Process of forming a heavier nucleus by binding two or more nuclei


• Two protons repel each other due to electric force; at high enough temperature (>10 million K) two protons can collide fast enough to overcome the electric force ("Coulomb barrier"), and fuse when the strong force takes over.


• Proton-Proton chain: hydrogen fusion that powers the Sun


• Extremely efficient way to generate energy:


Quiz 11A: fusion power...

• "Solar neutrino problem": solar neutrinos are low mass subatomic particles resulting from nuclear fusion, detected using underground "telescopes". They are difficult to detect, but neutrinos are direct a probe of fusion process deep inside the Sun. Only about 1/3 of the predicted number actually detected.


• Every element heavier than hydrogen (e.g. carbon and oxygen you are made of) is formed through nuclear fusion!!!


• Q: how is fusion different from fission?

Hydrostatic Equilibrium

• structure of the Sun depends on a balance between its internal forces


• Unless balanced by some force or pressure, the Sun will collapse on itself under its own gravity


• The Sun has not, thus a source of pressure exists


• higher pressure inside ==> higher T inside

• Solar Cycle and Climatic Changes:
• correlation between the solar cycle and earth temperature
• hypothesis: solar wind changes Earth's upper atmosphere, its temperature structure, and this in turn changes the atmosphere's circulation (e.g., ozone layer)
• Maunder minimum: few sunspots were observed between 1645 and 1715, coincident with abnormally cold winters in Europe and North America ("mini ice age").
• Greenhouse effect or increased solar activity?

• Solar Cycle and Space weather
• solar storms and Aurora (northern/southern light)
• fluctuations in atmospheric layer