The Sun: A Nuclear Powerhouse (Astronomy)

Recall cards on how the Sun actually shines. The nineteenth-century puzzle of the Sun's energy source: chemical burning would last only a few thousand years, and even Kelvin and Helmholtz's gravitational contraction could power it for only about 100 million years, far short of Earth's geological age. The answer came from Einstein's E = mc^2 and nuclear fusion: deep in the core, above about 12 million K, the proton-proton chain fuses roughly 600 million tons of hydrogen into helium each second, converting about 4 million tons of matter into pure energy. The solar interior in theory: hydrostatic equilibrium, the radiative and convective zones, and the hundreds of thousands of years a photon takes to escape while a neutrino leaves in seconds. And the observations that confirm it: the solar neutrino problem, neutrino oscillation, the Davis and Sudbury experiments, and helioseismology.