The source of the Sun’s energy
In the 19th century contraction was thought to power the Sun, but it could not explain the age of the Earth. In 1920 Eddington suggested that hydrogen turning into helium might be the source, and in 1939 Bethe worked out the reactions. In the Sun’s core (T ≈ 15 million K) four protons become one helium nucleus through the proton–proton chain; the resulting nucleus has ≈ 0.7 % less mass than the four original protons – this mass defect becomes energy: E = Δm · c². Each second the Sun converts ≈ 600 million tonnes of hydrogen to helium, and ≈ 4 million tonnes of mass become radiation energy (L = 3.8 · 10²⁶ W). The reaction also produces neutrinos, which pass freely through the Sun and are detected on the Earth (Super-Kamiokande, SNO, 2001). Energy in the core takes tens or even hundreds of thousands of years to reach the surface. The Sun will burn hydrogen for ≈ 10 billion years, i.e. about 5 billion more, and then become a red giant.
Calculation: if 1 kg of mass were converted entirely into energy, for how many years could it light a 100 W lamp? (E : 100 : 3.15 · 10⁷). Discuss the result in class.