Fusion: How Stars Shine: Practice Problem Solutions

1. Joining small nuclei into larger ones. In the Sun, hydrogen (protons) becomes helium.

2. Protons repel each other (both positive), so they need very high speeds and pressure to come close enough for the strong nuclear force to join them.

3. About 15 million kelvins.

4. (1) Two protons fuse, making deuterium, a positron and a neutrino. (2) Deuterium fuses with a proton to make helium-3 and a gamma ray. (3) Two helium-3 nuclei fuse to make helium-4 and two free protons.

5. If the core heats up, fusion speeds up and the core expands and cools, slowing fusion. If it cools, it shrinks and heats up, speeding fusion again.

6. Almost massless particles made in fusion that pass out of the Sun in seconds. Detecting them shows what is happening in the core right now.

7. About 10 billion years in total.