Physics · nuclei · NEET
| Fuel | Fusion in stars: hydrogen (protons) | Fission on Earth: uranium / plutonium |
| Process | Light nuclei join into a heavier one | Heavy nucleus splits into lighter pieces |
| Condition needed | Very high temperature (about 1.5 x 10^7 K in Sun) | A slow neutron and enough mass (critical mass) |
| Product | Helium-4 (plus positrons, neutrinos, gamma) | Two medium fragments (plus neutrons) |
Fusion. The Sun joins light nuclei (protons) into a heavier nucleus (helium). Fission is the opposite - a heavy nucleus like uranium splits into lighter pieces. The Sun has no uranium fuel in its core; its fuel is hydrogen. So remember: stars = fusion, reactors on Earth = fission.
Two protons both carry positive charge, so they push each other away by Coulomb repulsion. To get close enough for the strong nuclear force to pull them together, they need very high kinetic energy. High temperature means fast-moving protons. This is why it is called thermonuclear fusion - heat drives it. Note: the Sun's core is actually cooler than the value needed for average protons, so only the fastest protons fuse.
No. There is no oxygen and no chemical burning. 'Burning' here just means nuclei are being used up and changed into helium. The energy comes from mass being converted to energy (E = mc^2), not from a chemical reaction. Fusion releases about a million times more energy per reaction than chemical burning.
From lost mass. Four hydrogen atoms have slightly more total mass than one helium-4 atom (plus the other products). This tiny missing mass (mass defect) turns into energy by E = mc^2. The helium-4 nucleus is more tightly bound, so binding energy is released.
Four protons cannot collide at the exact same instant - that is almost impossible. So the Sun builds helium gradually: two protons make deuterium, then deuterium plus a proton makes helium-3, then two helium-3 nuclei make helium-4. For the last step to run, the first three steps must each happen twice.
It is the set of fusion reactions in which the Sun turns hydrogen into helium. Four protons combine step by step to form one helium-4 nucleus, and about 26.7 MeV of energy is released.
About 26.7 MeV per helium-4 nucleus formed from four hydrogen nuclei.
Hydrogen in the Sun's core. As hydrogen is used up, it turns into helium.
Fusion that is made possible by very high temperature. The heat gives particles enough kinetic energy to overcome the Coulomb repulsion between positive nuclei. It is the energy source inside stars.
The core cools and the star collapses under gravity, which raises the temperature again. Near 10^8 K, helium nuclei fuse into carbon, and heavier elements can form up to around iron, near the peak of the binding-energy curve.
Iron sits near the peak of the binding-energy-per-nucleon curve. Fusing nuclei heavier than iron would need energy instead of releasing it, so ordinary stellar fusion cannot produce them.
