The energy source inside stars is nuclear fusion, which changes hydrogen into helium. The reaction pathway differs according to the star's mass and temperature.
Basic Knowledge
The basis of nuclear fusion is the reaction in which four hydrogen nuclei (protons ¹H) are converted into one helium nucleus (⁴He). The mass defect Δm is released as energy according to E = Δm·c².
The pp chain (proton–proton chain) is the main reaction in stars about the size of the Sun (temperatures of approximately 10 million K or lower). In outline:
¹H + ¹H → ²H + e⁺ + ν (positron emission and neutrino emission) → ²H + ¹H → ³He → ³He + ³He → ⁴He + 2¹H
The CNO cycle is a cycle in which carbon (C), nitrogen (N), and oxygen (O) act as catalysts and becomes efficient at 15 million K or higher. In stars more massive than 1.3 times the Sun's mass, the CNO cycle is the main reaction.
In both pathways, the same net reaction occurs: “4¹H → ⁴He + energy.” The mass defect in one fusion reaction is approximately 0.7% of the hydrogen mass (4×1.0078u → 4.0026u, Δm ≈ 0.029u), producing an enormous amount of energy.
Nuclear fusion (light atomic nuclei combine to form a heavier atomic nucleus, releasing energy corresponding to the mass defect)
pp chain (the main process in stars about the size of the Sun; chain reaction beginning with reactions between protons)
CNO cycle (the main process in hot, massive stars; C, N, and O circulate as catalysts)
Mass defect (difference in mass before and after nuclear fusion; converted into energy by E=mc²)
Neutrino (generated in the pp chain; highly penetrating and reaches Earth)
Hydrogen burning (general term for nuclear fusion occurring during the main-sequence stage; converts hydrogen into helium)
Further Study (Background and Significance)
The Sun fuses approximately 600 million tons of hydrogen every second, of which approximately 4.25 million tons become energy through the mass defect (E = Δm·c²: Δm ≈ 4.25×10⁹ kg/s, c = 3×10⁸ m/s → P ≈ 3.83×10²⁶ W).
Nuclear fusion has an energy density approximately 10 million times that of ordinary chemical reactions. Fusion reactors (ITER and JT-60SA) use the deuterium–tritium (D-T) reaction instead of the CNO cycle.
Helium burning (the triple-alpha process) begins during the red-giant stage. Three ⁴He nuclei become ¹²C (carbon). Subsequently, heavier elements (C, O, Ne, Si, ... up to Fe) are synthesized. Elements heavier than iron are synthesized through neutron capture (the s-process and r-process) during supernova explosions.
- Net nuclear-fusion reaction: 4¹H → ⁴He + energy
- Mass defect ≈ 0.7% of the hydrogen mass
- pp chain: low-temperature, low-mass stars at or below the Sun's mass
- CNO cycle: stars at least 1.3 times the Sun's mass and high-temperature stars
- Sun's luminosity ≈ 3.83×10²⁶ W
- Helium burning (triple-alpha process) produces carbon
- Elements heavier than iron are produced by neutron capture during supernova explosions
Points of Caution (Often Confused)
① Distinguish the pp chain (low temperature and low mass) from the CNO cycle (high temperature and high mass). ② The mass defect Δm is converted entirely into energy according to E = Δm·c²; this differs by orders of magnitude from the chemical-reaction equation E = ΔH. ③ CNO acts as a catalyst and is not consumed. ④ “Hydrogen burning” is not combustion (oxidation), but nuclear fusion.
Practice
- In what types of stars do the pp chain and CNO cycle mainly occur, respectively?
- What is produced from four hydrogen nuclei through nuclear fusion?
- State the equation relating mass defect and energy.