CONTRACTION OF PRE-MAIN SEQUENCE STARS
- The interior heats due to gravitational contraction and radiates away this energy as black-body radiation
- At 106 K, fusion starts, pressure increases, and the star establishes hydrostatic equilibrium (the balance between gravity and gas pressure)
- As gravity pulls inwards (fusion releases energy, and maintains the core’s high temperature), gas pressure pushes outwards (high temperature prevents the star from collapsing under its own weight)
- When a star reaches hydrostatic equilibrium, it enters main sequence
* Energy produced more efficiently at core’s center
Difference Between Fission and Fusion
Nuclear Fission vs. Nuclear Fusion
Fission: splitting heavy nuclei into lighter ones (e.g. atomic bombs and nuclear reactors derive their energy from fission of uranium or plutonium)
Fusion: merging light nuclei into heavy nuclei (e.g. how stars shine, hydrogen bombs, “nuclear burning” – different from ordinary chemical burning processes)
Strong Nuclear Forces: protons in the nucleus repel by electrical forces, but strong nuclear forces, which can only occur at close distances, keep the atom together. As temperature rises, protons move faster. When 2 protons fuse, the output is 1 neutron, 1 positron, and 1 neutrino.
How Fusion Works: Proton-Proton Chain & CNO Cycle
Common Elements (and Their Isotopes) Involved in Fusion: ¹H (hydrogen) [1 proton], ²H (deuterium) [1 proton, 1 neutron], ³H (tritium) [1 proton, 2 neutrons], ³He (helium-3) [2 protons, 1 neutron], 4He (helium-4) [2 protons, 2 neutrons]
Step 1: 2 hydrogen nuclei –> deuterium nucleus => releases positron + neutrino
- Positron (e+): antimatter of electron
- Neutrino (ν): unchanged particle that only interacts very weakly with normal matter
Step 2: deuterium + hydrogen nuclei –> helium-3 => releases gamma ray
-> Repeat first two steps.
Step 3: 2 helium-3 –> helium-4 => releases two protons
Input: 6 protons
Output: 2 positrons, 2 neutrinos, 2 gamma rays, 1 helium nucleus, 2 protons
Net Output: 4 protons –> 1 helium-4 => releases 2 positrons, 2 neutrinos, 2 gamma rays
0.7% of the total mass of 4 protons is converted into energy, while 99.3% results in 1 helium nucleus. Some of the mass is converted into energy. Since E = mc², a little mass and release tremendous energy. While at rest, however, energy is equal to mass.
CNO (Carbon-Nitrogen-Oxygen) Cycle
The CNO Cycle is the main nuclear burning chain in main sequence stars hotter than the Sun. Using carbon as a catalyst to convert hydrogen into helium, the CNO cycle also converts 7% of hydrogen’s mass into energy; hydrogen fuses with carbon to form helium. 10% of the Sun’s nuclear fusion reactions is from the CNO Cycle. In 1967, Hans Bethe theorized on the energy production in stars.