Star Formation
Conditions for Fusion
- For nuclear fusion to occur, both nuclei must have sufficiently high kinetic energy to overcome the electrostatic repulsion between protons
- The conditions required to achieve this are:
- Very high temperature (on the scale of 100 million Kelvin)
- Very high pressure and density
- Four hydrogen nuclei (protons) are fused into one helium nucleus, producing two gamma-ray photons, two neutrinos and two positrons
- Massive amounts of energy are released
- The momentum of the gamma-ray photons results in an outward acting pressure called radiation pressure
Nuclear fusion of hydrogen nuclei to form helium nuclei
Equilibrium in Stars
- Once the core temperature of a star reaches millions of degrees kelvin and the fusion of hydrogen nuclei to helium nuclei begins
- The protostar’s gravitational field continues to attract more gas and dust, increasing the temperature and pressure of the core
- With more frequent collisions, the kinetic energy of the particles increases, increasing the probability that fusion will occur
- Eventually, when the core becomes hot enough and fusion reactions can occur, they will begin to produce an outward radiation pressure which balances the inward pull of gravity
- The star reaches a stable state where the inward and outward forces are in equilibrium
- As the temperature of the star increases and its volume decreases due to gravitational collapse, the gas pressure increases
- The gas pressure and the radiation pressure act outwards to balance the gravitational force (weight, F = mg) acting inwards
Equilibrium in stars occurs when the outward radiation pressure is balanced with the inward gravitational force
- If the temperature of a star increases, the outward pressure will also increase
- If outward pressure > gravitational force, the star will expand
- If the temperature drops the outward pressure will also decrease
- If outward pressure < gravitational force, the star will contract
- As long as these two forces are balanced, the star will remain stable
