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 pressureThe outward force exerted by photons produced during nuclear fusion in a star's core. It acts against the inward pull of gravity..

From nebula to protostar

• All stars form from a giant cloud of hydrogen gas and dust called a nebula.
• Gravitational attraction between individual atoms forms denser clumps of matter. This inward movement of matter is called gravitational collapse.
• The gravitational collapse causes the gas to heat up and glow, forming a protostar. Work done on the particles of gas and dust by collisions between the particles causes an increase in their kinetic energy, resulting in an increase in temperature.
• Protostars can be detected by telescopes that can observe infrared radiation.
• Eventually, the temperature will reach millions of degrees kelvin and the fusion of hydrogen nuclei to helium nuclei begins.

Equilibrium in stars

• Once the core becomes hot enough and fusion reactions can occur, the star begins 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.
• The gas pressure and the radiation pressure act outwards to balance the gravitational force (weight, $F = mg$) acting inwards.
• 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.
• Crucially, as long as these two forces are balanced, the star will remain stable. This is the defining feature of a main sequence star.