Star Formation (OCR A Level Physics)

• The life cycle of stars goes in predictable stages
• The exact route a star's development takes depends on its initial mass

Initial Stages for All Masses

• The first four stages in the life cycle of stars are the same for stars of all masses
• After these stages, the life-cycle branches depending on the whether the star is:
  • Low mass: stars with a mass between 0.5 and 10 times the mass of the Sun (0.5 M_Sun − 10 M_Sun)
  • High mass: stars with a mass more than about 10 times the mass of the Sun (> 10 M_Sun)

1. Nebula

• 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

2. Protostar

• 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

3. Nuclear Fusion

• Eventually, the temperature will reach 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
• 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

4. Main Sequence Star

• 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

Forces acting within a star. The centre red circle represents the star’s core and the orange circle represents the stars outer layers

• If the temperature of a star increases, the outward pressure will also increase
  • This will cause the star to expand

• If the temperature drops the outward pressure will also decrease
  • This will cause the star to contract

• As long as these two forces balance, the star will remain stable

• A star will spend most of its life on the main sequence
  • 90% of stars are currently on the main sequence
  • Main sequence stars can vary in mass from ~10% of the mass of the Sun to 200 times the mass of the Sun
  • The Sun has been on the main sequence for 4.6 billion years and will remain there for an estimated 6.5 billion years