Star Formation (OCR A Level Physics)

Revision Note

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Katie M


Katie M



Star Formation

  • 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 MSun − 10 MSun)
    • High mass: stars with a mass more than about 10 times the mass of the Sun (> 10 MSun)

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

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Katie M

Author: Katie M

Katie has always been passionate about the sciences, and completed a degree in Astrophysics at Sheffield University. She decided that she wanted to inspire other young people, so moved to Bristol to complete a PGCE in Secondary Science. She particularly loves creating fun and absorbing materials to help students achieve their exam potential.

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