Star Formation (DP IB Physics): Revision Note

Katie M

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Katie M

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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

5-10-2-nuclear-fusion_ocr-al-physics

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

hydrostatic-equilibrium, IGCSE & GCSE Physics revision notes

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

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Katie M

Author: Katie M

Expertise: Physics Content Creator

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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