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HSC Physics

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  1. Module 1: Kinematics
    1.1 Motion in a Straight Line
  2. 1.2 Motion on a Plane
  3. Module 2: Dynamics
    2.1 Forces
  4. 2.2 Forces, Acceleration and Energy
  5. 2.3 Momentum, Energy and Simple Systems
  6. Module 3: Waves and Thermodynamics
    3.1 Wave Properties
  7. 3.2 Wave Behaviour
  8. 3.3 Sound Waves
  9. 3.4 Ray Model of Light
  10. 3.5 Thermodynamics
  11. Module 4: Electricity and Magnetism
    4.1 Electrostatics
  12. 4.2 Electric Circuits
  13. 4.3 Magnetism
  14. Module 5: Advanced Mechanics
    5.1 Projectile Motion
  15. 5.2 Circular Motion
  16. 5.3 Motion in Gravitational Fields
    2 Topics
  17. Module 6: Electromagnetism
    6.1 Charged Particles, Conductors and Electric and Magnetic Fields
  18. 6.2 The Motor Effect
    1 Topic
  19. 6.3 Electromagnetic Induction
  20. 6.4 Applications of the Motor Effect
    1 Topic
  21. Module 7: The Nature of Light
    7.1 Electromagnetic Spectrum
    3 Topics
  22. 7.2 Light: Wave Model
  23. 7.3 Light: Quantum Model
    2 Topics
  24. 7.4 Light and Special Relativity
  25. Module 8: From the Universe to the Atom
    8.1 Origins of the Elements
    5 Topics
  26. 8.2 Structure of the Atom
    3 Topics
  27. 8.3 Quantum Mechanical Nature of the Atom
    2 Topics
  28. 8.4 Properties of the Nucleus
    2 Topics
  29. 8.5 Deep Inside the Atom
    4 Topics
Lesson 25, Topic 5
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Hertzsprung-Russel Diagram

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Hertzsprung Russel Diagram Colour

Hertzsprung-Russel diagrams draw relationships between the colour, temperature and luminosity of stars.

  • X-Axis: Colour / Temp, linked to Spectral Class
  • Y-Axis: Luminosity / Absolute Magnitude

Main sequence stars form a line from the top left to bottom right of the chart. The giants form bands near the top right, while white dwarfs are located in a line near the bottom left.

  • Main sequence stars undergo fusion of H –> He (proton-proton)
  • Heavier stars, i.e. giants, generally undergo CNO cycle fusion

Characteristics and Evolutionary Stage

Stars can be categorised into three main groups: main sequence, giants and white dwarves.

  • Main sequence stars include 90% of stars. They are cool and dim in the bottom right while hot and bright in the top right.
  • Giants tend to be massive in size and very bright despite their low temperatures. They are located in the upper-right.
  • White dwarves are small and dim but very hot. They form a line near the bottom-left.

Stars generally begin in the main sequence band. But once hydrogen in the core runs out, fusion stops which causes the core to contract. Temperatures rapidly rise, igniting an outer layer of hydrogen and causing the star to expand to a giant.

Rising temperatures allow the CNO cycle to occur until helium in the core depletes also. The core contracts again, causing temperatures to rise again also. An outer layer of helium ignites.

If the star is less than 8 solar masses, it will not reach a sufficient temperature to burn carbon, becoming unstable. It is now a white dwarf.

If the star is greater than 8 solar masses, fusion will continue until all fuel runs out, at which point it will either collapse into a blackhole or become a neutron star which undergoes a supernova.

Hertzsprung Russel Diagram Sizes