E.5
Fusion & Stars
Nuclear fusion in stars, stellar equilibrium, the Hertzsprung-Russell diagram, stellar parallax, Wien's law, luminosity and stellar spectra.
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Key Concepts, Fusion & Stars
Nuclear Fusion in Stars
Fusion is the combining of light nuclei to form heavier ones, releasing energy. It is the energy source of all stars. In main-sequence stars like the Sun, the dominant process is the proton-proton chain: four hydrogen nuclei (protons) fuse through a series of steps to produce one helium-4 nucleus, two positrons, two neutrinos and gamma radiation. Energy is released because the helium-4 nucleus has greater binding energy per nucleon than hydrogen, so its mass is slightly less than four protons. This mass defect converts to energy via E = mc². Fusion requires extreme temperature (around 10⁷ K) and pressure to overcome electrostatic repulsion and allow the strong nuclear force to act.
Stellar Equilibrium
A star is in equilibrium between two competing forces. Gravity acts inward, trying to collapse the star under its own weight. Radiation pressure (from photons produced by fusion in the core) and thermal gas pressure act outward. In a main-sequence star these forces exactly balance, maintaining a stable size. This is self-regulating: if fusion temporarily increases, outward pressure grows, the star expands slightly, the core cools and fusion slows. This mechanism keeps main-sequence stars stable for billions of years.
The Hertzsprung-Russell Diagram
The HR diagram plots stellar luminosity (vertical axis, increasing upward) against surface temperature (horizontal axis, increasing to the left). Most stars lie on the main sequence: a diagonal band from hot, luminous blue stars (top left) to cool, dim red stars (bottom right). Red giants and supergiants occupy the upper right (high luminosity, low temperature, very large radius). White dwarfs are in the lower left (low luminosity, high temperature, tiny radius). Lines of constant radius run diagonally across the diagram, since L = 4πR²σT⁴.
Stellar Parallax and the Parsec
Stellar parallax measures the apparent shift in a star's position when viewed from opposite sides of Earth's orbit (baseline = 2 AU). The parallax angle p is half the total angular shift. Distance in parsecs: d = 1/p, where p is in arcseconds. One parsec (pc) is defined as the distance at which 1 AU subtends 1 arcsecond: 1 pc ≈ 3.09 × 10¹⁶ m ≈ 3.26 light years. Parallax is only reliable for nearby stars (up to a few hundred parsecs). For greater distances, the parallax angle becomes too small to measure accurately and other methods (Cepheid variables, standard candles) are needed.
Surface Temperature, Luminosity and Stellar Radius
A star's surface temperature is determined from its spectrum using Wien's displacement law: λ_max × T = 2.90 × 10⁻³ m K. Hot stars peak in the ultraviolet (blue-white); cool stars peak in the infrared (red). Once T is known and luminosity L is measured, the stellar radius R can be calculated from the Stefan-Boltzmann law: L = 4πR²σT⁴, rearranging to R = √(L / (4πσT⁴)). This allows astronomers to determine stellar sizes without any direct measurement of angular diameter.
Chemical Composition from Stellar Spectra
When light from a star's hot interior passes through the cooler outer atmosphere, atoms there absorb photons at specific frequencies matching their discrete energy level transitions. This produces dark absorption lines (a Fraunhofer spectrum) on the continuous background. Each element produces a unique pattern of lines, acting as a fingerprint. By comparing dark lines in a stellar spectrum to known laboratory spectra, astronomers identify which elements are present. The Sun's spectrum reveals hydrogen and helium as dominant, along with trace metals. Doppler shifts in these lines also reveal the radial velocity of the star.
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Frequently Asked Questions, IB Physics Fusion & Stars
What is Fusion & Stars in IB Physics? ↓
Nuclear fusion in stars, stellar equilibrium, the Hertzsprung-Russell diagram, stellar parallax, Wien's law, luminosity and stellar spectra. This topic is part of Theme E (Nuclear & Quantum Physics) in the current IB Physics syllabus.
Is Fusion & Stars SL or HL in IB Physics? ↓
Fusion & Stars is covered by both SL and HL students in the current IB Physics syllabus. HL students study additional depth and extension content beyond the SL core.
What equations do I need for IB Physics Fusion & Stars? ↓
The key equations for Fusion & Stars are covered in the concept tutorial above. For a structured set of notes with all equations, conditions and worked examples, the GradePod Exam Pack includes a revision note template for every topic.
What are common exam mistakes in IB Physics Fusion & Stars? ↓
Common mistakes are covered in detail in the exam technique video above. The GradePod Exam Pack also includes exam-style questions with mark schemes so you can see exactly how marks are awarded and where students typically drop them.
How do I revise Fusion & Stars for the IB Physics exam? ↓
Follow the GradePod three-step method. First, watch the concept tutorial and tick off each learning objective on the checklist above as you go. Second, watch the exam technique video to see how IB-style questions are answered under exam conditions. Third, use the Exam Pack to practise independently with knowledge questions, exam questions and mark schemes. That's it. It works. I promise.