E.5 Fusion and stars

UNIT SYLLABUS

SL/HL

Nuclear fusion in stars, the H-R diagram, and stellar evolution.

1 Key Formulas

Wien's Law

\lambda_{max} T = 2.9 \times 10^{-3} mK

Stellar Parallax

d = \frac{1}{p}

2 Exam Preparation & Topic Explanations

Stellar Characteristics and Evolution

Stars maintain stability through an equilibrium between inward gravitational forces and outward radiation pressure from nuclear fusion. The Hertzsprung–Russell (HR) diagram is a key tool that plots a star's luminosity against its surface temperature, classifying stars into regions such as the main sequence, red giants, super giants, and white dwarfs. A star's mass determines its evolutionary path; for example, a low to medium mass star will eventually become a red giant, shed a planetary nebula, and leave behind a white dwarf. Distances to nearby stars can be determined using stellar parallax.

Pro Exam Strategy

Remember that on the HR diagram, the temperature axis (horizontal) is plotted backwards; it decreases from left to right.
Use the stellar parallax formula $d = \frac{1}{p}$ to find distances, ensuring distance $d$ is in parsecs and the parallax angle $p$ is in arc-seconds.
A star's radius can be determined from its luminosity and surface temperature using the Stefan-Boltzmann law.

3 MCQ Practice

Q1. What is the sequence for the evolution of a main sequence star of about 2 solar masses?

Red super giant $\rightarrow$ supernova $\rightarrow$ neutron star
Red giant $\rightarrow$ planetary nebula $\rightarrow$ white dwarf
Red giant $\rightarrow$ supernova $\rightarrow$ white dwarf
Red super giant $\rightarrow$ planetary nebula $\rightarrow$ neutron star

4 Short Answer Questions

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E.4 Fission

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