Fusion and Stars

Introduction

Fusion and stellar physics explain how stars shine, evolve, and reveal their properties to observers. The guide connects nuclear fusion, hydrostatic equilibrium, HR diagrams, parallax, spectra, and stellar radius calculations.

Guide Focus

• Explain fusion conditions and the equilibrium of stars.
• Use HR diagrams to classify stars and describe evolution.
• Apply stellar parallax and luminosity-temperature-radius relationships.

Key Concepts

1. Stellar equilibrium

A stable star balances inward gravitational force with outward radiation pressure produced by fusion in the core.

2. Fusion conditions

Fusion requires high temperature and high density so nuclei can get close enough for the strong nuclear force to bind them after overcoming electrostatic repulsion.

3. Stellar evolution and mass

A star’s mass strongly influences its lifetime and evolutionary path. More massive stars burn fuel faster and can evolve into very different final states from lower-mass stars.

4. HR diagrams

HR diagrams plot luminosity vertically and surface temperature horizontally. Required regions include main sequence stars, red giants, supergiants, white dwarfs, the instability strip, and lines of constant radius.

5. Distance and radius

Stellar parallax gives distance in parsecs using d = 1 / p when p is in arcseconds. Stellar radius can be determined from luminosity and surface temperature using black-body radiation ideas.

Common Mistakes

• Reading HR diagram temperature in the wrong direction.
• Assuming all bright stars are close rather than intrinsically luminous.
• Forgetting to convert between AU, light years, and parsecs when required.

Exam Tips

• Use spectra to infer surface temperature and composition.
• For parallax, smaller angle means larger distance.
• Fusion and fission both involve binding energy changes, but fusion joins light nuclei while fission splits heavy nuclei.

Practice Questions

Question 1 (Multiple Choice)

A star with a smaller parallax angle is generally:

A. Farther away. B. Closer. C. Cooler. D. Less massive.

Question 2 (Structured Paper 2 Style)

A star has measured parallax 0.020 arcseconds.

(a) Calculate its distance in parsecs. [2 marks]

(b) State two properties that can be inferred from a stellar spectrum. [2 marks]