Quick questions on Stellar evolution and nucleosynthesis: HSC Physics Module 8

<!-- Diagram: Hertzsprung-Russell diagram | reviewed 2026-05-21 --> <svg class="fig" viewBox="0 0 540 340" role="img" aria-labelledby="hr-t hr-d"> <title id="hr-t">Hertzsprung Russell diagram</title> <desc id="hr-d">A schematic Hertzsprung Russell diagram. Luminosity in solar units increases on the y axis. Surface temperature in kelvin decreases to the right. The main sequence runs diagonally from upper left to lower right with the Sun marked in the middle.

The first stages are the same (main sequence, red supergiant), but the higher core mass allows successive ignitions of heavier elements:

The continuous part of a stellar spectrum is a near-blackbody curve from the dense photosphere (see the Module 7 dot point on spectra and stars). The cooler outer atmosphere imprints absorption lines whose pattern reveals composition and (with line-ratio analysis) temperature. Nebulae and hot rarefied gas glow with emission lines instead. Together these spectra are the observational tool by which stellar nucleosynthesis is checked: the predicted abundances of elements in the surfaces of stars (and in interstellar gas clouds) can be matched against observations.

Sketch the main features of the Hertzsprung-Russell (H-R) diagram and identify the main sequence, red giant and white dwarf regions. [3 marks]

Calculate the surface temperature of a star whose spectrum peaks at $\lambda_{\max} = 290 \text{ nm}$. [2 marks]

A $20 M_\odot$ star ends its life as a supernova. (a) State the heaviest element typically formed by core nuclear fusion before collapse. (b) Explain why elements heavier than iron require the r-process.