Lecture Notes: Hertzsprung-Russell Diagram (HR Diagram)

Introduction

• Topic: HR Diagrams in Astrophysics
• Goals:
  • Understand the general shape of the HR diagram
  • Determine the stellar life cycle from the HR diagram
  • Determine the axes and scales of the HR diagram

HR Diagram Overview

• Definition: A plot of the luminosity of stars against their surface temperature.
• Historical Context: Developed independently by Hertzsprung and Russell (1911-1913).
• Purpose: Provides information on various properties of stars.

Structure of the HR Diagram

• Axes:
  • X-axis: Surface temperature (or spectral class) with hot on the left (50,000 Kelvin) and cool on the right (2,500 Kelvin).
  • Y-axis: Brightness (absolute magnitude) with bright stars at the top (-10) and dim stars at the bottom (+15).
• Scales: Both axes are logarithmic and non-linear.

Stellar Groupings on the HR Diagram

• Main Regions:
  • Red Giants: Top right; bright but cool.
  • Main Sequence: Diagonal band; stars fusing hydrogen into helium.
  • White Dwarfs: Bottom left; hot but dim.

Main Sequence Stars

• Characteristics:
  • Long-lived phase where stars fuse hydrogen.
  • Our Sun is in this category with an absolute magnitude of +5 and 5700 Kelvin.
  • 90% of observable stars are in the main sequence.

Red Giants and Supergiants

• Characteristics:
  • Bright and cool with large surface area.
  • Example: Betelgeuse with magnitude -5 and 3700 Kelvin.
  • Fusion reactions beyond hydrogen-helium.
  • Supergiants: 10-100 times the Sun's mass, highly luminous; engage in fusion creating heavier elements.

White Dwarfs

• Characteristics:
  • Hot but dim with small surface area.
  • End stage of stellar life cycle with no active fusion.
  • Eventually cool to become black dwarfs.

Importance of the HR Diagram

• Significance: Reveals different star types and stellar life cycles.
• Stellar Life Cycle Stages:
  • Start on main sequence.
  • Transition to red giant phase.
  • End as white dwarfs.

Examination Focus

• Key Elements to Remember:
  • General shape and regions on the HR diagram.
  • Axis scales: -10 to +15 for magnitude, 50,000 to 2,500 Kelvin for temperature.
  • Position of the Sun and other reference stars like Vega and Betelgeuse.

Conclusion

• Understanding the HR diagram helps in studying stellar evolution and characteristics of different star types.
• Essential for AQA A-level Physics in the astrophysics topic.