Lecture on The Balmer Thermometer and Stellar Classification

Introduction to the Balmer Thermometer

• The concept relates to stellar temperature.
• Balmer Series:
  • Deals with hydrogen atoms in stars.
  • Transitions to/from the second energy level.
  • Emission Lines: Occur when electrons drop to the second energy level.
  • Absorption Lines: Occur when electrons in the second energy level are further excited.

The Balmer Series of Lines

• Composed of four lines in the visible spectrum due to transitions:
  • H-alpha: 3 to 2 (Red line)
  • H-beta: 4 to 2
  • H-gamma: 5 to 2 (Violet line)
  • H-delta: 6 to 2
• Significance: All lines are visible.

Temperature Effects on Balmer Lines

• Cool Stars:

  • Difficult to excite electrons to the second energy level.
  • Few emission lines due to lack of electrons in higher energy states.
  • Stars too cool: around 3000 Kelvin.

• Hot Stars:

  • Electrons are at energy levels higher than 2.
  • Few electrons transitioning to energy level 2.
  • Stars too hot: around 20000 Kelvin.

• Balmer Thermometer:

  • Concept that weak Balmer lines appear in both very hot and very cool stars.

Historical Classification of Stars

• Original Classification: Harvard System

  • Categorized stars by strength of Balmer lines (A to Z).
  • A stars: Strongest Balmer lines.
  • B, C, etc.: Gradually weaker lines.

• Modern Spectral Classification:

  • Reduced to seven letters: O, B, A, F, G, K, M.
  • Rearrangement: Based on temperature from hottest to coolest.
  • Color Coding:
    • O/B: Bluish-white
    • A: Pure white
    • F: White with hint of yellow
    • G (e.g., Sun): Yellow
    • K: Orange
    • M: Red

Mnemonics for Spectral Classification

• Mnemonic Example: "Oh Be A Fine Girl/Guy, Kiss Me."
• Encouragement to create personal mnemonics.

Subdivision of Spectral Classes

• Further detailed into ten subclasses (e.g., B0, B1, etc.).
• Allows precise differentiation in temperature and elemental abundance.

Stellar Spectrum and Surface Temperature

• Modern Implications:
  • Computers easily determine star temperature from spectra.
  • No need for detailed plotting of wavelength flux.

Conclusion

• Spectral Lines and Star Temperature:
  • More lines in cooler stars.
  • Strongest Balmer lines in A stars.
• Final Observation:
  • Spectra complexity increases as stars get cooler.
  • Visual representation of spectral classes from O to M available.