Reading Starlight: Spectra and Spectral Classification

Learning Goals

1. Explain the OBAFGKM spectral sequence as a temperature sequence, not a composition sequence.
2. Describe why hydrogen absorption lines are strongest around spectral type A rather than in the hottest or coolest stars.
3. Use spectral type and luminosity class together to identify a star’s evolutionary state.

Reading anchor: OpenStax Astronomy 2e, Chapter 17.1-17.4 (the brightness/spectra of stars and spectral classification) and Chapter 5.5 (formation of spectral lines).

Why This Matters

Annie Jump Cannon and colleagues at Harvard classified hundreds of thousands of stellar spectra by eye in the early twentieth century, originally in an alphabetical sequence (A, B, C, ...) that seemed to track line strength. Only later did Cecilia Payne-Gaposchkin show, in her 1925 PhD thesis, that the sequence is really a temperature sequence and that stars are overwhelmingly hydrogen and helium — a result initially doubted because it contradicted the prevailing assumption that stars had roughly Earth-like compositions.

Opening Phenomenon

Vega (spectral type A0V, T_eff ≈ 9602 K) shows very strong hydrogen absorption lines. The Sun (G2V, T_eff ≈ 5778 K) shows much weaker hydrogen lines but strong lines of ionized calcium. Rigel (B8Ia, T_eff ≈ 12100 K) shows weaker hydrogen lines than Vega despite being hotter. All three stars are overwhelmingly hydrogen by mass.

First question: what here is directly observed, and what is inferred from a physical model?

Evidence We Need to Explain

• Hydrogen (Balmer) absorption lines peak in strength around spectral type A (T_eff ≈ 9,000-10,000 K), not at the hottest O stars or the coolest M stars.
• The hottest O and B stars show strong ionized helium lines instead, because their higher temperatures ionize most hydrogen atoms, removing the bound electron needed to produce a Balmer absorption line.
• The coolest K and M stars show strong molecular bands (e.g., titanium oxide) because low temperatures allow molecules to survive without being dissociated by collisions.

Model

• Spectral type is set by surface temperature: which atomic transitions are visible depends on what fraction of atoms are in the right excitation and ionization state to absorb a given wavelength, and that fraction is a strong, non-monotonic function of temperature.
• A star’s line strengths therefore measure temperature (and, more weakly, pressure/density) even though essentially all stars share the same dominant composition (about 70% hydrogen, 28% helium by mass).
• Luminosity class (from line widths, sensitive to surface gravity) distinguishes a main-sequence dwarf from a giant or supergiant of the same temperature, resolving the ambiguity that temperature alone cannot fix size.

Quantitative Tool

Worked Example

1. Rank by T_eff: Rigel (12100 K, B8Ia) > Vega (9602 K, A0V) > Sun (5778 K, G2V) > Betelgeuse (3600 K, M1-2Ia).
2. Yet Balmer line strength peaks at Vega’s A0 type, not at hotter Rigel: above about 10,000 K, collisions with free electrons increasingly ionize hydrogen past neutral, removing the electron needed for a Balmer transition.
3. Below about 5,000-6,000 K (Sun and cooler), most hydrogen remains in its unexcited ground state, unable to absorb visible-light Balmer photons, so Balmer lines weaken again even though the gas is still overwhelmingly hydrogen.
4. This non-monotonic behavior is exactly why the historical alphabetical line-strength ordering (A, B, C, ...) had to be reshuffled into the temperature-ordered OBAFGKM sequence once the physics were understood.

Common Misconception

Assuming that strong hydrogen lines mean "more hydrogen" and weak hydrogen lines mean "less hydrogen." Line strength primarily reflects excitation/ionization state (i.e., temperature), not elemental abundance.

Active Learning Segment

Given three unlabeled spectra (strong Balmer lines; strong ionized helium lines; strong molecular bands), assign each to hot/intermediate/cool without additional information, and justify each assignment.

Lab Connection

Lab 01 uses each sample star’s spectral type and T_eff to place it correctly on the Hertzsprung-Russell diagram’s temperature axis.

Synthesis

The OBAFGKM sequence orders stars by temperature, using ionization and excitation physics, not composition, to explain why line strengths rise and fall non-monotonically across it.

References

• OpenStax Astronomy 2e, Chapter 17.1-17.4 (the brightness/spectra of stars and spectral classification) and Chapter 5.5 (formation of spectral lines).
• Local reference copy: references/openstax-astronomy-2e-extracted.txt.
• Course dataset used in this lecture’s worked example: materials/ASTR230/data/.