Spectral Classification of Stars

Why This Matters

Spectral classification is the astronomer's fingerprinting system—it transforms the light from distant stars into a detailed profile of their temperature, composition, size, and evolutionary stage. You're being tested on your ability to connect observable properties (color, spectral lines, luminosity) to physical characteristics (temperature, mass, chemical makeup) and ultimately to evolutionary status. The classification schemes aren't arbitrary; they encode the physics of stellar atmospheres and nuclear processes.

When you see a star labeled G2V, you should immediately understand what that tells you about its temperature, size, and where it sits on the H-R diagram. Exam questions will ask you to move fluidly between classification systems, spectral features, and physical interpretations. Don't just memorize the sequence O-B-A-F-G-K-M—know why different spectral lines dominate at different temperatures and how luminosity class reveals a star's evolutionary state.

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Temperature and the Harvard Sequence

The Harvard classification system organizes stars by surface temperature, which directly controls which spectral lines appear strongest. At higher temperatures, atoms become increasingly ionized, changing which electron transitions are possible and therefore which absorption lines dominate the spectrum.

Harvard Spectral Classification (O, B, A, F, G, K, M)

• Temperature is the organizing principle—O-type stars exceed 30,000 K while M-type stars fall below 3,700 K, spanning nearly a factor of ten in surface temperature
• Each class shows characteristic absorption lines based on which atoms and ions can exist at that temperature; remember the mnemonic "Oh Be A Fine Girl/Guy, Kiss Me"
• The sequence reflects ionization physics—helium lines dominate hot O and B stars, hydrogen peaks in A stars, and molecular bands appear only in cool K and M stars

Temperature Sequence and Ionization States

• The sequence runs from hottest to coolest: O (>30,000 K) → B (10,000–30,000 K) → A (7,500–10,000 K) → F (6,000–7,500 K) → G (5,200–6,000 K) → K (3,700–5,200 K) → M (<3,700 K)
• Ionization equilibrium determines line strength—hydrogen lines peak at ~10,000 K (A stars) not because hydrogen is most abundant there, but because that's where the optimal excitation conditions exist
• Subdivisions (0-9) provide finer temperature resolution; G2 is hotter than G8, with the Sun classified as G2

Stellar Color and Temperature

• Color directly indicates temperature—hot stars appear blue-white, cool stars appear orange-red, following blackbody radiation principles
• Color indices like B-V quantify this relationship; negative B-V indicates blue (hot) stars, positive B-V indicates red (cool) stars
• Wien's displacement law connects peak wavelength to temperature: $\lambda_{max} = \frac{2.898 \times 10^{-3} \text{ m·K}}{T}$

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Spectral Lines as Diagnostic Tools

Absorption and emission lines in stellar spectra act as chemical and physical fingerprints. Each element produces a unique pattern of lines at specific wavelengths, and the strength of these lines depends on temperature, pressure, and abundance.

Spectral Lines and Their Significance

• Line patterns are element-specific—the wavelengths of absorption lines identify which elements are present in the stellar atmosphere
• Line strength and width reveal physical conditions; broader lines indicate higher pressure or rotation, stronger lines suggest optimal excitation temperature
• Doppler shifts in spectral lines reveal stellar motion toward or away from us, enabling radial velocity measurements

The Balmer Series

• Hydrogen's visible spectrum transitions ($n = 2$ to higher levels) produce the Balmer series: $H\alpha$ (656 nm, red), $H\beta$ (486 nm, blue-green), $H\gamma$ (434 nm, violet)
• Maximum strength in A-type stars (~10,000 K) where hydrogen atoms are excited but not fully ionized
• Critical for classification—Balmer line strength is a primary criterion distinguishing A stars from neighboring spectral types

Metallic Lines Across Spectral Types

• "Metals" in astronomy means all elements heavier than helium; their lines strengthen in cooler stars where atoms remain neutral
• F, G, and K stars show prominent metal lines including calcium H and K lines, iron lines, and sodium D lines
• Metal abundance variations reveal stellar population and galactic chemical evolution—Population II stars show weaker metal lines than Population I

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Luminosity and the Two-Dimensional Classification

Temperature alone doesn't fully describe a star. The Morgan-Keenan system adds luminosity class, creating a two-dimensional classification that places stars precisely on the H-R diagram. Stars of the same temperature can have vastly different luminosities depending on their size and evolutionary stage.

Luminosity Classes (I to V)

• Class V (main sequence) includes most stars, fusing hydrogen in their cores; the Sun is a main-sequence star
• Classes III-IV (giants and subgiants) are evolved stars with expanded envelopes and higher luminosity than main-sequence stars of the same temperature
• Classes I-II (supergiants and bright giants) are the most luminous, representing massive stars in late evolutionary stages or intermediate-mass stars after core helium exhaustion

The Morgan-Keenan (MK) System

• Combines spectral type and luminosity class—a complete classification like G2V tells you temperature (~5,800 K) and evolutionary status (main sequence)
• Line width distinguishes luminosity classes—giants have narrower lines than dwarfs at the same temperature due to lower atmospheric pressure
• Standard stars define each class—classification is done by comparing unknown spectra to well-characterized reference stars

The Hertzsprung-Russell Diagram

• Plots luminosity vs. temperature (or spectral type), revealing stellar populations and evolutionary tracks
• Main sequence runs diagonally from hot, luminous O stars to cool, dim M dwarfs; giants and supergiants occupy the upper right
• White dwarfs appear in the lower left—hot but dim due to their tiny size; they represent stellar remnants

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Mass, Evolution, and Classification Changes

Spectral classification isn't static—stars change their position in classification space as they evolve. A star's mass determines its evolutionary path, and that path is traced through changing spectral types and luminosity classes.

Mass-Luminosity Relationship

• Luminosity scales steeply with mass: approximately $L \propto M^{3.5}$ for main-sequence stars
• High-mass stars burn hot and fast—an O-type star exhausts its fuel in millions of years, while an M-dwarf can shine for trillions of years
• This relationship only applies to main-sequence stars—giants and supergiants have evolved off this correlation

Stellar Evolution and Classification

• Stars move through spectral classes as they evolve; a massive star might begin as O-type and end as a red supergiant (M-type, class I)
• Post-main-sequence evolution increases luminosity and typically decreases temperature, moving stars rightward and upward on the H-R diagram
• Luminosity class tracks evolutionary stage—a star progresses from class V to IV to III as it exhausts core hydrogen and expands

Spectroscopic Parallax

• Distance determination method using spectral classification to estimate absolute magnitude, then comparing to apparent magnitude
• Distance modulus equation: $m - M = 5 \log_{10}(d) - 5$, where $d$ is distance in parsecs
• Relies on accurate classification—errors in luminosity class lead to large distance errors, especially for giants vs. dwarfs

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Unusual and Extreme Stars

Not all stars fit neatly into the standard classification scheme. These outliers reveal extreme physical conditions and rare evolutionary stages. Peculiar stars and exotic spectral types test the boundaries of classification systems and probe unusual stellar physics.

Wolf-Rayet Stars

• Massive, hot stars with strong stellar winds showing broad emission lines rather than absorption lines
• Temperatures exceed 20,000 K with surfaces stripped down to helium or heavier elements; classified as WN (nitrogen-rich) or WC (carbon-rich)
• Supernova progenitors—represent a late evolutionary stage of the most massive stars before core collapse

Peculiar Stars

• Deviate from standard spectral characteristics for their temperature class; designated with "p" suffix (e.g., Ap stars)
• Chemically peculiar stars show unusual abundances due to diffusion, magnetic fields, or binary mass transfer
• Magnetic Ap stars exhibit strong, organized magnetic fields that create surface abundance patches and spectral variability

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Quick Reference Table

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Self-Check Questions

1. Why do hydrogen Balmer lines peak in A-type stars rather than in hotter O-type stars where hydrogen is equally abundant?

2. Two stars have identical spectral type G2 but different luminosity classes (V and III). Which is more luminous, and what physical property explains the difference?

3. Compare and contrast how temperature affects the spectra of B-type stars versus K-type stars—what types of spectral features dominate in each, and why?

4. If you observe a star with strong TiO molecular bands, what can you immediately conclude about its temperature, and why can't these molecules exist in hotter stars?

5. An FRQ asks you to determine the distance to a star using its spectrum. Outline the spectroscopic parallax method and identify the classification information you need.