The Spectra of Stars (and Brown Dwarfs)

Core Concepts

Where dark lines come from

A star's visible "surface" is the photosphere — dense, hot gas emitting a smooth continuous spectrum. Above it lie cooler, thinner layers. As photons stream outward, atoms there absorb light at very specific wavelengths — the energies at which electrons jump between allowed orbits (atomic physics of Chapter 5). Every absorbed wavelength is missing from the light we receive, so the rainbow arrives crossed by dark absorption lines. The mirror image, emission lines, appears when hot, thin gas glows at those same wavelengths, as in a nebula. Because each element has a unique set of electron jumps, each element leaves a unique line pattern.

Why the lines change with temperature

An atom can only absorb light if its electrons sit in the right states. Hydrogen's Balmer lines (visible wavelengths) require electrons to start in the second energy level. In very hot stars (say 30,000 K), most hydrogen is ionized — stripped of electrons — so little remains in the second level and lines are weak. In very cool stars (3,000 K), almost all hydrogen sits in the ground state, so again few atoms can absorb. Only near 10,000 K, where a good fraction of hydrogen is in the second level, do the Balmer lines reach maximum strength. This rise-and-fall pattern means no single line tells the temperature; the pattern of many lines together does.

The spectral sequence: O B A F G K M

In the early 1900s, Annie Jump Cannon and colleagues at Harvard classified hundreds of thousands of stellar spectra by their line patterns. The original lettering was alphabetical, but the sequence was later re-ordered by temperature — most famously through Cecilia Payne-Gaposchkin's 1920s thesis work showing stars are mostly hydrogen and helium and that the classes differ by temperature. Hottest to coolest:

• O (roughly 30,000 K and up): ionized helium lines, weak hydrogen.
• B (about 10,000–30,000 K): neutral helium lines, hydrogen strengthening.
• A (about 7,500–10,000 K): hydrogen Balmer lines at maximum strength.
• F (about 6,000–7,500 K): hydrogen weakening; Ca II lines appearing.
• G (about 5,200–6,000 K): strong Ca II and metal lines; the Sun is G2 (commonly taught).
• K (about 3,700–5,200 K): metal lines dominate; titanium oxide (TiO) bands beginning.
• M (about 2,400–3,700 K): strong molecular bands such as TiO.

The mnemonic "Oh Be A Fine Guy/Girl, Kiss Me" preserves the hot-to-cool order; classes divide into tenths (G0, G2, G5), so G2 is slightly hotter than G5.

Temperature, not abundance

The classic trap is reading line strength as abundance. Hydrogen dominates essentially all stars, yet its lines are weak in O and M stars alike — not because hydrogen is scarce, but because its atoms are in the wrong states to absorb those wavelengths. Payne-Gaposchkin's great insight: a huge variety of stellar spectra are one composition viewed at many temperatures. Strong iron lines, for example, mean the star is cool enough for many iron electrons to be in useful states — not that it is iron-rich. Astronomers call all elements heavier than helium metals, terminology that still surprises newcomers.

Brown dwarfs: classes L, T, and Y

A brown dwarf forms like a star — by gravitational collapse — but is too low in mass to sustain steady hydrogen fusion: below roughly 0.075–0.08 solar masses (a commonly taught boundary), cores never quite reach the fusion threshold. Brown dwarfs shine from leftover formation heat (plus a brief deuterium-burning phase), so they slowly cool and fade. Their cool atmospheres extend the classification sequence:

• L (about 1,300–2,400 K): dust and metal hydride features; TiO gone.
• T (about 700–1,300 K): methane (CH₄) and water vapor absorption.
• Y (below about 700 K): even cooler; ammonia and water dominate.

Because they are cool, brown dwarfs radiate mostly in the infrared — which is why they were not found until infrared surveys matured in the mid-1990s (the first confirmed examples, Teide 1 and Gliese 229B, were announced in 1995, a commonly taught milestone). The lithium test helps: brown dwarfs never get hot enough to destroy lithium, while low-mass stars burn it away early, so lithium in a cool spectrum marks a substellar candidate (caveats for very young objects).

How It Works / Step-by-Step Process

1. Record the spectrum and identify the most prominent absorption features; match them to known atomic patterns (ionized helium, neutral helium, Balmer lines, Ca II, TiO bands).
2. Place the pattern on the temperature ladder O B A F G K M; a star sits at the class where its strongest features match.
3. If the object is very cool with methane, water, or ammonia absorption, consider an L, T, or Y brown dwarf; check for lithium as supporting evidence.
4. Cross-check the class against color and luminosity when those measurements exist.

Common Confusions