Stellar Classes (OBAFGKM) and Sub-Stellar Classes (L/T/Y) — Definitions, Temperatures, Examples

Stellar Classes (OBAFGKM) and Sub-Stellar Classes (L/T/Y)

A compact, practical reference for spectral classes: what each class means physically, typical effective temperature ranges, and real examples you can look up in any star catalog. This includes the main stellar sequence (O, B, A, F, G, K, M) and the sub-stellar regime dominated by brown dwarfs (L, T, Y).

Stellar: O B A F G K M Sub-stellar: L T Y (brown dwarfs) Key metric: T_eff (effective temperature) Subclasses: 0–9 (e.g., G2, M5)

How the classification works (in one minute)

The letter class is a spectral label based on absorption features in the object’s light, which correlate strongly with surface temperature. The canonical sequence is O–B–A–F–G–K–M (hottest to coolest). Subclasses run from 0 (hotter) to 9 (cooler) within each letter (e.g., G2 is warmer than G8).

Temperature ranges below are typical for T_eff and vary by luminosity class (dwarfs vs giants vs supergiants), metallicity, and measurement method. Treat them as useful “working ranges,” not hard boundaries.

Stellar spectral classes (O, B, A, F, G, K, M)

Class	Typical T_eff	Defining traits (plain language)	Real examples
O O-type	≈ 30,000–50,000 K	Extremely hot, massive, and short-lived. Strong ultraviolet output; ionizes surrounding gas (bright H II regions). Spectra show ionized helium lines in many O stars. Often found in young star-forming regions and associations.	ζ Puppis (Zeta Puppis) θ¹ Ori C (Theta-1 Orionis C) HD 93250 (Carina region)
B B-type	≈ 10,000–30,000 K	Very hot, bright, and still relatively short-lived. Strong blue-white color; prominent neutral helium lines (cooler than O, so ionized helium fades). Many are rapid rotators; some exhibit emission lines (Be stars).	Rigel (β Orionis) Spica (α Virginis) Regulus (α Leonis)
A A-type	≈ 7,500–10,000 K	White to blue-white. Strong hydrogen Balmer absorption lines are a hallmark in many A stars. Common “bright star” examples in the night sky; lifetimes typically hundreds of millions to ~1–2 billion years.	Sirius A Vega Altair
F F-type	≈ 6,000–7,500 K	Yellow-white. Hydrogen lines weaken relative to A stars; metal lines become more prominent. Often considered a “sweet spot” for long-lived, stable stars that still provide substantial visible light.	Procyon A Polaris (F7 Ib) γ Virginis (Porrima)
G G-type	≈ 5,200–6,000 K	Yellow. Good balance of lifespan and energy output; many discussions of habitability center here because the Sun is a G star. Spectra show many metal lines; hydrogen is present but not dominant.	The Sun (G2 V) α Centauri A Tau Ceti
K K-type	≈ 3,700–5,200 K	Orange. Long-lived, typically quieter than many M dwarfs (though activity varies), making K dwarfs attractive targets in exoplanet searches. Spectra emphasize neutral metals and molecular features begin to appear.	Arcturus (K1.5 III) Aldebaran (K5 III) ε Eridani
M M-type	≈ 2,400–3,700 K	Red. The most common stars in the Milky Way by number (especially M dwarfs). Strong molecular bands (notably titanium oxide in many M stars). Very long lifetimes; many are flare-active.	Proxima Centauri (M5.5 V) Barnard’s Star Betelgeuse (M1–2 Ia)

Sub-stellar classes (L, T, Y): brown dwarfs and planetary-mass objects

“Sub-stellar” objects are not massive enough to sustain long-term hydrogen fusion like true stars. The most common sub-stellar spectral classes you will see are L, T, Y, which primarily describe brown dwarfs (and, at the coldest end, objects that blur into gas-giant planet temperatures).

A practical mental model: M is “cool star,” L is “hot brown dwarf,” T is “cool methane brown dwarf,” and Y is “ultra-cool brown dwarf” approaching planetary temperatures.

Class	Typical T_eff	Defining traits (plain language)	Real examples
L L-type	≈ 1,300–2,400 K	Brown dwarfs hotter than the T/Y classes. Many show dusty atmospheres; spectra often feature metal hydrides and strong alkali lines. Visually they are extremely dim; most are detected via infrared surveys.	Teide 1 Luhman 16A 2MASSW J1507476−162738
T T-type	≈ 700–1,300 K	Cooler brown dwarfs where methane absorption becomes prominent in the infrared; atmospheres are less dusty than many L dwarfs. These are often described as “methane dwarfs.”	Gliese 229B Luhman 16B WISE J0458+6434
Y Y-type	≈ 250–700 K	Ultra-cool brown dwarfs with temperatures comparable to gas-giant planets. Extremely faint; detected almost exclusively in mid-infrared. Atmospheric chemistry is complex, with features consistent with very cold conditions.	WISE 0855−0714 WISE 1828+2650 WISE J0350−5658

Important notes (to prevent common misunderstandings)

1) Spectral class vs luminosity class

Spectral class (OBAFGKM/LTY) is primarily about temperature and atmospheric features. Luminosity class (Roman numerals like V, IV, III, I) describes whether the object is a main-sequence dwarf, subgiant, giant, or supergiant. Two objects can share a spectral class but have radically different sizes: a K1 V dwarf and a K1 III giant do not “feel” similar in brightness or radius even if their spectral letter matches.

2) The boundary between stars and brown dwarfs

The dividing line is not “temperature.” It is primarily mass: below roughly 0.075–0.080 M☉ (about 75–80 Jupiter masses, depending on composition) an object cannot sustain stable hydrogen fusion. Many brown dwarfs briefly burn deuterium early on, then cool over time—meaning a single object can move through temperature regimes as it ages.