Step by Step
O/B
The hottest classes — O and B
O-type stars exceed 30,000 K, appear blue, and show ionized helium lines (example: Rigel). B-type stars run 10,000-30,000 K, appear blue-white, and show helium lines (example: Spica).
A/F
The middle-hot classes — A and F
A-type stars run 7,500-10,000 K, appear white, with strong hydrogen lines (examples: Vega, Sirius). F-type stars run 6,000-7,500 K, appear yellow-white, with calcium lines (example: Procyon).
G/K
Sun-like and cooler — G and K
G-type stars run 5,200-6,000 K, appear yellow, showing calcium and metal lines — our Sun is a G2 star. K-type stars run 3,700-5,200 K, appear orange, showing molecular absorption bands (example: Arcturus).
M
The coolest class — M
M-type stars run 2,400-3,700 K, appear red, showing titanium oxide (TiO) molecular bands (example: Betelgeuse). The letter "V" after a spectral class (like G2V) denotes the star is on the main sequence. Beyond M, the classes L, T, and Y are used for brown dwarfs (objects too small to sustain full hydrogen fusion). Each letter class is further subdivided using numbers 0-9, from hotter to cooler.
Applied Walkthrough
1
Looking up at the night sky, a blazing blue-white star like Rigel is an O-type star, among the hottest and most luminous stars known, exceeding 30,000 K at its surface.
2
By contrast, our own Sun, a fairly ordinary G2 star, sits solidly in the middle of the temperature range, with a yellow color and calcium/metal absorption lines in its spectrum.
3
A red giant like Betelgeuse, despite being enormous in physical size, is actually a cooler M-type star — its reddish color and titanium oxide absorption bands reveal that its cooler surface temperature (below 3,700 K), not its size, is what determines its spectral classification.
4
The mnemonic "Oh Be A Fine Girl/Guy, Kiss Me" locks in the correct order from hottest to coolest: O, B, A, F, G, K, M — useful for quickly recalling both the temperature sequence and the associated colors.
Exam Application
Exams test whether you can recite the OBAFGKM sequence in the correct hot-to-cool order, match each class to its approximate temperature range and color, and correctly classify the Sun as a G2 main-sequence star.
⚠ Common Trap
The most common trap is assuming spectral class correlates directly with a star's size or luminosity — spectral class is fundamentally about surface temperature (revealed through absorption lines), not size; a cool, enormous red giant and a small, cool main-sequence star can share the same spectral class despite vastly different sizes.
✓ Quick Self-Check
1. What does the OBAFGKM mnemonic represent, and in what order?
The stellar spectral classes, in order from hottest to coolest.
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2. What spectral class is our Sun, and what does the letter after the class number mean?
G2V — the "V" indicates it's a main-sequence star.
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3. What color and approximate temperature range characterizes an M-type star?
Red; below about 3,700 K.
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4. What spectral classes are used for brown dwarfs, beyond the main OBAFGKM sequence?
L, T, and Y.
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5. What does spectral class fundamentally reveal about a star?
Its surface temperature, based on absorption line patterns — not directly its size or luminosity.
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