Step by Step
1
Temperature determines color
A star's surface temperature directly determines both its apparent color and the specific absorption lines visible in its spectrum — hotter stars appear blue, cooler stars appear red.
2
O through F — the hot end
O-type stars (over 30,000 K) appear blue, showing ionized helium lines. B-type stars (10,000-30,000 K) appear blue-white, with neutral helium lines. A-type stars (7,500-10,000 K) appear white, with strong hydrogen Balmer lines. F-type stars (6,000-7,500 K) appear yellow-white, with ionized calcium lines.
3
G through M — the cool end
G-type stars (5,200-6,000 K) appear yellow, showing calcium and iron lines — our Sun is a G2 star specifically. K-type stars (3,700-5,200 K) appear orange, showing molecular absorption bands. M-type stars (below 3,700 K) appear red, showing titanium oxide bands — and are actually the most common spectral type in the entire galaxy.
4
Subdivisions within each class
Each spectral class is further subdivided using numbers 0 through 9, running from hotter (0) to cooler (9) within that class — which is exactly why our Sun is specifically labeled G2, not just "G."
Applied Walkthrough
1
Looking at two stars side by side in the night sky, a blue-white star is immediately identifiable as hotter than a reddish star, purely from color alone — this visual difference directly reflects each star's surface temperature.
2
Our own Sun, at around 5,778 K, falls into the G-type category, and its specific subdivision — G2 — places it toward the hotter end of the G class, appearing yellow with calcium and iron absorption lines in its spectrum.
3
Despite red M-type stars being relatively dim and cool compared to hotter classes, they're actually the most common spectral type throughout the entire galaxy — most stars we can't see with the naked eye from Earth are faint, common M-type red dwarfs.
4
This entire spectral sequence — captured by the mnemonic "Oh Be A Fine Girl/Guy, Kiss Me" — allows astronomers to immediately estimate a star's temperature and likely color just from its spectral classification alone.
Exam Application
Exams test whether you know the specific temperature range and color for each spectral class in detail, whether you can place the Sun precisely as G2, and whether you know that M-type stars — despite being cool and dim — are actually the most common spectral type in the galaxy.
⚠ Common Trap
The most common trap is assuming the most visually striking or famous stars (often hot O and B types) are representative of stars generally — in reality, the vast majority of stars in the galaxy are cool, dim M-type red dwarfs, simply too faint to be prominent in the night sky.
✓ Quick Self-Check
1. What color and temperature characterizes an O-type star?
Blue, over 30,000 K.
Tap to reveal / hide
2. What color and temperature characterizes an A-type star, and what is its distinguishing spectral feature?
White, 7,500-10,000 K, with strong hydrogen Balmer lines.
Tap to reveal / hide
3. What is the Sun's specific spectral subclass, and what does the number indicate?
G2 — the number 2 indicates it's toward the hotter end of the G class (subdivisions run 0-9, hotter to cooler).
Tap to reveal / hide
4. What spectral feature distinguishes M-type stars?
Titanium oxide (TiO) molecular absorption bands.
Tap to reveal / hide
5. Which spectral type is the most common in the galaxy, despite being cool and dim?
M-type.
Tap to reveal / hide