☰ Contents · Astronomy

Stellar spectra and the Hertzsprung–Russell diagram

Lessons 50–51 · 2 lessons · M. Mamadazimov. Astronomy Grade 11, 1st edition. DAVR NASHRIYOTI, Tashkent, 2018
50

Stellar spectra and spectral classes

Textbook: p. 108
GoalKnow the spectral classes (O, B, A, F, G, K, M), their temperatures and spectral features.
New words
spectral class · spektral sinfBalmer lines · Balmer chiziqlariabsorption line · yutilish chizig‘ibrown dwarf · jigarrang mitti
Explanation

Stellar spectra are classified in the Harvard system: O, B, A, F, G, K, M, in order of decreasing temperature (O > 30,000 K, B 10,000–30,000, A 7500–10,000, F 6000–7500, G 5200–6000, K 3700–5200, M < 3700 K). Each class is divided into subclasses 0–9; the Sun is G2. Class O is blue, class M red; in class A the hydrogen Balmer lines (Hα 656.3 nm, Hβ 486.1 nm) are strongest (Sirius, Vega); class G shows many lines of ionised calcium and metals; class M has molecular absorption bands in a cool atmosphere (Betelgeuse, Proxima). In 1925 Cecilia Payne showed that the differences between spectra are mainly due to temperature and that stars are mostly hydrogen and helium. Luminosity classes (I–V) show the type of star: V – main sequence, III – giant, I – supergiant (so the Sun is G2V). Objects cooler than M are L, T and Y brown dwarfs.

Worked examples
A star at 9500 K belongs to class A (7500–10,000 K) and is white; for example Vega.
The gap between Hα (656 nm) and Hβ (486 nm) is 656 – 486 = 170 nm – in class A stars both lines are clearly visible.
Class activity

Invent your own mnemonic for O B A F G K M (like “Only Bright Astronomers Find Great Kepler’s Mistakes”) and share it with the class; next to each letter write a colour and one star.

Practice
1
In what order are the spectral classes arranged by temperature?
2
Hα is 656 nm and Hβ 486 nm. What is the difference in nm? (656 – 486)
3
To which class does a star of T = 4500 K belong, and what colour is it?
4
Why do cool M stars show molecular bands while hot O stars do not?