The HertzsprungRussell Diagram Stellar Classification Spectral Types Stars

The Hertzsprung-Russell Diagram

Stellar Classification: Spectral Types * Stars classified according to appearance of spectrum. * Originally believed differences due to composition – Not! 40, 000 K O 3000 K B A F G Temperature K B 0, B 1, B 2, . . . , B 9, A 0, A 1, . . . M

Sun: G 2 p. 258

The Hertzsprung-Russell (HR) Diagram * E. Hertzsprung & H. N. Russell (1911 - 13) * Result of seeking correlations between observable properties of stars. * Correlates luminosity (or absolute magnitude) and temperature (or spectral type).

* “Scoop out” a representative sample of stars – how do they arrange themselves in terms of luminosity and temperature? Supergiants Giants Main Sequence White Dwarfs

For ~ 41, 400 stars near the sun. * Overall, we find: 90% of stars lie on Main Sequence. L 1% of stars are Giants & Supergiants. 9% of stars are White Dwarfs. T

* Giants & supergiants are actually big; white dwarfs are small. . . G 2 Supergiant G 2 Giant L Sun (G 2) G 2 stars: * All have same temp. * But luminosities differ. . . T. . . So radii must differ! Eh?

B 8 supergiant B 8 main sequence star Luminosity Class: I: Supergiants II: Bright Giants III: Giants IV: Subgiants V: Main sequence

Rigel (B 8 Ia) Betelgeuse (M 2 Ia) Aldebaran (K 5 III) Sirius (A 1 V) Sun (G 2 V) Proxima Centauri (M 5 V) p. 266

The Mass-Luminosity Relation

Masses of Main Sequence Stars 100 (Sun’s mass = 1) L p. 271 0. 08 Temp

Mass-Luminosity Relation for Main Sequence Stars L M 3. 5 L Small change in mass results in large change in luminosity. p. 271 Mass

What is the luminosity (relative to the sun) of a star 3 times more massive than the sun? L M 3. 5 L = 33. 5 47 This star is 47 times more luminous than the sun!

Summary of Stellar Properties (All stars on HR diagram) Temperature 2500 – 50, 000 K Luminosity 0. 001 – 1, 000 L Mass 0. 08 – 100 M Radius 0. 01 – 1000 R

A final word about the stars in the night sky: * Majority of stars in the galaxy are low-luminosity cool stars (“red dwarfs”) Most nearby stars are red dwarfs. * All red dwarfs in the night sky are invisible to the naked eye! * Night sky dominated by distant (& relatively rare) high-luminosity stars.

Stars within 12 light years of the Sun

60 ly

. 700 ly (!)