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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 B0, B1, B2, . . . , B9, A0, A1, . . . M Sun: G2 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. . . G2 stars: G2 Supergiant G2 Giant L Sun (G2) * All have same temp. * But luminosities differ . . . T . . . So radii must differ! Eh? B8 supergiant B8 main sequence star Luminosity Class: I: Supergiants II: Bright Giants III: Giants IV: Subgiants V: Main sequence Rigel (B8 Ia) Betelgeuse (M2 Ia) Aldebaran (K5 III) Sirius (A1 V) Sun (G2 V) Proxima Centauri (M5 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 – 1,000,000 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 (!)