The Hertzsprung-Russell Diagram

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Presentation transcript:

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 3000 K O B A F G K M  Temperature B0, B1, B2, . . . , B9, A0, A1, . . .

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.  1% of stars are Giants & Supergiants.  9% of stars are White Dwarfs. L T

* Giants & supergiants are actually big; white dwarfs are small. . . G2 stars: * All have same temp. * But luminosities differ . . . L G2 Supergiant G2 Giant Sun (G2) 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

100 Masses of Main Sequence Stars (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

 This star is 47 times more luminous 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 Luminosity Mass Radius 2500 – 50,000 K 0.001 – 1,000,000 L 0.08 – 100 M 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.

Red Dwarfs

Stars within 12 light years of the Sun

60 ly

. 700 ly (!)

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