1. The Hertzsprung-Russell Diagram

2. 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, . . .

3. Sun: G2
p. 258

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

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

6. 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

7. * 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?

8. B8 supergiant
B8 main sequence star
Luminosity Class:
I: Supergiants
II: Bright Giants
III: Giants
IV: Subgiants
V: Main sequence

9. Rigel (B8 Ia)
Betelgeuse (M2 Ia)
Aldebaran (K5 III)
Sirius (A1 V)
Sun (G2 V)
Proxima
Centauri (M5 V)
p. 266

10. The Mass-Luminosity Relation

11. 100
Masses of Main
Sequence Stars
(Sun’s mass = 1) L
p. 271
0.08
Temp

12. 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

13.  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  L =  47
 This star is 47 times more luminous
than the sun!

14. 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

15. 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.

16. Red Dwarfs

17. Stars within 12 light years of the Sun

18. 60 ly

19. .
700 ly (!)