Transcript Hertzsprung2 - courses.psu.edu

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 (!)