The Hertzsprung-Russell Diagram
Outline
1. Stellar Classification
* O, B, A, . . . temperature sequence
2. Hertzsprung-Russell (HR) Diagram
* E. Hertzsprung & H. N. Russell (1911 - 13)
* Correlates luminosity & temperature
* Main sequence (most stars), giants, supergiants, white dwarfs
* Giants , supergiants really are big
* Luminosity class (I, II, . . . )
- From width of spectrum lines
3. Mass-Luminosity Relation
* Mass increases up the main sequence
* L a M3.5
4. Summary of Stellar Properties
* Range of temperature, luminosity, mass & radius
5. Stars in the Night Sky
* Nearest stars dim & cool
* Brightest-appearing stars distant & luminous
Questions
1. Draw an HR diagram and sketch the regions occupied by main sequence stars, giant stars,
supergiant stars and white dwarf stars.
2. What kinds of stars are most numerous in the Galaxy? Why don't you see (with the naked
eye) any of these stars in the night sky?
3. The figure above shows an HR diagram with five stars (A - E) along with the main sequence
indicated.
a)
b)
c)
d)
e)
f)
g)
h)
Which star has the largest diameter?
Which star has the smallest diameter?
Which is the hottest star?
Which is the star most like the sun?
Which star is a cool supergiant?
Which star has strong lines of ionized helium in its spectrum?
Which star is the white dwarf?
Which star has spectrum lines due to molecules?
4. Explain how you know that a K5 giant star must be larger in diameter than a K5 main
sequence star. Hint: plot both stars on an HR diagram and note how their temperatures and
luminosities compare.
5. Suppose the following stars all have the same apparent brightness. Which is farthest away?
Which is nearest? Why?
(a) G2 V
(b) B3 III (c) F0V
(d) B0 I (e) White dwarf
6. A star 10 times more massive than the sun (spectral type B3 on the main sequence) would be
how much more luminous than the sun?
Answers
1.
2. The most numerous stars are the “red dwarfs” – the coolest stars on the main sequence. Even
though they are, on average, the closest stars to Earth, none are visible to the naked eye because
of their extremely low luminosities.
3.
b)
c)
d)
e)
f)
g)
h)
a) B
C
A appears to be slightly hotter than C.
D
B
A, and very likely C
C
B and E
4. K5 giants and main sequence stars have the same temperature. Temperature dictates the
brightness (energy radiated) per unit area (e.g., per square meter). So both giant and main
sequence K5 stars radiate the same amount of energy from each square meter. So, in order for
the K5 giant to be more luminous, it must have more square meters; i.e, it must have a larger
surface area, thus a larger radius.
5. The B0 I star is farthest away, as it is the most luminous among the stars listed. The white
dwarf is nearest, as it has the lowest luminosity among the stars listed.
6. The mass-luminosity relation goes like this: L ~ M3.5. In this case, this means: (10)3.5 =
3160. Thus, a 10 solar-mass star would be 3160 times the Sun's luminosity.