H-R Diagram
Integrated Science (H)
The Hertzsprung - Russell diagram – a graph, actually - sorts stars based upon brightness,
energy, and temperature. Brightness is measured as absolute magnitude, the inherent brightness
of a star regardless of its distance from earth. (Note the inverse scale for magnitude.) Luminosity
is a measure of the total radiant energy output of a star. Temperature is plotted in degrees Kelvin
and is also reflected in the spectral class each star is assigned. All three factors, which serve to
distinguish one star from another, are determined by a star’s mass.
Using the H-R diagram, astronomers are able to assess what stage of stellar evolution any single
star is in, as well deduce the time spent in each stage by stars in general.
A. Answer the following using the Figure I: The H - R diagram.
1. What type of star are most of those plotted on the H-R diagram?
• Which stage of the star life cycle are these in?
• What can you infer about the length of this stage, based upon the number of stars on the
diagram?
2. In which half of the diagram are found the ...
• hottest stars?
• brightest stars?
• most energetic stars?
3. Which type of radiating source is ...
• hot, yet dim?
• low temperature, yet bright?
• high temperature and bright?
4. Which spectral classes contain ...
• main sequence stars?
• white dwarfs?
• giants?
B. Apply the H-R diagram to the brightest stars from Earth, listed in Table I.
5. Plot any 5 stars onto the H-R diagram. Have your partner do a different 5. You’ll need to add
in increments 1-9 for each spectral class.
• Which stage of life are most of these brightest stars in? Infer why there are few dwarfs
in the group.
6. Apparent magnitude describes how bright a star appears to us on Earth.
• Which of these stars looks brightest to us?
• Compare and contrast the apparent and absolute brightness of Vega and Capella.
Infer why these two stars differ.
7. Use the spectral class and apparent magnitude data to determine which star ...
• is hottest?
• has the greatest absolute magnitude?
• is least luminous (energetic)?
Follow Up:
1. What accounts for the fact that some main sequence stars are hot & bright, and others cool &
dim?
2. How can a main sequence star be as bright as a giant?
3. Why are most stars on the main sequence?
4. Your cousin, a neophyte stargazer and the family know-it-all, claims that Rigil Kentaurus is
the biggest and brightest star in the sky. Diplomatically correct him.
5. Create a mnemonic to remember the sequence of the spectral classes.
Extension: HR interactive: http://astro.unl.edu/naap/hr/animations/hr.html
FIGURE I: The Hertzsprung-Russel Diagram
TABLE I: Brightest Stars as Seen From Earth
Name
Sirius
Canopus
Rigil Kentaurus
Arcturus
Vega
Capella
Rigel
Procyon
Betelgeuse
Achernar
Hadar
Altair
Aldebaran
Acrux
Spica
Antares
Fomalhaut
Pollux
Deneb
Beta Crucis
Apparent magnitude
(negative = brighter)
-1.47
-0.72
-0.01
-0.06
0.04
0.05
0.14
0.37
0.41
0.51
0.63
0.77
0.86
0.90
0.91
0.92
1.15
1.16
1.26
1.28
Spectral
class
A1
F0
G2
K2
A0
G8
B8
F5
M2
B3
B1
A7
K5
B2
B1
M1
A3
K0
A2
B0.5
Absolute magnitude
(negative = brighter)
1.4
-3.1
4.4
-0.3
0.5
-0.6
-7.1
2.7
-5.6
-2.3
-5.2
2.2
-0.7
-3.5
-3.3
-5.1
2.0
1.0
-7.1
-4.6
Distance
(ly)
8.7
98
4.3
36
26.5
45
900
11.3
520
118
490
16.5
68
260
220
520
22.6
35
1600
490