What is a Hertzsprung-Russell Diagram?

1
What is a Hertzsprung-Russell Diagram?
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An H-R diagram plots the luminosity (light-energy
emitted/time) and temperature of stars
Luminosity
Note stars do not uniformly fill the diagram!
Temperature
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Normal hydrogen- burning stars reside on the main
sequence of the H-R diagram
Stars spend the majority of their life on the
main sequence.
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High-Mass Stars
Normal hydrogen- burning stars reside on the main
sequence of the H-R diagram
Low-Mass Stars
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Large radius
Stars with low temperature and high luminosity
must have large radius
Small radius
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SUPERGIANTS
Stars with low temperature and high luminosity
must have large radius
GIANTS
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SUPERGIANTS
Stars with low temperature and high luminosity
must have large radius
GIANTS
WHITE DWARFS
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H-R diagram depicts Temperature Color
Spectral Type Luminosity Radius Mass
Lifespan Age
Luminosity
Temperature
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C
B
Which star is the hottest?
D
Luminosity
A
Temperature
10
C
B
Which star is the hottest?
A
D
Luminosity
A
Temperature
11
C
B
Which star is the most luminous?
D
Luminosity
A
Temperature
12
C
B
Which star is the most luminous?
C
D
Luminosity
A
Temperature
13
C
B
Which star is burning hydrogen in its core?
D
Luminosity
A
Temperature
14
C
B
Which star is burning hydrogen in its core? (Its
a main sequence star)
D
Luminosity
D
A
Temperature
15
C
B
Which star has the largest radius?
D
Luminosity
A
Temperature
16
C
B
Which star has the largest radius?
C
D
Luminosity
A
Temperature
17
A
Which star is most like our Sun?
D
Luminosity
B
C
Temperature
18
A
Which star is most like our Sun?
D
B
Luminosity
B
C
Temperature
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A
Which of these stars will have changed the least
10 billion years from now?
D
Luminosity
B
C
Temperature
20
A
Which of these stars will have changed the least
10 billion years from now?
D
Luminosity
B
C
C
Temperature
21
A
Which of these stars can be no more than 10
million years old?
D
Luminosity
B
C
Temperature
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A
Which of these stars can be no more than 10
million years old?
D
A
Luminosity
B
C
Temperature
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How can we know this if stars live millions or
billions of years?
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Stars in a Star Cluster

1. Born at the same time
2. All about the same distance
3. Same chemical composition
4. Because they formed from the same gas cloud

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How do we measure the age of a star cluster?
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Pleiades now has no stars with life expectancy
less than around 100 million years
Main-sequence turnoff
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Main-sequence turnoff point of a cluster tells us
its age
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Detailed modeling of the oldest globular clusters
reveals that they are about 13 billion years old