Download Hertzsprung-Russell Diagram

Lecture 20
The Hertzsprung-Russell
Diagram
November 11, 2015
1
2
Hertzsprung-Russell Diagram
• Hertzsprung and Russell found a correlation
between luminosity and spectral type
(temperature)
10000
Hot, bright stars
100
Luminosity
(Solar units)
Sun
1
Cool, dim stars
0.01
Main Sequence
0.0001
hot
O B A F G K M
cooler
3
4
• Most stars (~90%) are on the main sequence
– The greater the temperature, the more luminous
the star
– M type stars are the most common.
– O type stars are the least common.
5
Sizes of Stars
• Size of star is related to the temperature and
the luminosity by:
Luminosity  Temperature  Radius
4
L   T  4 R
4
2
2
– If you know the position on HR diagram you
know the size of the star.
6
Mira is _____ than Barnard’s star because it is ______.
A.
B.
C.
D.
hotter … larger
smaller … more
88% luminous
more luminous … larger
more luminous … hotter
10%
1%
A.
0%
B.
C.
D.
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100 R
10 R
Supergiants
10000
100 1 R

Luminosity
(Solar units)
Giants
1
0.01
0.0001
White
Dwarfs
Sun
Cool, dim stars
0.1 R
Main Sequence
0.01 R
hot
O B A F G K M
cooler
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• Supergiants -- cool, bright, red, large stars
• Giants -- cool, bright red, less large stars
• Main Sequence -- spans range from hot,
bright stars to cool, dim stars.
• White dwarfs -- hot, small, dim stars.
• These classifications will give clues to
stages in the evolution of stars.
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Stars come in
many sizes!
10
Sizes of Objects in our Universe
Images from webisto.com/space
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Sizes of Objects in our Universe
12
Sizes of Objects in our Universe
13
Sizes of Objects in our Universe
14
Sizes of Objects in our Universe
15
A star has a high luminosity (100 solar luminosities) and a
surface temperature of 3500 K. What type of star is it?
A.
B.
C.
D.
A high mass–main sequence star
A low mass–main sequence star
A red giant
A white dwarf
75%
18%
6%
0%
A.
B.
C.
D.
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Masses of Stars
• We cannot directly measure the mass of an
isolated star.
• If something is orbiting the star, can use the
general form of Kepler’s Third Law
M 1  M 2 P
2
a
3
• Luckily, 2/3 of all stars are binary stars, two
stars that orbit one another
17
A binary star consists of two identical stars orbiting each
other. If they are 4 AU apart and orbit with a period of
4 years, how massive is each star?
A.
B.
C.
D.
4M
95%
2M
1M
0.5M
3%
A.
2%
B.
C.
0%
D.
18
A binary star consists of two identical stars orbiting each
other. If they are 4 AU apart and orbit with a period of
4 years, how massive is each star?
A.
B.
C.
D.
4M
2M
1M
0.5M
2
3
M

M
P

a
 1 2
3
a
 2M   2
P
4 AU 

a
M

 2M
2
2
2P
2  4 yr 
3
3
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Mass-Luminosity Relation
• True ONLY for
Main Sequence stars
• As the mass
increases, the
luminosity increases
rapidly
Luminosity  Mass3
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HR Diagram:
Main sequence
stars labeled by
mass in units of
solar masses.
21
Which of the following entries is incorrect when comparing
different spectral classes of main sequence stars with the Sun?
Temperature*
O
Higher
Mass A. A
Higher
B. B
Size
C. C
Color*
D. D
Luminosity
G
Same
56%
M
Lower
A
Same
Lower
Smaller B
Same
Same
C
24%
Blue
Yellow
Red
11%
Higher
9%
Same
A.
Lower
B.
C.
D
D.
22
Which of the following entries is incorrect when comparing
different spectral classes of main sequence stars with the Sun?
Same
M
Lower
A
Same
Lower
Smaller B
Same
Same
Color*
Blue
Yellow
Red
Luminosity
Higher
Same
Lower
A. A
Temperature*
B. B
Mass
C. C
D. D
Size
O
Higher
Higher
G
C
D
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Apparent Brightness
• The brightness an object appears to have.
• The further away the object, the dimmer it
looks
Luminosity
Apparent Brightness 
d = distance
2
4 d
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Spectroscopic Parallax
• If luminosity and apparent brightness are
known, distance can be determined.
m  M  5log d  5
• Difficult to accurately measure luminosity
for one star
– Use spectra to get spectral type and class
• Can use a cluster of stars
• Distance to the cluster can be determined by
comparing the HR diagram of the cluster
with a template HR diagram
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Spectroscopic Parallax
Observations
10000
Template
100
1
0.01
hot
O B A F G KM
cooler
0.0001
hot
O B A F G K M
cooler
26
Spectroscopic Parallax
10000
100
1
0.01
0.0001
hot
O B A F G K M
cooler
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The Distance Ladder
• See Figure 16.17 on p. 416
• First rung: parallax
• Second rung: spectroscopic parallax, or
“main-sequence fitting”
• Third rung: standard candles/inverse square
law (Cepheids/supernovae)
• Fourth rung: Hubble’s Law
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The Distance Ladder
We will be discussing the longer distance “rungs” of the ladder in subsequent lectures.
Figure 26.12, Freedman and Kaufmann,
7th ed. Universe, © 2005 W. H. Freeman & Company