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The H-R Diagram
I bet my dwarf can beat your giant.
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Goals
•
•
•
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Look at parts of H-R diagram.
Lifetimes?
Sizes?
Clusters?
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Main Sequence & Thermal Radiation
• The Main Sequence
makes sense!
• Hotter stars are bluer.
• Hotter stars are
brighter
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The Main Sequence
• Stars characterized by
what holds them up.
• 90% held up by heat of
Hydrogen fusion
4H  He + Energy
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M.S. Lifetime
• More Massive 
Hotter
• Hotter  More
luminous
• More luminous 
shorter life
TLife
Mass ( M Sun )

 TSun
L( LSun )
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Clusters
• Groups of stars all at the
same distance and age.
• Watch evolution along the
H-R diagram.
• Get age of cluster.
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Open
Clusters
• These are the new stars.
• Small groups of young
stars.
• Slowly drifting apart.
Jewel Box – copyright MichaelBessell
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Angular Size
• Linear size = how big something really is
– Meters, inches, light years, feet
• Angular size = how big something looks
– Degrees, arcminutes, arcseconds, milliarcseconds
Circle = 360 degrees
1 degree = 60 arcmin
1 arcmin = 60 arcsec
1 arcsec = 1000 mas
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Angular vs. Real Size
Diameter
Diameter 2 R
tan q 

Distance
D
q
Distance
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Stellar Radii
How big are stars?
• We see stars have different
luminosities and different
temperatures.
• Stars have different sizes.
• If you know:
50 mas
– Distance
– Angular size
• Learn real size.
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Angular versus Real
Supergiants, Giants and Dwarfs
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Betelgeuse
• Angular size = 50 mas
• Parallax = 7.6 mas = 0.0076 arcsec
• Apparent mag = 0.6
• Distance = 1/parallax = 132 pc
• True size = distance * angular size = 1400 Rsol
• Absolute Mag = m – 5log(d/10pc) = -5
– Our sun M ~5, Betelgeuse = 10,000x luminosity
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Globular Cluster
12 billion years old
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M13
M13 – Natalie Redfield ‘06
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Luminous
Giants
Why are Red Giants more luminous than
main sequence stars?
– Hotter things are brighter per square inch.
– The larger the star, the more square inches.
– Luminosity is total light given off.
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Concept Test
Which star is:
1. Hottest?
2. Most luminous?
3. Least likely to be
found in an open
cluster?
4. Least likely to be
found in a globular
cluster?
Of Main Seq. Stars?
Star
Spec
Type
m
M
A
B0 V
0.0
0.0
B
G2 V
10.0
4.4
C
K5 III
0.0
-2.0
D
F7 I
-1.0
-5.0
E
K3 V
5.0
6.5
1. Most massive?
2. Longest Life span?
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Homework #10
• For Oct 13:
• Reread B17.1 – B17.2,
• Do B16 Problem 20, and B17 Problem 1, 18
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