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The H-R Diagram
How light from the stars can tell us (almost) everything about them!
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How Hot Is It?
• Photometry: measuring a star’s temperature by studying its color
• What color is the hottest? The coldest?
• Hottest: Blue/Violet
• Coldest: Red
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NO!!! YES!!!
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Stellar Spectroscopy
• Can identify the chemical composition ANDthe temperature of stars!
• How?
• Excitation energy of electrons in their shells –Quantum Mechanics
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Spectral Classification
• Using spectroscopy to relate directly to stellar surface temperature.
• Fixed by Annie Jump Cannon
• Classifies stars by temperature from hottest to coolest
• Directly related to a star’s color
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O B A F G K M
40,000 K
2,000 K
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O B A F G K M• There are sub-
ranges
0 – 10
0 = hottest
10 = coolest
For example:
B4 is a hotter star than B8
F9 is hotter than G2
• Our Sun?
A G2 star
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Patterns Emerge When Luminosity (power) is plotted against Temperature
1911 – Ejnar Hertzsprung 1913 – Henry Russell
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Stars don’t have random temperatures and colors! They are directly related!
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Luminosity Class
• The measure of Watts produced by the star
• Also able to be seen by spectroscopy
• White Dwarfs usually not included (no fusion)
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The H-R Diagram plots FOUR aspects of stars
The 2 Pairs are Directly Related:
Spectral Class & Temperature
Luminosity & Magnitude
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Remember Stellar Lifecycles?
• Interactive HR Diagram
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HR Diagram Project• Create your own HR Diagram
using people or objects!
• Follow a theme: a family, a TV show, movie, or book characters, etc.
• Can be digital or on paper
• Must demonstrate four different stages of star life:
1. Protostar
2. Main Sequence
3. Giant/Supergiant
4. White Dwarf/NS/Black Hole