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Astronomy
Grade 9-12
Hertzsprung-Russell Diagram Master Cheat Sheet
A printable reference covering H-R diagram axes, luminosity, temperature, spectral classes, star color, main sequence, giants, and white dwarfs for grades 9-12.
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The Hertzsprung-Russell diagram is a graph that shows how stars compare by luminosity, surface temperature, color, and spectral class. Students need this cheat sheet because the H-R diagram connects many astronomy ideas in one visual tool. It helps explain how stars are classified, how they change over time, and why stars of different sizes and temperatures look different.
Key Facts
- On an H-R diagram, luminosity increases upward and surface temperature usually decreases from left to right.
- The main sequence runs from hot, bright blue stars in the upper left to cool, dim red stars in the lower right.
- Spectral classes from hottest to coolest are O, B, A, F, G, K, M.
- Star color indicates surface temperature, with blue stars hottest, white and yellow stars intermediate, and red stars coolest.
- Luminosity is the total energy a star emits per second, often compared to the Sun using L_sun.
- The Stefan-Boltzmann relationship is L = 4 pi R^2 sigma T^4, so luminosity depends on radius squared and temperature to the fourth power.
- Giants and supergiants are very luminous because they have very large radii, even when their surface temperatures are cool.
- White dwarfs are hot but dim because they have very small radii.
Vocabulary
- Hertzsprung-Russell diagram
- A graph that plots stars by luminosity and surface temperature to show patterns in star types and evolution.
- Luminosity
- The total amount of energy a star gives off each second.
- Main sequence
- The diagonal band on the H-R diagram where stars spend most of their lives fusing hydrogen into helium.
- Spectral class
- A category based on a star's temperature and absorption lines, ordered O, B, A, F, G, K, M from hottest to coolest.
- Absolute magnitude
- A measure of how bright a star would appear if it were placed at a standard distance of 10 parsecs.
- White dwarf
- A small, hot, dim stellar remnant left after some stars lose their outer layers.
Common Mistakes to Avoid
- Reading temperature left to right as increasing is wrong because most H-R diagrams place hotter stars on the left and cooler stars on the right.
- Assuming all bright stars are hot is wrong because cool giants and supergiants can be very luminous due to their large size.
- Confusing apparent brightness with luminosity is wrong because apparent brightness depends on distance, while luminosity is the star's actual energy output.
- Thinking white dwarfs are cool because they are dim is wrong because they are dim mainly due to their small radius, not low temperature.
- Placing the Sun among giants is wrong because the Sun is a G-type main sequence star with luminosity about 1 L_sun.
Practice Questions
- 1 A star is plotted in the upper left of an H-R diagram. Is it likely hot or cool, and is it likely bright or dim?
- 2 A red giant has a surface temperature of 3500 K but a luminosity of 1000 L_sun. What property mainly explains its high luminosity?
- 3 List the spectral classes from hottest to coolest.
- 4 Two stars have the same surface temperature, but one is much more luminous. Explain what this suggests about their relative sizes.