Lesson Plan

• Describe luminosity as the total power emitted by a star.
• Explain the inverse‑square relationship between luminosity, flux, and distance.
• Apply the period‑luminosity relation for Cepheid variables to determine a star’s luminosity.
• Calculate stellar distances using the standard‑candle method.
• Evaluate sources of error such as interstellar extinction and calibration uncertainties.

• Projector and screen
• Whiteboard and markers
• Printed worksheet with flux‑luminosity problems
• Scientific calculators
• Graph paper
• Data tables for Cepheid, RR Lyrae, and Type Ia supernovae
• Diagram of the inverse‑square geometry
• Clicker/quiz app for exit ticket

Begin with a striking image of a distant galaxy and ask, “How do astronomers know how far away it is?” Connect this to students’ prior work on flux and the inverse‑square law. State that by the end of the lesson they will be able to use luminosity and the standard‑candle method to determine astronomical distances.

1. Do‑now (5 min): Quick written question – “What is flux and what are its units?”
2. Mini‑lecture (10 min): Define luminosity, discuss intrinsic vs. observed properties, and introduce units (watts).
3. Derivation activity (10 min): In pairs, derive F = L/(4πd²) and rearrange to solve for d; share results.
4. Standard‑candle examples (10 min): Present Cepheids, RR Lyrae, and Type Ia supernovae with the diagram; highlight why their luminosities are known.
5. Guided problem (15 min): Using a given Cepheid period and measured flux, calculate L via the period‑luminosity relation and then the distance.
6. Error‑analysis discussion (5 min): Identify extinction and calibration issues that affect distance estimates.
7. Exit ticket (5 min): Write one sentence answering “What information is required to turn a star into a standard candle?”

Summarise that luminosity is the intrinsic power of a star and that, when combined with measured flux, it enables distance calculations via the inverse‑square law. Collect the exit tickets and remind students to complete the worksheet on additional standard‑candle calculations for homework.