Lesson Plan

• Describe what a standard candle is and why its intrinsic luminosity is known.
• Explain how the inverse‑square law and distance modulus are used to calculate astronomical distances.
• Apply the step‑by‑step method to determine the distance to a Type Ia supernova.
• Identify common sources of error that affect standard‑candle distance measurements.

• Projector and screen for slides
• Printed worksheet with standard‑candle calculations
• Scientific calculators or spreadsheet software
• Access to telescope image database (e.g., Hubble archive)
• Whiteboard and markers

Begin with a striking image of a distant supernova and ask students how we know its distance. Review the concepts of luminosity and flux from previous lessons. State that by the end of the class they will be able to use a standard candle to calculate cosmic distances and evaluate the reliability of their results.

1. Do‑now (5') – short quiz on luminosity vs. flux to activate prior knowledge.
2. Mini‑lecture (10') – define standard candle, present the inverse‑square law and distance‑modulus formula.
3. Guided example (12') – work through the Type Ia supernova calculation on the worksheet.
4. Group activity (10') – each group selects a different standard candle (Cepheid, RR Lyrae, TRGB) and outlines the steps to find its distance.
5. Class discussion (8') – share group findings and discuss limitations and sources of error.
6. Exit ticket (5') – write one sentence explaining why standard candles are essential for the cosmic distance ladder.

Summarise how known intrinsic luminosity combined with observed brightness yields distance, reinforcing the key formulas. Collect exit tickets to gauge understanding, and assign homework to complete a second worksheet problem using a Cepheid variable. Remind students to read the textbook section on the cosmic distance ladder for the next lesson.