Lesson Plan

• Describe the concept of a standard candle and the distance‑modulus relationship.
• Explain how Cepheid variables and Type Ia supernovae are used to measure extragalactic distances.
• Apply the distance‑modulus formula to calculate distances from given apparent and absolute magnitudes.
• Identify major sources of uncertainty and their impact on distance estimates.

• Projector and screen
• PowerPoint slides with diagrams of standard candles
• Worksheet with example calculations
• Graphing calculator or spreadsheet software
• Printed handout of standard‑candle tables
• Digital light‑curve images (Cepheid & Type Ia)
• Whiteboard and markers

Begin with a short video clip showing how astronomers map the universe, then ask students how we can know the size of something we cannot touch. Review the difference between apparent and absolute magnitude as prior knowledge. State that by the end of the lesson they will be able to use standard candles to calculate galaxy distances and evaluate uncertainties.

1. Do‑now (5') – Quick quiz on apparent vs. absolute magnitude.
2. Mini‑lecture (10') – Introduce standard candles, distance modulus, and show Cepheid & Type Ia examples.
3. Guided practice (15') – Pairs work through the Cepheid calculation worksheet, inserting period, deriving M, then distance.
4. Interactive activity (10') – Use a spreadsheet to analyse a Type Ia supernova light curve and compute its distance.
5. Think‑pair‑share (5') – Discuss sources of uncertainty (extinction, metallicity, calibration).
6. Exit ticket (5') – Write one sentence summarising how a standard candle yields a distance.

Recap the key steps: know the absolute magnitude, measure apparent magnitude, apply the distance modulus, and consider uncertainties. Collect exit tickets and remind students to complete the additional problem set on other standard candles for homework.