• Quick retrieval from earlier learning (The life cycle of stars; The electromagnetic spectrum). Students answer on mini whiteboards.
• Show answers together and note who needs support in the main task.

1. In the life cycle of a star much more massive than the Sun, what comes straight after "supernova"? (protostar / neutron star or black hole / nebula / red supergiant) → neutron star or black holePrompt: Very massive stars end their lives far more violently than the Sun.
2. In the life cycle of a star like the Sun, what comes straight after "main sequence star"? (nebula (cloud of dust and gas) / red giant / protostar / white dwarf) → red giantPrompt: The Sun is in its main sequence stage now.
3. In the life cycle of a star much more massive than the Sun, what comes straight after "protostar"? (nebula / supernova / main sequence star / neutron star or black hole) → main sequence starPrompt: Very massive stars end their lives far more violently than the Sun.

• Ask why we see the Moon change shape over a month.
• Share the objective: The expanding universe and the Big Bang. Read the success criteria together.

• Work through the example out loud, saying why you do each step, not just what you do.
• Example: About how old is the universe?
• Step 1: Measurements of the CMB and expansion give an age of about 13.8 billion years.
• Step 2: So the answer is: about 13.8 billion years
• Answer: about 13.8 billion years
• Pause after each step and ask a student to say what comes next.

• Do these together. Students try each step on whiteboards before you reveal it.
• Use the hints as prompts rather than giving the answer.

1. What is red-shift? (galaxies turning red because they are hot / light from distant galaxies is stretched to longer wavelengths / the Sun setting / light becoming bluer) → light from distant galaxies is stretched to longer wavelengthsPrompt: Red light has a longer wavelength than blue.
2. The Big Bang: which statement is true? (The cosmic microwave background is evidence for the Big Bang. / The Big Bang was an explosion at one point in an existing empty space. / The Earth is the centre of the universe. / Red-shift means galaxies are moving towards us.) → The cosmic microwave background is evidence for the Big Bang.Prompt: Which way are distant galaxies moving?

• Students work on the student sheet. Circulate and check the first two answers of every student early.
• Students who finish: write a question of their own for a partner, with the answer.

The 6 questions are on the student sheet below, with answers in the key.

• Two questions on their own. Sort answers into got it / nearly / not yet to plan the next lesson.

1. More distant galaxies show greater red-shift. What does this suggest? (Galaxies are standing still / Light slows down over time / The universe is shrinking / The universe is expanding) → The universe is expandingPrompt: Further galaxies are moving away faster.
2. A spectral line normally at 500 nm is observed at 525 nm in light from a galaxy. By how many nanometres has it been red-shifted? → 25 nmPrompt: Find the difference between the observed and normal wavelengths.

Free online practice with hints and printable worksheets for this skill: eurekami.com/science/grade-11.