Question Constellation Sign up for free AQA · GCSE · Physics · Higher
Comparing Galactic Red-Shifts
The light spectrum from every galaxy includes dark lines. The lines have the same pattern. Figure 7 shows the position of dark lines in the visible spectra of light from the Sun and from two distant galaxies.
Explain what these light spectra tell us about the velocities of galaxy A and galaxy B.
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Marking points
- 1 Infers from red-shift that both galaxies are moving away from us.
- 2 Identifies that galaxy A has the greater red-shift.
- 3 Concludes that galaxy A is travelling faster than galaxy B.
Full-mark answer
The dark-line patterns for both galaxies are shifted towards the red compared with the Sun, so both galaxies are moving away from us. Galaxy A has a larger red-shift than galaxy B, so galaxy A is moving away faster.
Why this answer loses marks
Both galaxies are moving away.
This gives the shared direction only; full credit also compares the shift sizes and converts that comparison into relative speed.