Cosmology
4 marks · EasyA student is looking at the light spectra from two galaxies, Galaxy A and Galaxy B, using a spectrometer. She notices that the absorption lines for Galaxy B are further towards the red end of the spectrum than those in Galaxy A.
What is the name for the shift of absorption lines towards the red end of the spectrum, and what does it indicate about a galaxy’s motion relative to us?
If Galaxy B’s absorption lines have shifted more than Galaxy A’s, what can you deduce about the relative speeds and distances of the two galaxies from us?
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(a) Red-shift; it shows the galaxy is moving away from us.
(b) Galaxy B is further away and moving faster from us than Galaxy A.
Common mistakes: Saying blue-shift instead of red-shift; Mixing up which galaxy is further/faster
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Marking points
B1 (a) · 1 mark · Identifies the effect as 'red-shift'
B2 (a) · 1 mark · States it indicates the galaxy is moving away from us
B3 (b) · 1 mark · Correctly states Galaxy B is further away
B4 (b) · 1 mark · States Galaxy B is moving faster than Galaxy A
Worked solution
(a) The shift towards the red is called red-shift. This shows the galaxy is moving away from us.
(b) As Galaxy B's absorption lines are shifted more, it means it is further away and moving away faster than Galaxy A.