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AQA GCSE · Question 03.4 · Space physics
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.
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.
How to approach this question
1. Compare the position of the dark lines for Galaxy A and B to the lines for the Sun. Are they shifted towards the blue or red end of the spectrum?
2. What does this shift (redshift or blueshift) tell you about the motion of the galaxies relative to us?
3. Compare the size of the shift for Galaxy A with the size of the shift for Galaxy B. What does a larger shift imply about the galaxy's speed?
Full Answer
Both galaxies A and B show their spectral lines shifted towards the red end of the spectrum (redshift). This indicates that both galaxies are moving away from us. The lines for galaxy B are shifted more towards the red end than the lines for galaxy A. This means that galaxy B has a greater redshift and is therefore moving away from us at a higher velocity than galaxy A.
The dark lines in the spectrum of light from stars and galaxies correspond to specific frequencies of light being absorbed by elements in the star's atmosphere.
1. **Redshift:** When we compare the spectra of Galaxy A and Galaxy B to the Sun's spectrum, we see that the pattern of dark lines is shifted towards the longer wavelength (red) end. This phenomenon is called redshift.
2. **Velocity:** Redshift is evidence for the Doppler effect with light. It tells us that the light source (the galaxy) is moving away from the observer (us). Therefore, both galaxies are receding from Earth.
3. **Comparison:** The lines in Galaxy B's spectrum are shifted further to the red than those in Galaxy A's spectrum. The size of the redshift is proportional to the recessional velocity. A larger redshift means a greater speed. Therefore, Galaxy B is moving away from us faster than Galaxy A.
Common mistakes
✗ Confusing redshift with blueshift. Blueshift would indicate movement towards us.\n✗ Only stating that the lines have moved, without specifying the direction (towards red).\n✗ Failing to compare the two galaxies and conclude that B is faster than A.
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