Consider a spiral galaxy that is moving directly away from Earth with a speed V
ID: 1457984 • Letter: C
Question
Consider a spiral galaxy that is moving directly away from Earth with a speed V = 4.900×105 m/s at its center, as shown in the figure(Figure 1) . The galaxy is also rotating about its center, so that points in its spiral arms are moving with a speed v = 5.600×105 m/srelative to the center.
If light with a frequency of 8.130×1014 Hz is emitted in both arms of the galaxy, what frequency is detected by astronomers observing the arm that is moving toward the Earth? (Measurements of this type are used to map out the speed of various regions in distant, rotating galaxies.)
If light with a frequency of 8.130×1014 Hz is emitted in both arms of the galaxy, what frequency is detected by astronomers observing the arm that is moving away from Earth?
Both answers in Hz.
Explanation / Answer
Consider v/c<<1 so use non-relativistic doppler effect f' = f(c/(c+v))
where v = (4.9 +- 5.6) x 105 m/s , f = 8.13 x 1014 Hz , c= 3 x108 m/s
1) The frequency detected by astronomers observing the arm that is moving toward the Earth =
f' = (8.13 x 1014 x 3 x108 )/(3 x108 + 10.5 x 105 ) =8.1016 x 1014 Hz
2) The frequency detected by astronomers observing the arm that is moving away from the Earth=
f' = (8.13 x 1014 x 3 x108 )/(3 x108 + 0.7 x 105 ) = 8.1281 x 1014 Hz