What is the role of the Doppler effect in constructing a three-dimensional map of the spiral arms?
Answer
It measures the radial velocity of clouds (speed toward or away from Earth), which is used in kinematic modeling to infer distance.
By observing redshift (moving away) or blueshift (moving toward), astronomers calculate radial velocity, which, when combined with models of galactic rotation, yields the crucial third dimension: distance.
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