A Radio Astronomer Points a Radio Telescope Through the Milky

A radio astronomer points a radio telescope through the Milky Way's disk in a direction directly away from the center of the Galaxy. If the astronomer measures the Doppler shift of the 21-cm radio line of hydrogen from interstellar clouds at several different distances from the Sun along this line of sight, what should be the result? Refer to Figure 16-8 from the text.

A) The Doppler shift should have a constant value, regardless of distance, as a result of the Sun's speed of motion around the center of the Galaxy.
B) The Doppler shift should be almost zero for all clouds, regardless of distance.
C) The Doppler shift should rise to a maximum at some distance, then decrease again.
D) The Doppler shift should be larger the farther away the cloud is.