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? (Figure 13-7 of Comins,Discovering the Essential Universe,6th ed.,may be helpful.)

Question

Quizplus · Accepted Answer

Since the telescope is pointed directly away from the center of the Galaxy, the radial velocity component of the clouds along the line of sight will be almost zero, and therefore the Doppler shift will be almost zero for all clouds, regardless of distance. This is because the circular motion of stars around the center of the Galaxy causes the hydrogen gas in the interstellar medium to also move in a circular pattern, which has no component of motion along the telescope's line of sight.

A Radio Astronomer Points a Radio Telescope Through the Milky