When astronomers measure the narrow line emissions of hydrogen at 21-cm radio wavelengths along a particular line of sight through the disk of the Milky Way Galaxy, they can tell the distances to different hydrogen clouds because
A) absorption of extragalactic radiation at this wavelength is greater the farther away the absorber is from the Sun.
B) clouds that are farther away have smaller angular sizes.
C) the emission is weaker from clouds that are farther away.
D) clouds at different distances have different Doppler shifts because of the rotation of the Galaxy.

Question

Quizplus · Accepted Answer

The 21-cm line emissions from hydrogen clouds in the Milky Way are used to determine distances based on the Doppler shifts caused by the rotation of the Galaxy. Clouds at different distances move at different velocities relative to us, leading to variations in the observed wavelength due to the Doppler effect. This allows astronomers to map the structure and motion of the Galaxy.

When Astronomers Measure the Narrow Line Emissions of Hydrogen at 21-Cm