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

question 14

Multiple Choice

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.

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