What observational tool allows astronomers to calculate the precise line-of-sight velocity of gas clouds?
Answer
Measuring the Doppler shift of the neutral hydrogen ($ ext{H} ext{I}$) radio signal
The most effective method involves observing the faint radio wave emitted by neutral hydrogen atoms at about 21 centimeters. A shift toward a shorter wavelength (blue shift) means movement toward Earth, and a shift toward a longer wavelength (red shift) means movement away.
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Astrophysics: Galaxy Rotation Curves and Dark Matter - YouTube
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