What specific spectral line is famously measured in rest frame to determine radial velocity?
Answer
The neutral hydrogen (HI) line at 21 centimeters
Astronomers commonly target the neutral hydrogen (HI) line, which has a known rest wavelength of 21 centimeters, to measure the Doppler shift and calculate line-of-sight velocity.
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Astrophysics: Galaxy Rotation Curves and Dark Matter - YouTube
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