Astronomers aim a radio telescope at a distant region of the Milky Way Galaxy and detect 21-cm radio waves with no Doppler shift. Which one of these possible explanations is NOT correct?
A) The neutral hydrogen in this region is not moving relative to Earth.
B) The neutral hydrogen in this region is moving perpendicularly across the line of sight.
C) The neutral hydrogen in this region is in a circular orbit around the galactic center at the same radius as the solar system.
D) The neutral hydrogen in this region is moving away from Earth instead of toward Earth.

Question

Quizplus · Accepted Answer

Explanation: The absence of a Doppler shift indicates that the neutral hydrogen is not moving toward or away from Earth along the line of sight. If it were moving away from Earth, a redshift (indicative of a Doppler shift) would be observed. Choices A, B, and C all describe scenarios where no Doppler shift toward or away from Earth would be observed, making them plausible explanations for the lack of observed Doppler shift.

Astronomers Aim a Radio Telescope at a Distant Region of the Milky