We detect neutral gas in the interstellar medium by looking for radiation at 21 cm that arises when
A) an electron moves from the n = 1 to n = 2 state in a hydrogen atom.
B) an electron is ionized from a hydrogen atom.
C) carbon monoxide (CO) gas is excited by stellar radiation.
D) an electron's magnetic poles flip and align with the magnetic poles of a proton in a hydrogen atom.
E) an electron combines with a proton to make a hydrogen atom.

Question

Quizplus · Accepted Answer

The 21 cm radiation is due to the hyperfine transition in neutral hydrogen, where the electron's spin flips to align with the proton's spin.

We Detect Neutral Gas in the Interstellar Medium by Looking