We detect neutral gas in the interstellar medium by looking for radiation at 21 cm that arises when

Question

Quizplus · Accepted Answer

The correct answer is D because neutral hydrogen atoms in the interstellar medium can emit radiation at a wavelength of 21 cm when the spin of an electron flips and aligns with the spin of a proton. This emission is known as the 21 cm line and is a common way of detecting neutral hydrogen gas in the universe. A and B are incorrect because they describe processes that result in ionized hydrogen, which would not emit radiation at 21 cm. C is also incorrect because carbon monoxide does not emit radiation at 21 cm.

We Detect Neutral Gas in the Interstellar Medium by Looking