If Astronomers Aim a Radio Telescope at a Distant Spiral $\alpha$

question 25

Multiple Choice

If astronomers aim a radio telescope at a distant spiral arm of the Milky Way Galaxy, they will probably observe a 21-cm line. If they point a large optical telescope at this same region, they will probably not be able to detect the neutral hydrogen that gives rise to the 21-cm radio signal. Why not?

• A) Neutral hydrogen is incapable of emitting visible radiation.
• B) The H _\(\alpha\) line-the primary emission of neutral hydrogen-has a much smaller energy than the 21-cm radio wave and thus is harder to detect.
• C) There is little energy in the depths of space to excite visible radiation from neutral hydrogen.
• D) The visible light emitted by neutral hydrogen is all absorbed by H II gas regions before reaching the solar system.

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