Why Is It That Astronomers Rely Upon the 21-Centimeter Line

question 43

Multiple Choice

Why is it that astronomers rely upon the 21-centimeter line associated with the spin flip in hydrogen and not the more familiar hydrogen Balmer lines to gather information from the far side of the galactic plane?

• A) The Balmer lines are associated with neutral hydrogen, and most of the hydrogen astronomers need to observe is molecular hydrogen.
• B) The Balmer lines are associated with molecular hydrogen, and most of the hydrogen astronomers need to observe is neutral hydrogen.
• C) The Balmer lines are in the visible and ultraviolet regions of the spectrum, and these do not penetrate the gas and dust clouds easily.
• D) Photons released in the spin-flip transition have much more energy than those that come from the Balmer transitions.