In Order to Emit a 21-Cm Photon,an Atom of Neutral

question 178

Multiple Choice

In order to emit a 21-cm photon,an atom of neutral hydrogen in interstellar space must absorb energy to place it in the excited state (the parallel spin configuration) .What is the MOST likely mechanism to achieve this?

• A) thermal energy
• B) collisions
• C) "falling" from an even higher energy state
• D) cosmic rays

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