In Order for a Cold Atom of Hydrogen to Emit

question 27

Multiple Choice

In order for a cold atom of hydrogen to emit a 21-cm wave, it must first be in a slightly higher energy state. What event usually "kicks" the hydrogen atom up to this higher state?

• A) waves of ultraviolet energy from hot stars pass by a cold gas cloud
• B) a gas cloud collapses under its own gravity until it is much smaller than it originally was
• C) gas atoms within the cloud collide
• D) hydrogen is ionized by other waves
• E) some alcohol molecules within the cloud lead to a wild party

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