Hydrogen is the most abundant gas in the Universe, and provides important clues about its properties. The cosmic microwave background was created through interactions with ionized hydrogen, and emissions from hydrogen ions help us identify energetic events like star formation. They do, that is, if the events are far enough away. Ironically, the Universe conspires to keep us from seeing events in our own galaxy. Or, more accurately, the Sun has kept us from seeing them until very recently, when the Voyager probes finally got far enough from the Sun to see what our galaxy is up to.

Hydrogen's lone electron normally resides in the ground state, but various energetic events can move it to higher states. Two types of emission result from these processes. Balmer-alpha occurs when the electron drops from the second excited state to the first (and has nothing to do with Steve or thrown chairs). Lyman-alpha emissions occur when the electron drops from the first excited state back to the ground. The Lyman emissions, which occur in the UV, rarely reach us directly. There's so much hydrogen out there that it's often absorbed and re-emitted multiple times before getting very far from its source.

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