USAAAO — 2023 First Round

Fill in the blanks. The Balmer lines are a series of emission spectrum of hydrogen. In the low temperature interstellar medium, despite hydrogen being the most commonly occurring element in such environment, the Balmer series is not observed as absorption lines. That happens because the transitions that require such spectrum to appear as absorption lines correspond to transitions from the (1) hydrogen level to higher states. In low temperature environments, the (1) level is (2) .