IMAGING THE DIFFUSE FOG OF
Hydrogen is the most abundant element in the Universe. It is also the simplest atom possible, containing a proton and an electron held together by their mutual electric attraction. Because of its simplicity, the detailed understanding of the hydrogen atom structure played an important role in the development of quantum mechanics.
Since the lifetime of energy levels with principal quantum number n greater than 1 is far shorter than the typical time it takes to excite them in the rarefied environments of the Universe, hydrogen is commonly found to be in its ground state (lowest energy level) with n = 1. This implies that the transitions we should focus on are those that involve the n = 1 state. Below we describe two such transitions, depicted in figure 7.1.