A relatively recent experiment claims to have discovered that the Milky Way galaxy is surrounded by an enormous halo of highly ionized gas. One of the spectral lines detected is the n = 2 to n = 1 transition line for an oxygen atom so highly ionized that only one electron remains in orbit around the nucleus. Oxygen is the eighth element in the periodic table, with nuclear charge 8e (i.e. the number of protons is Z = 8). What is the energy, in electron volts, of a photon emitted by this transition?