The carbon atom can be easily ionized, more easily than hydrogen atoms for example. In star forming regions, where massive young stars emit ultraviolet light capable of ionizing atoms, all the neutral carbon nearby becomes ionized. The singly-ionized carbon atom (abbreviated CII) emits a strong line in the far infrared that is both very intense and consequently a reliable proxy for the ultraviolet flux from star formation activity. In some extreme star forming galaxies, the energy in this one infrared CII line alone can be as much as one percent of the entire energy budget of the galaxy. The extreme brightness of the line makes it a very powerful tool for studying cosmically remote galaxies in the early universe because it is one of the easiest lines to detect and its measured wavelength, shifted by expansion of the universe, provides a precise measure of the galaxy's distance. All this means that astronomers are working towards a more precise understanding of how and where carbon is ionized by young stars. One major outstanding puzzle is that in some bright star-forming galaxies the strength of the CII emission is as much as one hundred or more times weaker than it is in the strongest cases, and the reason is not well understood.