Molecular clouds in interstellar space can sometimes produce natural masers (the radio wavelength analogs of lasers) that shine with bright, narrow beams of radiation. Regions of active star formation generate some of the most spectacular such masers—in one case radiating as much energy in a single spectral line as does our Sun in its entire visible spectrum. In these sources, the maser radiation comes from molecules like water or OH that are excited by collisions and the radiation environment around the young stars.