The broadening or splitting of lines in a spectrum by the electric field of electrons and ionized atoms. The effect is much more pronounced in main-sequence stars, which have higher densities of ions and electrons in their atmospheres (hence raising the frequency of collisions between atoms), than in the rarefied atmospheres of red giants. The hydrogen lines are sensitive to Stark broadening, and the resulting linewidths can be used to assign a luminosity class to A stars. The effect is named after the German physicist Johannes Stark (1874–1957).