Quasar
Quasars are extremely luminous. They were first identified as being high redshift sources of electromagnetic radiation, including radio waves and visible light. The light (and other energy) appeared to be similar to stars, rather than large sources like galaxies. On the other hand, their spectra had very broad emission lines, unlike any known from stars, hence "quasi-stellar". Their luminosity can be 100 times greater than that of the Milky Way.