About a third of massive stars lose their hydrogen-rich envelopes through mass exchange with a companion star; a process that completely changes the life of the star. Instead of growing large, cool and red, the star becomes small, hot and blue since it now only consists of the exposed helium core. The deaths of stripped stars are also different from their single star counterparts. While a massive single star explodes as a hydrogen-rich supernova, most stripped stars explode as stripped-envelope supernovae (type Ib/c) and a fraction of them form the double neutron star systems the merge in gravitational wave events. Because stripped stars are so hot, they emit a large fraction of their radiation as ionizing, which makes them a stellar source of ionizing radiation that rarely is accounted for when modeling feedback from stellar populations.

I will focus on the ionizing properties of stripped stars. I will start on a small scale and describe how we used detailed evolutionary and spectral models to identify candidate stripped stars. We combine the detailed models with population synthesis to estimate the contribution from stripped stars to the integrated spectra of stellar populations. The models show a delayed emission of ionizing radiation because stripped stars are formed over an extended period in time. The late ionizing emission mimics the presence of massive stars and can affect the determination of mass and age of galaxies and stellar clusters. When accounting for the ionizing emission from stripped stars over cosmic history, we discovered that stripped stars could have been one of the main sources that drove the reionization of the Universe.