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The discovery of an excess of light in the far-ultraviolet (UV) spectrum inelliptical galaxies was a major surprise in 1969. While it is now clear thatthis UV excess is caused by an old population of hot helium-burning starswithout large hydrogen-rich envelopes rather than young stars, their origin hasremained a mystery. Here we show that these stars most likely lost theirenvelopes because of binary interactions, similar to the hot subdwarfpopulation in our own Galaxy. This has major implications for understanding theevolution of the UV excess and of elliptical galaxies in general. Inparticular, it implies that the UV excess is not a sign of age, as had beenpostulated previously, and predicts that it should not be strongly dependent onthe metallicity of the population

Binary Stars as the Source of the Far-UV Excess in Elliptical Galaxies