NASA’s Hubble Space Telescope has identified an ultra-massive white dwarf, designated WD 0525+526, only 128 light-years from Earth. Ultraviolet measurements show the star is about 1.2 times the Sun’s mass and roughly 21,000 kelvins—both hotter and heavier than most white dwarfs. Using Hubble’s Cosmic Origins Spectrograph, an international team led by the University of Warwick detected carbon lines in the star’s atmosphere. The carbon signature indicates the object formed when two white dwarfs, or a white dwarf and a sub-giant star, merged—a violent origin that stripped away the usual hydrogen-helium envelope. The study, published 13 August in Nature Astronomy, is the first to confirm a merger remnant through ultraviolet spectroscopy and brings the tally of known white-dwarf merger products to seven. Researchers say the finding implies such mergers may be more common than previously thought, refining models of stellar evolution and the pathways that lead to Type Ia supernovae.