Hypervelocity stars are so called because, uh, they travel really fast—enough to break free from the galaxy. Astronomers have long known that this can happen when a binary star system gets caught in a black hole’s grip. Under the right circumstances, one of the stars spirals in toward the black hole while the other is violently flung outward at a tremendous speed.

Over the past decade, astronomers have found 18 such speedy blue stars in the Milky Way that could have been created this way, all tracing their birthplace to the supermassive black hole at the Galactic center. But an international team of researchers has now discovered a new batch of 20 sun-sized hypervelocity stars that don’t appear to originate from the Galactic core, presenting a mystery to astronomers about their origins. The study was presented at the annual meeting of the American Astronomical Society last week.