Mysterious high-speed “bullets”—clumps of possibly oxygen-rich gas traveling at up to 20 million miles per hour—have been discovered shooting out of the rarest stellar explosion in our galaxy.

They were spotted after a cloud of debris surrounding the Milky Way’s only known helium nova finally cleared after more than 20 years, revealing that an unusual stellar system was to blame for the extraordinary explosion.

But the origin of the “bullets” is an enigma that has left astronomers puzzled—nothing of their kind has ever been observed in other novae throughout the universe.