Be stars are rapidly rotating luminaries with bright gas disks. Usually, a companion star spins them up: mass flowing from the neighbor whirls the star like a bullet in a rifled barrel. But there are also lone Be stars without a partner: their origin long remained a mystery.
New research shows that solitaries can be born from a supernova explosion. In a binary system, a massive helium star "feeds" its neighbor hydrogen fuel, spinning it up. Upon reaching the Chandrasekhar limit (about 1.4 solar masses), the helium star detonates as a type Ia supernova. The blast acts like a gunpowder charge and launches the already-spun-up star into free flight — exactly like a bullet from a barrel.
Such runaway stars carry the "memory" of the explosion: their high speed betrays a turbulent past. Astronomers can track them down by measuring the motion of helium witnesses to bygone cataclysms.
The 'e' in Be stars stands for 'emission'—radiation. It occurs because rapid rotation tears gas from the star's equator, forming a bright disk.