In space, an astonishing pair has been found: a tiny but monstrously dense white dwarf and its puffy, lightweight neighbor — a red dwarf. They race around their common center every 1.37 hours. The white dwarf's gravity rips gas from its companion — mostly hydrogen and helium — and that gas, spiraling inward, slams into the magnetic poles, heating up to millions of degrees. Each time the pole faces our way, a radio flash erupts — just like a lighthouse beam piercing the darkness.

This mechanism resembles pulsars — rapidly spinning neutron stars, discovered by Jocelyn Bell Burnell in 1967 — but here the period is much longer. And the key discovery: the white dwarf's mass is teetering on the Chandrasekhar limit. Add just a pinch more matter, and the star will collapse, igniting a supernova that outshines an entire galaxy. Right now, this lighthouse is operating on the brink of its own destruction.