Gamma-Ray Bursts

Seconds-long flashes that outshine galaxies, discovered by satellites watching for nuclear tests.

US Vela satellites monitoring the test-ban treaty stumbled on them in 1967; the find stayed classified for years.
Long bursts mark collapsing massive stars; short ones, neutron-star mergers — confirmed by GW170817.
For their brief peak they are the most luminous events known in the universe.