In the 1960s, the United States launched a series of satellites to verify a treaty by watching the skies for nuclear bombs set off by the Soviet Union. The Vela satellites didn’t sniff out any bombs. But they did find some big explosions: titanic blasts by dying stars. And the first one was detected 50 years ago today.

The discovery remained a secret until 1973, when project scientists reported on 16 such explosions in a scientific journal. They called the blasts “gamma-ray bursts.” It took decades for astronomers to get a pretty good idea of what caused them. And even today, their story is incomplete.

We do know that all the bursts seen so far have occurred far outside our home galaxy. Yet when they appear, they far outshine entire galaxies of stars. Something that’s so bright yet so far away must be extremely powerful — so powerful that it took a long time to figure out the source.

The longest-lasting bursts probably represent the death throes of massive stars. At the end of such a star’s life, its core collapses to form a black hole or a neutron star. The star’s outer layers fall inward, with some of their gas forming a superhot disk around the collapsed core. Some of the material in the disk blasts its way through the poles of the dying star, forming powerful jets of gamma rays. We see the outburst only if one of the jets aims our way.

This model isn’t complete. But it’s the best idea so far to explain outbursts that were first seen 50 years ago today.

Script by Damond Benningfield