The discovery filled a long-known chink in the accepted explanation of how the chemistry of the universe evolved from pure hydrogen and helium into the diverse place it is today. Stars and supernovas could manufacture the elements up to iron or so, according to classic papers in the 1950s but heavier elements required a different thermonuclear chemistry called r-process and lots of free neutrons floating around. Where would they have come from?

One idea was neutron star collisions, or kilonovas, which now seem destined to take their place on the laundry list of cosmic catastrophes along with the supernova explosions and black hole collisions that have shaped the history of the universe.

Until now there was only indirect evidence of kilonovas. Astronomers found a fireball from a gamma-ray burst in 2013, but there was no proof that neutron stars were involved. Now astronomers know they are, completing the picture of the origin of bling.

One burning question is what happened to the remnant of this collision. According to the LIGO measurements, it was about as massive as 2.6 suns. Scientists say that for now they are unable to tell whether it collapsed straight into a black hole, formed a fat neutron star that hung around in this universe for a few seconds before vanishing, or remained as a neutron star. They may never know, Dr. Kalogera said.

Neutron stars are the densest form of stable matter known. Adding any more mass over a certain limit will cause one to collapse into a black hole, but nobody knows what that limit is.

Future observations of more kilonovas could help physicists understand where the line of no return actually is.

Dr. Holz, the University of Chicago astrophysicist, said, “I still can’t believe how lucky we all are,” reciting a list of fortuitous circumstances. They had three detectors running for only a few weeks, and it was the closest gamma-ray burst ever recorded and the loudest gravitational wave yet recorded. “It’s all just too good to be true. But as far as we can tell it’s really true. We’re living the dream.”