Neutron stars are the "trash-compacted" surviving cores of massive stars that exploded. Weighing more than our Sun, they would fit inside New York City. At this unimaginable density, one teaspoon of surface material would weigh at least 4 billion tons on Earth.

If that doesn't stagger the imagination, just think of what happens when two of these cannon balls collide head-on. They ripple the very fabric of time and space in a phenomenon called gravitational waves, which can be measured by detectors on the ground.

The explosive event, named GW170817, was observed in August 2017. The blast released the energy comparable to that of a supernova explosion. It was the first combined detection of gravitational waves and gamma radiation from a neutron star merger.

In the aftermath of the smashup a blowtorch jet of radiation was ejected at nearly the speed of light, slamming into material surrounding the obliterated pair. Hubble was on the scene of the explosion just two days after the collision. Astronomers used Hubble to measure the motion of a blob of material the jet slammed into. As the jet rocketed away from the site of the explosion, the blob moved outward like a leaf caught on a stream of water from a garden hose. The incredible precision, gleaned from Hubble and radio telescopes, needed to measure the blob's trajectory, was equivalent to measuring the diameter of a 12-inch-diameter pizza placed on the Moon as seen from Earth. This was a major watershed in the ongoing investigation of neutron star collisions that keep ringing throughout the universe.

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