For only the second time in history, the LIGO and Virgo detectors have picked up gravitational waves produced by two neutron stars spiralling into each other. The signal, designated GW250508, was detected last week and has already yielded new measurements that sharpen our understanding of matter under extreme conditions.

Neutron star mergers are among the most energetic events in the observable universe. Two stellar remnants, each roughly the mass of the Sun compressed into a sphere the size of a city, lose orbital energy as gravitational waves until they collide in a fraction of a second. The resulting explosion — a kilonova — is thought to be one of the primary factories for heavy elements including gold, platinum and uranium.

The first confirmed neutron star merger, GW170817 in 2017, came with an electromagnetic counterpart detected by dozens of telescopes worldwide. That event transformed multi-messenger astronomy from a theoretical aspiration into a working science. GW250508 is already being followed up by space and ground-based observatories across the spectrum.

Preliminary analysis suggests the two stars in this event were unusually massive — combined, they exceeded three solar masses, near the upper boundary of what neutron stars are thought to be capable of. That raises a provocative question: did the merger produce another neutron star, or did the combined mass push it over the threshold into a black hole?

The electromagnetic follow-up data, still coming in, should help resolve this. A short gamma-ray burst accompanying the merger would suggest black hole formation. Early reports from the Fermi satellite are ambiguous, which itself is scientifically interesting.

For the broader community, the most immediate payoff is another measurement of the Hubble constant — the rate at which the universe is expanding. GW170817 gave one estimate; GW250508 provides a second, independent data point in an ongoing effort to resolve a stubborn tension between different measurement methods.

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Physicists Detect a Second Gravitational Wave from a Neutron Star Merger