In May 2020, detectors picked up a strange signal from space — ripples in spacetime from the merger of a ultradense sphere and a black hole. In such pairs, the objects usually spiral in on a nearly circular path. This time, though, the orbit was stretched into an ellipse. Astronomers call this “eccentricity”—a measure of how much an orbit deviates from a circle. And it turns out that elongation acts like a magnifying glass: it makes the system’s details far sharper.

The first example is spin. Black holes and neutron stars have spin — how fast they rotate and in which direction. With a circular orbit, it’s hard to tell if the black hole is spinning opposite to the pair’s motion. But if the orbit is elongated, it becomes glaringly obvious. That kind of backward spin is a sign that the pair wasn’t born in a quiet binary system, but rather in the chaos of a star cluster, where objects collide and capture each other. So an eccentric neutron star–black hole merger lets us peer into a violent past.

Eccentricity also helps weigh neutron stars. There’s a hint that they fall into two groups — light ones and heavy ones, around 1,3 and 1,8 solar masses. Ordinary mergers can’t confirm this, even with 30 events. But with elongated orbits, confidence jumps by a factor of 7,5—a fog bank lifting. And a neutron star’s mass tells us how tightly matter is squeezed in its core, one of the deepest puzzles in physics.

Finally, eccentricity helps us trace which stars gave rise to black holes. Astronomers use computer models to track stellar lives and their chemical makeup — the fraction of elements heavier than helium, or metallicity. Different metallicity gives birth to different black holes. Eccentric mergers can tell these scenarios apart with stunning confidence: the data favor one model over another by factors of billions.

That’s how an elongated orbit transforms fuzzy blobs into sharp portraits. Future detectors like the Einstein Telescope will catch more of these events, and gravitational waves will reveal how matter behaves in the universe’s most extreme conditions.