Three milliseconds before the merger. A gargantuan black hole spinning at breakneck speed drags in a neutron star — a ball the size of a city but with 1.5 times the Sun's mass. Tidal forces stretch the star into a thread as thin as a needle and fling its matter outward. Thus opens a celestial forge, where temperatures skyrocket to hundreds of billions of degrees and matter accelerates to 0.3 times the speed of light — faster than a trip from Earth to the Moon in a couple of seconds.

In a series of six computer reenactments of this cataclysm, astrophysicists varied the black hole’s spin and the mass ratio. Each simulation solves Albert Einstein's equations of gravity, enriched with magnetic fields and nuclear physics. The computations ran on supercomputers, modeling every millisecond of the collapse. Fritz Zwicky had suggested back in the 1930s that such cataclysms spawn neutron stars and cosmic rays, but the process details stayed mysterious until numerical methods arrived.

The ejected material compresses into a scorching crescent stretched along the equator. This crescent is only a few degrees thick, yet its speed reaches 0.3c. For comparison, such a jet would travel from the Sun to Earth in half an hour. Inside this cloud, nucleosynthesis kicks off instantly — rapid neutron capture, or the r-process. Atomic nuclei, like a sponge, soak up neutrons and promptly convert them into protons, step by step climbing the periodic table toward gold, platinum, uranium. This forging takes mere seconds.

The light born in this furnace, a kilonova, shines as bright as a million Suns, but not for long. Unlike ordinary supernovae, whose glow lasts months, a kilonova flares and fades within a week or two. Its brightness depends heavily on viewing angle: edge-on, the crescent's infrared flash is brilliantly visible; seen from above, it can elude even the sharpest telescopes. That's exactly what happened with event S190814bv: LIGO detected gravitational waves, but there was no light. For the famous flash AT2017gfo, however, the theoretical light curves fit the observations like a glove.

Now, with such templates in hand, astronomers are gearing up for the era of new sky surveys. The infrared eyes of JWST and ground-based giants will purposefully hunt kilonovae at the very viewing angle where their emission peaks. From subtle details in these light curves, we can weigh the accretion disk born after the merger and determine whether it launches a jet — a narrow beam of particles that may ignite a short gamma-ray burst. Meanwhile, inside computer memory, next-generation numerical simulations are spinning: they add neutrino-driven winds, jets, and disk evolution over tens of seconds. When these calculations incorporate all ingredients, we'll be able to read the story of each merger from its multicolor spectrum, stretched by cosmological redshift. And then, perhaps, we’ll find that the gold atoms in a wedding ring are ashes scattered by a celestial forge long before the Sun was born.