Three milliseconds before the collision, black hole (a region with unimaginable gravity that traps even light) and neutron star (the ultradense core of a dead star) spiral in for their final dance. Tidal forces rip the star apart, scattering its material. The temperature instantly skyrockets to hundreds of billions of degrees — thousands of times hotter than the center of the Sun. Thus the celestial forge opens.

In this inferno, nucleosynthesis kicks off — rapid neutron capture. Atomic nuclei greedily absorb neutrons and transform into ever heavier elements: from iron to gold, platinum, and uranium. The whole process takes seconds. The ejected cloud takes the shape of a crescent, flying at one-third the speed of light. If such a jet went from Earth to the Moon, it would get there in a couple of seconds.

The light from this cosmic forge — a flash shining like a hundred million Suns, but fading in weeks. Unlike supernovae (stellar explosions) that are visible for months, this flash is fleeting. Its brightness strongly depends on the angle from which we view the cloud. If the cloud is edge-on, we see a bright glow; if face-on, it is nearly invisible. That's why one recent gravitational wave alert had no light counterpart: the cloud was facing the wrong way.

Now, armed with computer numerical simulations, astronomers know where to point telescopes. The infrared eyes of JWST and the detectors of LIGO jointly search for such mergers, catching both gravitational waves (ripples in space) and light. After the catastrophe, around the black hole sometimes forms an accretion disk of infalling material and a narrow jet (stream of particles), and light from distant events is stretched by the redshift of the Universe. All this information helps read the story of each merger. And then it becomes clear: the atoms of gold in a wedding ring were once born in the celestial forge long before Earth existed.