On November 9, 2024, astronomers reported the first detailed observations of heavy element formation resulting from a neutron star collision, located 130 million light-years from Earth. This event led to a colossal explosion, creating the smallest black hole ever observed and providing a chronological account of heavy atom formation.
Neutron stars are remnants of massive stars (7 to 19 solar masses) that collapse after exhausting their nuclear fuel. Their outer layers are expelled during supernova explosions, leaving behind a hyperdense core containing about two solar masses within a sphere approximately 20 kilometers in diameter. Gravitational collapse forces electrons and protons to combine, forming neutrons.
Some neutron stars exist in binary systems, orbiting either a normal star or another neutron star. If the latter survives the supernova explosion of the first, they generate gravitational waves due to their extreme densities as they spiral closer together. Eventually, their collision produces a kilonova explosion, believed to create heavy elements like gold and platinum.
This process had not been characterized in detail until now. Researchers from the Cosmic DAWN Center at the Niels Bohr Institute, University of Copenhagen, combined light measurements from the kilonova AT2017gfo using multiple telescopes, marking the first observation of these elements' formation. Co-author Rasmus Damgaard stated, "We can now see the moment when atomic nuclei and electrons unite in this residual luminescence." He added, "For the first time, we are witnessing the creation of atoms and measuring the temperature of the matter."
The team analyzed the light from AT2017gfo, which resulted from the catastrophic collision of two neutron stars, producing a small black hole and ejecting neutron-rich material in a plasma sphere expanding at nearly light speed. The kilonova's brightness equaled that of hundreds of millions of suns due to the immense radiation from radioactive decay of the elements involved.
In the immediate aftermath of the collision, the ejected material reached temperatures of several billion degrees, a thousand times hotter than the Sun's core and comparable to conditions in the universe just one second after the Big Bang. These extreme conditions caused electrons to detach from atomic nuclei, forming a continuously moving ionized plasma.
As time passed, the material cooled, similar to the universe post-Big Bang. About 370,000 years after the Big Bang, the material had cooled enough for electrons to bind with atomic nuclei, forming the first atoms. A similar rapid neutron capture process occurs during a kilonova explosion, creating elements heavier than iron.
Albert Sneppen, the study's lead author, noted that the kilonova's development is so rapid that no single telescope can capture its entire history due to Earth's rotation. Therefore, the team combined measurements from telescopes in Australia, South Africa, and the Hubble Space Telescope.
This collaborative effort provided a timeline of heavy atom formation. After the kilonova explosion, the expanding sphere of material takes hours for light to traverse completely, allowing researchers to trace the explosion's chronology from the sphere's edge. In the part closest to Earth, electrons are already attached to atomic nuclei, while the black hole is still forming at the far side.
Damgaard remarked, "It's like observing three radiations from the cosmic microwave background around us, but here we can see everything from the outside. We witness before, during, and after the moment of atom birth." The researchers observed the formation of heavy elements like strontium and yttrium and suspect that other unlisted heavy elements may have also formed.