“For the first time, we see the creation of atoms; we can measure the temperature of the matter and see the microphysics in this remote explosion.”
Astronomers have observed a monumental collision between two neutron stars, leading to the formation of the smallest black hole ever detected and the creation of precious metals such as gold, silver, and uranium.
The team’s depiction of this dramatic event, which occurred 130 million light-years away in the galaxy NGC 4993, was produced using various instruments, including the Hubble Space Telescope. It aims to shed light on the “past, present, and future” of these dense star mergers, potentially revealing how elements heavier than iron, which cannot form even in the largest stars, originate.
The collision and merger of neutron stars trigger a luminous phenomenon called a “kilonova.” As the remnants from this cataclysm expand at nearly the speed of light, the kilonova brightens the surrounding space with light comparable to hundreds of millions of suns.
A team of researchers led by scientists from the Cosmic DAWN Center at the Niels Bohr Institute developed this new understanding of neutron star mergers while delving into the enigmatic nature of kilonovas.
“We can now witness the moment when atomic nuclei and electrons unite during the afterglow,” stated Rasmus Damgaard, a researcher at the Cosmic DAWN Center. “For the first time, we observe the creation of atoms, measure the matter’s temperature, and analyze the microphysics of this distant explosion.”
“It’s akin to admiring three-dimensional cosmic background radiation surrounding us from every direction, but here, we see everything from an external perspective. We observe before, during, and after the atoms’ formation.”
The gold in your jewelry originates from some of the universe’s most violent events.
Neutron stars form when stars at least eight times the mass of the sun burn through their nuclear fusion fuel and can no longer resist their gravitational pull.
As these stars explode in supernovae, their outer layers are blasted into space, leaving behind a dense core with a mass equivalent to 1–2 suns compressed into a sphere about 12 miles (20 kilometers) in diameter.
This core collapse forces electrons and protons to combine, forming a dense sea of neutrons. The density is so extreme that a sugar cube-sized portion of neutron star material would weigh a billion tons on Earth—comparable to squeezing 150 million elephants into a sugar cube-sized space.
Given this extreme density, it’s not surprising that neutron stars play a crucial role in forming elements heavier than iron.
Neutron stars aren’t always isolated. Some exist in binary systems alongside a companion star. In rare cases, the companion star is also large enough to become a neutron star without being flung away by the initial supernova explosion.
This results in a binary neutron star system where both dense stars orbit each other. Due to their immense density, as they orbit, they create ripples in spacetime, known as gravitational waves, which propagate through space and carry away angular momentum.
As the system loses angular momentum, the stars spiral closer to each other. This speeds up the emission of gravitational waves, further accelerating the loss of angular momentum until the stars are close enough for their gravity to force a merger.
The resulting collision ejects neutron-rich matter at billions of degrees—thousands of times hotter than the sun—and similar in temperature to the universe just a second after the Big Bang.
During this violent event, particles like electrons and neutrons swirl around the merged body. The neutron stars quickly collapse into a black hole, enveloped in a plasma cloud that cools over the next few days.
Within this cooling plasma cloud, atoms rapidly capture free neutrons through the rapid neutron capture process (r-process) and bind to electrons. This results in the formation of heavy but unstable elements that decay rapidly, emitting light observed as kilonovas. This decay also yields lighter elements still heavier than iron, such as gold, silver, and uranium.
The team detected the afterglow of particles forming heavy elements like Strontium and Yttrium, suggesting that other heavy elements likely formed in the aftermath of this neutron star collision.
“The matter expands so rapidly that within hours, light takes time to cross the explosion,” explained Kasper Heintz, a researcher at the Niels Bohr Institute. “This allows us to observe different moments of the explosion by simply looking at its edges. Closer to us, electrons have already attached to nuclei, while further away, near the nascent black hole, the present moment remains in the future.”
The team’s achievement was made possible through the combined efforts of telescopes worldwide.
“This astrophysical explosion evolves dramatically over time, making it impossible for a single telescope to monitor its entire progression. The Earth’s rotation blocks individual telescopes’ views of the event,” said Albert Sneppen, team leader and researcher at the Niels Bohr Institute. “By integrating data from observatories in Australia, South Africa, and the Hubble Space Telescope, we could track the event’s evolution in remarkable detail.”
The team’s findings were published on Wednesday (Oct. 30) in the journal Astronomy & Astrophysics.
