The recent neutron star merger produced what is currently the smallest known black hole. The collision unleashed an explosive fireball expanding at near-light speeds, illuminating with an intensity comparable to hundreds of millions of suns in the days following the impact. This massive release of energy, known as a kilonova, glowed due to the radioactive decay of heavy elements synthesized in the explosion.

Through a coordinated effort involving telescopes across the globe, an international team led by the Cosmic DAWN Center at the Niels Bohr Institute has drawn closer to answering a fundamental question in astrophysics: How are elements heavier than iron formed?

Global Telescope Network Captures Event in Detail
"This astrophysical explosion unfolds dramatically with each passing hour, making it impossible for any one telescope to document the entire event," explained Albert Sneppen, a PhD student at the Niels Bohr Institute and lead author of the study. By integrating observations from Australia, South Africa, and the Hubble Space Telescope, researchers have been able to trace the explosion's evolution in rich detail. "We show that the whole offers much more insight than the sum of individual data sets," Sneppen added.

Moments after the neutron star collision, fragments of stellar matter reached temperatures of several billion degrees - thousands of times hotter than the Sun's core and comparable to conditions one second after the Big Bang. Such extreme temperatures force electrons to detach from atomic nuclei, creating an ionized plasma. The cooling process that follows mirrors the gradual cooling of the Universe after its inception.

Detecting the Fingerprint of Strontium Among Heavy Elements
Approximately 370,000 years after the Big Bang, the Universe had cooled enough for electrons to bind to atomic nuclei, forming the first atoms and allowing light to travel unimpeded. This epoch of recombination produced the "cosmic background radiation," which offers us a view into the early Universe. Now, a similar recombination process is observable in the stellar matter of this kilonova.

One significant finding is the presence of heavy elements such as strontium and yttrium. While their detection was straightforward, it suggests that many other heavy elements whose origins remained unknown were likely formed in the explosion. "We now have direct evidence of atomic nuclei and electrons merging in the kilonova's afterglow," said Rasmus Damgaard, a PhD student at the Cosmic DAWN Center and co-author of the study. "For the first time, we're witnessing atom formation, measuring matter's temperature, and observing the microphysics of this distant explosion. It's like observing a cosmic backdrop of light from all sides but seeing it unfold from the outside."

Kasper Heintz, assistant professor at the Niels Bohr Institute and study co-author, added, "The matter in the kilonova expands so rapidly that light needs hours to traverse the fireball. This means that, by observing its farthest reaches, we glimpse back in time to earlier stages of the explosion. Closer to us, electrons have already bound to atomic nuclei, but farther out - on the opposite side of the nascent black hole - this binding remains a future event."

Research Report:Rapid kilonova evolution: Recombination and reverberation effects

Related Links
University of Copenhagen - Faculty of Science
Stellar Chemistry, The Universe And All Within It