Astrophysicists have, for the first time, observed the birth of heavy elements during the collision of neutron stars.

In 2017, astronomers recorded the event AT2017gfo, which resulted from the merger of two neutron stars. This rare phenomenon, known as a "kilonova," is accompanied by the release of an immense amount of energy and matter, serving as an ideal "laboratory" for studying nucleosynthesis processes—the formation of new atomic nuclei from protons and neutrons.

It is important to note that kilonovas are observed in binary systems during the merger of two neutron stars or a black hole with a neutron star. The energy emitted during these astronomical events can exceed that of novas (stars whose brightness suddenly increases by approximately 1,000 to 1,000,000 times) by 1,000 times. Kilonovas also act as sources of strong electromagnetic radiation, gravitational waves, and elements heavier than iron.

The authors of a new study, published in the journal Astronomy & Astrophysics, concluded that the bright cosmic cataclysm AT2017gfo resulted in the formation of a fireball expanding at a speed of about 40-45 percent of the speed of light. Interestingly, in the following days, the kilonova shone with a brightness comparable to that of hundreds of millions of suns.

A team led by Albert Sneppen from the University of Copenhagen (Denmark) collected and analyzed observational data of the AT2017gfo event obtained from telescopes around the world, covering a period from 0.5 to 9.4 days after the merger, and tracked the changes in the chemical characteristics of the kilonova over time.

"We can now observe the moment when atomic nuclei are formed in the glow following the kilonova explosion. For the first time, we are witnessing the creation of atoms in this way, allowing us to measure the temperature and see the microphysics of the process in this distant explosion," explained one of the authors of the study, Rasmus Damgaard.

The team noted that one of the key discoveries was the observation of a spectral line at a wavelength of one micrometer (the wavelength of electromagnetic radiation in the optical range), which suddenly appeared 1.17 days after the merger, indicating the presence of heavy elements such as strontium (Sr II) and yttrium (Y) in the kilonova ejecta.

Moreover, the timing of the appearance of strontium spectral features aligns with predictions based on local thermodynamic equilibrium models (i.e., equilibrium within a mass volume of the system), confirming the correlation between the observed emission temperature and the ionization temperature. The latter turned out to be almost uniform in all directions, with a variation of less than five percent. This suggests that the processes occurring during the merger of neutron stars are more symmetrical than previously thought.

Since the merger of neutron stars is considered one of the primary sources of elements heavier than iron, including gold and platinum, the ability to observe these processes in real-time allows for the testing and refinement of existing nucleosynthesis theories. Thus, the results of the study provided new insights into the dynamics of kilonova ejecta and the conditions under which heavy elements are formed.