The collision of two neutron stars has created the smallest black hole ever seen. This dramatic event also produced a massive fireball that expanded nearly at the speed of light, shining as brightly as hundreds of millions of Suns. This bright object, known as a kilonova, glows intensely due to the radiation from heavy, radioactive elements formed in the explosion.
By studying the light from the kilonova with telescopes around the world, an international team of researchers, led by The Cosmic DAWN Center at the Niels Bohr Institute, is getting closer to understanding this explosion and answering an important question: Where do elements heavier than iron come from?
Scientists have noticed the temperature of tiny particles during the bright light that happens when two neutron stars crash into each other and form a black hole. This is the first time they could measure these space events’ small, physical details. It also shows that quick observations can capture how something changes over time.
Albert Sneppen, a PhD student at the Niels Bohr Institute, explains that astrophysical explosions evolve rapidly, making it impossible for a single telescope to capture the entire event due to the Earth’s rotation. However, researchers can closely track the explosion’s development by integrating data from telescopes in Australia, South Africa, and the Hubble Space Telescope. Sneppen emphasizes that this combined approach reveals insights beyond what each dataset offers individually.
Following the collision, the fragmented star matter reaches temperatures of billions of degrees—about a thousand times hotter than the Sun’s core and similar to the Universe’s temperature just after the Big Bang.
At these extreme temperatures, electrons exist in an ionized plasma free from atomic nuclei. As time passes—over minutes, hours, and days—the star matter begins to cool, mirroring the cooling process of the Universe after the Big Bang.
About 370,000 years after the Big Bang, the Universe cooled enough for electrons to bond with atomic nuclei, forming the first atoms. This allowed light to travel freely, leading to the emergence of “cosmic background radiation,” which fills the night sky.
A similar process is observed in the star matter from the explosion, where electrons reunite with atomic nuclei. This has resulted in the detection of heavy elements like Strontium and Yttrium, and likely, many other heavy elements of uncertain origin were also produced in the explosion.
Rasmus Damgaard, PhD student at Cosmic DAWN Center and co-author of the study, said, “We can now see the moment where atomic nuclei and electrons are uniting in the afterglow. 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.”
“It is like admiring three cosmic background radiation surrounding us from all sides, but here, we get to see everything from the outside. We see before, during, and after the moment of birth of the atoms.”
Kasper Heintz, co-author and assistant professor at the Niels Bohr Institute continues: “The matter expands so fast and gains in size so raidly, to the extent where it takes hours for the light to travel across the explosion. This is why, just by observing the remote end of the fire ball, we can see further back in the history of the explosion.”
Journal Reference:
- Sneppen, Watson, Damgaard, Heintz et al. Emergence hour-by-hour of r-process features in the kilonova AT2017gfo. Astronomy & Astrophysics. arXiv:2404.08730v2