A team of international researchers, including astrophysicists from the HUN-REN Research Center for Astronomy and Earth Sciences and the University of Szeged, has determined the timeline for the Sun's formation. Their findings indicate that the Sun formed relatively quickly, within 10 to 20 million years, from the gas material of its birthplace and was not born in isolation but within a larger stellar family.
During their investigation, researchers first measured the decay of fully ionized thallium ions using a specialized particle accelerator at the GSI/FAIR laboratory in Germany. Based on the experimental results, they calculated the amount of radioactive lead (205Pb) produced in the interiors of stars. The researchers established a timeframe for the Sun's formation from its parent gas material, falling between 10 and 20 million years, as reported in a recent HUN-REN CSFK press release.
To study the Sun's formation timeline, astronomers utilize the decay of long-lived radioactive nuclei, which were created in other stars before the Sun's birth. Although these radioactive nuclei have decayed over the 4.6 billion years since the Sun's formation, their decay products can still be detected in meteorites.
Radioactive lead (205Pb) serves as an ideal subject, being the only radioactive nucleus that can form exclusively in medium-mass stars, which are about two to four times the mass of the Sun, through neutron capture.
On Earth, radioactive lead (205Pb) decays into thallium (205Tl) by capturing an atomic electron, converting one of its protons into a neutron. In stars, where temperatures can reach hundreds of millions of degrees Celsius and all electrons are stripped from atoms, the reverse process can occur: 205Tl decays into 205Pb. This decay mode is exceptionally rare.
Due to the complex behavior of these two nuclei, estimating the quantity of radioactive lead produced in stars is only feasible if researchers understand how the decay rates of the two nuclei change with temperature within the star's interior. Measuring these decays under normal laboratory conditions is problematic, as thallium is stable on Earth.
To overcome this challenge, a group of researchers from 37 institutions across 12 countries measured the decay of fully ionized thallium ions using the specialized particle accelerator at GSI/FAIR in Germany. This required stripping thallium of all its electrons and maintaining it in this special state for several hours.
Astrophysicists, including those from the HUN-REN CSFK and the University of Szeged, then estimated the amount of radioactive lead expelled from medium-mass stars by calculating new stellar models.
“The refined decay rates allow us to estimate with high certainty how much radioactive lead (205Pb) was produced in stars and incorporated into the Sun's parent gas cloud,” explained Balázs Szányi, a PhD student at the University of Szeged.
“By comparing the amount of radioactive lead derived from meteorites, we concluded that the Sun formed relatively quickly, within 10 to 20 million years, from the gas material of its birthplace. This aligns with other radioactive nuclei formed in the same stars, indicating that our Sun did not form in isolation but as part of a large stellar family, along with numerous siblings that have long since dispersed and lost each other,” summarized Maria Lugaro, a researcher at the Institute of Astronomy.
“The pioneering experimental setups, the interdisciplinary collaboration of global nuclear physics and astrophysics research teams, and extensive hard work contribute to understanding the nuclear processes occurring within stars. Our new experiment revealed the timeline of events 4.6 billion years ago that led to the formation of our Sun,” emphasized Guy Leckenby, a PhD student at the Canadian national particle accelerator center (TRIUMF) and the first author of the publication.
The researchers dedicate their work to their late colleagues, Fritz Bosch, Roberto Gallino, Hans Geissel, Paul Kienle, Fritz Nolden, and Gerald J. Wasserburg, who supported this research for several decades.