Astronomers discover rare ‘fossil star’ in outer regions of Milky Way

When the universe began, only hydrogen and helium existed. All other elements had to be created in the center of massive stars — and it took millennia for those heavier elements to spread throughout the cosmos.

Theories suggest “fossil stars” might have included many massive stars that were 140 times more massive than the sun. The intense ultraviolet radiation and energetic supernovae, or explosions, of those massive stars are thought to have considerably impacted the formation of the second generation of stars.

However, clear observational evidence of these ancient stars has remained elusive. At least until now.

In a groundbreaking study, a team of astronomers — with the help of the Subaru Telescope atop Maunakea on the Big Island — has discovered an ancient star bearing a unique chemical makeup unlike anything ever before observed in the Milky Way galaxy.

Among the 400 candidates of chemically pristine stars observed by researchers from the National Astronomical Observatory in Japan, National Astronomical Observatories of China and other institutions as part of the new study, they found one — called LAMOST J101051.9+235850.2 (J1010+2358) — in the outer regions of the Milky Way that displayed distinct chemical makeups that offer the clearest evidence yet of such supernovae.

The research team has worked nearly 10 years to study early generation stars identified using the Chinese survey telescope LAMOST. They were able to measure the detailed chemical compositions of those stars using the Subaru Telescope.

“Like archaeologists excavate fossils to study an ancient history of Earth, we identify a ‘fossil star’ in the Milky Way galaxy and examine details of chemical patterns to know the nature of the first stars that exploded more than 10 billion years ago,” explained professor Wako Aoki with the National Astronomical Observatory in Japan, who led the observing programs with the Subaru Telescope. “We are excited to find one of the strongest links between a nearby star and the extremely massive first-generation stars that only existed in the early universe.”

This exceptional find bolsters theories that some of the first generation of stars in the universe included unusually massive objects. The discovery also has critical implications in understanding the nature of the first stars in the universe and how they were formed in the aftermath of the Big Bang.

“We believe the star, J1010+2358, we have discovered provides the most convincing evidence that the first stars with mass larger than 100 times that of the sun existed in the early universe,” said Subaru Telescope assistant professor Miho Ishigaki.

“It provides an essential clue to constraining the initial mass function in the early universe,” added professor Zhao Gang, a co-author of the study. “Before this study, no evidence of supernovae from such massive stars has been found in the metal-poor stars.”

An article about the new study — “A metal-poor star with abundances from a pair instability supernova” — was published in the journal Nature on June 7.

The focus will now shift to understanding the percentage of the first stars that were very massive — a question that can be answered by exploring more stars and measuring their chemical compositions.