🌟 Rare: a “dead” star briefly lights up again

A star reaching the end of its life has transformed at a speed rarely accessible to astronomers.

Known as Sakurai’s Object, this star suddenly became active again in 1996. Before this episode, it was evolving toward becoming a white dwarf, the small, extremely dense core left behind by some stars after their death. Its temperature was then comparable to that of the Sun, around 5,500°C to 6,000°C. Thirty years later, it has reached approximately 30,200°C.

A layer of helium began producing energy again after the main nuclear activity had ceased. The star expanded and ejected a large amount of matter into space. Astronomers then refer to it as a “born-again” star.

This phenomenon is so rare that only two stars have been directly observed during such a transformation. After its outburst, Sakurai’s Object also disappeared behind thick clouds of gas and dust. Tracking its evolution therefore became particularly difficult.

The researchers used the Very Large Telescope, installed in Chile, to analyze its light. Each chemical element leaves distinctive signatures. The observations reveal carbon and helium in particular, along with a powerful stellar wind. These clues now make it possible to identify the star as a Wolf-Rayet star.

The star is currently ejecting matter at high speed from its surface. Models applied to the new observations yield a temperature of nearly 30,200°C. The value remains within an estimated range of approximately 26,700 to 35,700°C.

This evolution is remarkable for its speed. Major changes in a star generally unfold over periods far longer than a human lifetime. Astronomers usually have to compare several stars observed at different ages to infer a common evolutionary path. Here, they can follow the same object over the course of a few decades.

The measurements also bring a surprise for the researchers. Even after this evolution, Sakurai’s Object remains much cooler than V605 Aquilae, the other born-again star observed directly, whose comparable episode occurred about 80 years earlier. Its surface temperature had risen from approximately 5,000°C to more than 95,000°C.

Observations will now continue to measure this warming more precisely. Sakurai’s Object should subsequently resume its evolution toward becoming a white dwarf. Above all, monitoring it will allow researchers to test, year after year, the calculations describing this very brief phase in a star’s life.