Belgian Team Watches a Dead Star Spring Back to Life
Astronomers led by the Royal Observatory of Belgium have found that a rare “born-again” star has become roughly six times hotter over the past 30 years, one of the fastest temperature increases ever recorded in a dying star. The object, known as Sakurai’s Object, has entered a new phase of its evolution and now radiates the powerful, carbon-rich stellar wind characteristic of a Wolf-Rayet star, according to findings published on 16 September in Monthly Notices of the Royal Astronomical Society.
Because stars normally evolve over millions or billions of years, the discovery offers scientists a once-in-a-generation chance to watch stellar evolution unfold in real time.
A star that died, then woke up
Sakurai’s Object, also catalogued as V4334 Sagittarii, sits in the constellation Sagittarius at the centre of a planetary nebula. Astronomers believe it once resembled our Sun but had already ended nuclear burning and was cooling towards becoming a white dwarf — the hot, dense core left behind after an ordinary star dies.
Then, in 1996, it unexpectedly burst back to life. As the Royal Observatory of Belgium explains, the star underwent a rare event called a “very late thermal pulse,” in which a layer of helium deep inside the dead star suddenly reignited. The pulse caused it to expand, cool and hurl large amounts of material into space, temporarily returning it to an earlier stage of its life. Astronomers call such objects “born-again” stars.
The outburst was first spotted by Japanese amateur astronomer Yukio Sakurai, who initially mistook it for a slow nova. It later became the object’s namesake. Only two stars have ever been directly observed undergoing this kind of dramatic rebirth: Sakurai’s Object and V605 Aquilae, which experienced a similar event about 80 years ago.
Hidden, then measured with the VLT
Following its eruption, Sakurai’s Object vanished behind thick clouds of gas and dust it had ejected, making direct observation difficult. To investigate its current state, the team analysed light collected by the European Southern Observatory’s Very Large Telescope (VLT) in Chile and compared it with sophisticated computer models that simulate the atmospheres and winds of Wolf-Rayet stars.
The observations revealed distinctive signatures of carbon and helium, allowing researchers to determine the star’s temperature, chemical composition and the character of its stellar wind. Their analysis puts the surface temperature at roughly 27,000 to 36,000 Kelvin — compared with a solar-like temperature of about 5,800 Kelvin three decades ago.
“For most stars it is impossible to see them evolve on a human timescale,” said Dr. Peter van Hoof of the Royal Observatory of Belgium. “Sakurai’s Object offers something far rarer… giving us the opportunity to watch stellar evolution unfold in real time.”
A test of competing theories
The study’s significance extends beyond the star itself. Sakurai’s Object evolved far more quickly than earlier models predicted, prompting scientists to develop a new generation of theories about how dying stars behave.
According to The National Tribune, which carried the Royal Astronomical Society release, the new measurements show the star is reheating more gradually than some of those newer models anticipated. That mismatch is valuable: it gives astronomers a natural laboratory for testing which theories best describe what happens when a dying star briefly springs back to life.
“Our measurements show that the star is reheating more gradually than some of these new models predicted,” said Dr. Griet Van de Steene of the Royal Observatory of Belgium. “That gives us an important way of testing which theories best describe what happens when a dying star briefly springs back to life.”
The findings also suggest that Sakurai’s Object is at an earlier stage of its evolution than V605 Aquilae, effectively providing two snapshots of the same process separated by roughly half a century.
What to watch for
The team will continue monitoring Sakurai’s Object as it reheats and resumes its journey towards becoming a helium-rich white dwarf. Each new observation refines a picture that would otherwise require thousands or millions of years to assemble.
“As we continue to monitor the star over the coming years, we expect to learn much more about this remarkable phase of stellar evolution,” Van de Steene added. For a field accustomed to inferring slow cosmic change from scattered evidence, a star that transforms within a single human lifetime remains a rare and welcome anomaly.
According to VRT NWS, the research underscores Belgium’s continuing contribution to astrophysics — a fitting note as the Royal Observatory of Belgium, founded in 1826, marks its 200th anniversary.