Born-Again Star Sakurai’s Object Lets Astronomers Watch Stellar Evolution in Real Time

In a universe where stars typically change on timescales measured in millions or billions of years, astronomers have been granted an extraordinary exception. Sakurai’s Object, a star that stunned observers when it unexpectedly reignited in 1996, has now entered a new and revealing stage of its evolution. Using the European Southern Observatory’s Very Large Telescope in Chile, a research team involving scientists from The University of Manchester and The Valongo Observatory has confirmed that the star has developed the powerful stellar wind characteristic of Wolf-Rayet stars, a hallmark of a brief but crucial phase in the lives of dying stars. The findings, published in Monthly Notices of the Royal Astronomical Society, offer scientists something almost never available in stellar astronomy: the chance to watch the final phases of a star’s life unfold within a single human lifetime.

The story of Sakurai’s Object begins with a star that, by all appearances, had already finished its journey. Astronomers believe it was once similar to our own Sun, a modest star that steadily burned hydrogen in its core for billions of years before exhausting its nuclear fuel. Having completed its normal evolution, the star had already ended nuclear burning and begun its transformation into a white dwarf, the hot, dense remnant core left behind after an ordinary star dies. At this point, the star should have simply cooled quietly for eternity, fading slowly into the darkness of space. Instead, something remarkable happened deep within its interior.

The star underwent an exceptionally rare event known as a very late thermal pulse. In this phenomenon, a layer of helium buried deep inside the dying star suddenly reignites, restarting nuclear fusion in what was supposed to be a dead ember. The consequences are dramatic. The reignition causes the star to rapidly expand and cool, and it ejects large amounts of material into space. In effect, the star temporarily returns to an earlier, puffier stage of its life, reversing billions of years of evolution in a cosmic blink of an eye. This dramatic reversal is why astronomers describe Sakurai’s Object as a born-again star, and it is precisely this behavior that makes the object such a valuable natural laboratory.

Such events are extraordinarily difficult to catch in action. Only two stars have ever been directly observed undergoing this type of dramatic rebirth: Sakurai’s Object and V605 Aquilae, which experienced a similar event roughly eighty years earlier. Because very late thermal pulses are predicted to occur in a small fraction of low- and intermediate-mass stars as they approach the white dwarf stage, each observed example carries enormous scientific weight. Theoretical models of stellar evolution make detailed predictions about how these events should unfold, how quickly the star should expand, cool, eject material, and then reheat, but without real observations, those predictions remain largely untested. Sakurai’s Object provides the test case that theorists have long needed.

Studying the star, however, has never been straightforward. Following its outburst in 1996, Sakurai’s Object became hidden behind thick clouds of gas and dust released during the eruption. This shroud of ejected material obscured the star itself, making direct observation extremely difficult and forcing astronomers to rely on indirect signatures of its changing state. The material thrown off during the rebirth continues to expand outward, and radio observations with the Atacama Large Millimeter/submillimeter Array have shown that the ejected envelope now extends over a region comparable in size to our entire solar system, a striking reminder of just how much mass the star shed in so short a time.

To determine the star’s current condition, the research team took a different approach. They analysed light collected by the Very Large Telescope and compared it with sophisticated computer models that simulate the atmospheres and powerful winds of Wolf-Rayet stars. Wolf-Rayet stars are known for their extreme properties: they are hot, luminous objects that drive dense, fast outflows of material from their surfaces. By matching the observed spectrum against these models, the researchers were able to identify distinctive signatures of carbon and helium in the light from Sakurai’s Object. These chemical fingerprints allowed the team to pin down the star’s temperature, its chemical composition, and the characteristics of its stellar wind, effectively reconstructing the state of a star that cannot be seen directly.

The results reveal that Sakurai’s Object is once again on the move. The analysis suggests that the star’s surface temperature currently lies between roughly 27,000 and 36,000 degrees Kelvin, a significant increase that shows the star is reheating following its dramatic eruption nearly three decades ago. This reheating is a critical part of the predicted sequence: after a very late thermal pulse, the born-again star should gradually shrink and grow hotter as it burns through the freshly ignited helium, eventually returning to the white dwarf cooling track. Measuring how fast that reheating proceeds is one of the most important ways to discriminate between competing theoretical models of the process.

Professor Albert Zijlstra of the Jodrell Bank Centre for Astrophysics at The University of Manchester emphasized why this measurement matters. Most stars evolve so slowly that major changes take place over timescales far longer than a human lifetime, which means astronomers usually have to piece together snapshots of stellar evolution by comparing different stars at different stages of their lives. Sakurai’s Object offers something far rarer: one of the very few stars known to have changed dramatically within just a few decades, allowing researchers to watch stellar evolution unfold in real time. According to Zijlstra, the observations make it possible to test theories of how stars evolve and to gain new insights into one of the shortest and least understood phases in the life of a dying star.

The pace of the recovery is itself a scientific result. Zijlstra noted that one of the key questions is how quickly Sakurai’s Object should recover after its dramatic eruption, and the new measurements show that the star is reheating more gradually than some earlier models predicted. That discrepancy gives astronomers an important way of testing which theories best describe what happens when a dying star briefly springs back to life. Models that predicted a faster return to high temperatures must now be reconsidered, while those that anticipate a slower climb may prove more accurate. In this sense, every additional measurement of the star’s temperature and wind properties narrows the field of viable theoretical descriptions of very late thermal pulses.

The comparison with V605 Aquilae adds another dimension to the findings. The new analysis suggests that Sakurai’s Object is at an earlier stage of its evolution than its counterpart, which underwent its own rebirth around eighty years ago. Because the two stars experienced the same rare phenomenon at different times, they effectively provide two snapshots of the same evolutionary sequence, separated by decades. As Sakurai’s Object continues to reheat and resumes its journey toward becoming a white dwarf, it should pass through stages that V605 Aquilae has already traversed, allowing astronomers to check whether the two objects follow the same path. The team plans to keep observing the star over the coming years, expecting to learn much more about this remarkable phase of stellar evolution, one of the most rapid and unusual ever observed, and one that transforms a slow-motion cosmic story into an event that can be followed within a working astronomer’s career.

Subject of Research: The rebirth and reheating of Sakurai's Object following a very late thermal pulse

Article Title: 'Born-again' star offers rare chance to watch stellar evolution in real time

Article References: 'Born-again' star offers rare chance to watch stellar evolution in real time. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: Sakurai's Object, born-again star, very late thermal pulse, stellar evolution, white dwarf, Wolf-Rayet stars, Very Large Telescope, stellar wind, V605 Aquilae, University of Manchester, Monthly Notices of the Royal Astronomical Society, helium reignition