One of the most rapidly changing stars ever seen, the "born-again" star called Sakurai's Object has entered a new stage of its evolution. This stage involves the star shedding material in a powerful stellar wind while heating up and extending over a region similar in size to our solar system.
The star, also known as V4334 Sagittarii, is located around 10,000 light-years away and became an object of intense fascination for astronomers in 1996 when it sprang back to life after earlier entering the final stages of its evolution.
Most stars evolve over millions or even billions of years, timescales far longer than a human lifetime. That means astronomers usually have to build a picture of stellar evolution from snapshots of different stars at different stages of their lives. By occurring in a figurative blink of a cosmic eye, the evolution of Sakurai's Object offers scientists a rare opportunity to investigate the stellar life cycle.
"Sakurai's Object offers something far rarer. It is one of the very few stars known to have changed dramatically within just a few decades, giving us the opportunity to watch stellar evolution unfold in real time," Albert Zijlstra from Jodrell Bank Centre for Astrophysics at The University of Manchester said in a statement. "With our observations, we can test theories of how stars evolve and gain new insights into one of the shortest and least understood phases in the life of a dying star."
The pathology of a dying star
Sakurai's Object was once like the sun, but it long ago exhausted its fuel needed for nuclear fusion, meaning it was on its way to collapsing to become a white dwarf stellar remnant. That is the kind of dense stellar corpse the sun will leave behind when it dies in around 4.6 billion years.
That was until Sakurai's Object underwent a rare event called a very late thermal pulse, which occurs when a layer of helium inside a star reignites suddenly. This reignition causes the star to rapidly expand and cool while also ejecting vast amounts of stellar material into space. This results in the star effectively returning to an earlier stage in its evolution, giving it a second chance at life, hence the "born again" tag.
Scientists have only witnessed this very late thermal pulse process occurring in two stars, Sakurai's Object and V605 Aquilae. However, the fact that Sakurai's Object ejected vast amounts of material as it sprang back to life means that it is surrounded by a vast envelope of gas and dust, making it difficult to study.
To perform their study, this team turned to the Very Large Telescope (VLT), making observations of Sakurai's Object that could then be compared to stellar evolution models simulating the atmospheres and powerful winds of Wolf-Rayet stars.
This investigation revealed that Sakurai's object has a temperature between 48,000 and 35,700 degrees Fahrenheit (27,000 and 36,000 degrees Celsius). This means the born again star is now reheating after its expansive resurrection seen in 1996, but not quite how scientists expected.
"One of the key questions is how quickly Sakurai's Object should recover after its dramatic eruption. Our measurements show that the star is reheating more gradually than some earlier models predicted," Zijlstra said. "That gives us an important way of testing which theories best describe what happens when a dying star briefly springs back to life."
The team's findings indicate Sakurai's Object is at an earlier stage of its evolution than V605 Aquilae, which underwent its own very late thermal pulse episode around 80 years ago.
The team will now continue to observe Sakurai's Object as it continues its journey toward becoming a white dwarf.
"As we continue to monitor the star over the coming years, we expect to learn much more about this remarkable phase of stellar evolution," Zijlstra added.
The team's research was published on Wednesday (Sept. 16) in the journal Monthly Notices of the Royal Astronomical Society.
