We think we understand how stars form, live, and die, but sometimes looking out in the universe we discover things that don’t like to behave as they should. This is the case of SAO 244567, a star 2,700 light-years away that has undergone a sudden rebirth.
This object has been visibly evolving for the last 45 years, changing its temperature by almost 40,000°C (72,000°F). And new Hubble observations performed by an international team suggest that the star is now cooling again, going back to the original evolutionary sequence.
In a paper, published in the Monthly Notices of the Royal Astronomical Society, the team looked for confirmation of the rare helium-shell flash scenario they proposed in 2014. This is when the pressure in the star is suddenly enough to fuse helium around the core, expanding and brightening the star.
“SAO 244567 is one of the rare examples of a star that allows us to witness stellar evolution in real time,” said lead author Nicole Reindl from the University of Leicester, UK, in a statement. “Over only twenty years the star has doubled its temperature and it was possible to watch the star ionizing its previously ejected envelope, which is now known as the Stingray Nebula.”
A helium flash occurs in about one-quarter of low-to-medium-mass stars. When a star becomes old, its core runs out of hydrogen and the star turns into a red giant. The star then burns the helium at its core until it becomes depleted too, and hydrogen in the other shells start fusing. When all this material is spent, the helium around the core is suddenly and rapidly ignited, creating the helium flash.
Shown is how the star's rebirth is predicted to play out. ESA/HST/NASA
“The release of nuclear energy by the flash forces the already very compact star to expand back to giant dimensions – the born-again scenario,” Reindl said. Although there have been a couple of observations of similar objects, this is the first time the heating and cooling phase of a star have both been observed.
“We need refined calculations to explain some still mysterious details in the behavior of SAO 244567,” added Reindl. “These could not only help us to better understand the star itself but could also provide a deeper insight in the evolution of central stars of planetary nebulae.”