Stellar evolution takes place over millions – often billions – of years. So when a star is seen undergoing major change in only a few decades, astronomers sit up and take notice.
Between 1971 and 2002 the surface temperature of the star SAO 244567, which sits in the centre of the Stingray nebula, skyrocketed by almost 40,000 degrees Celsius. Now new observations made with the Cosmic Origins Spectrograph on the Hubble Space Telescope have revealed that SAO 244567 has started to cool and expand.
“SAO 244567 is one of the rare examples of a star that allows us to witness stellar evolution in real time,” explains Nicole Reindl from the University of Leicester, UK, and lead author of a study outlining the phenomenon which will be published in the Monthly Notices of the Royal Academy of Sciences.
It’s weird behaviour but not unheard of – a star three to four times the mass of the sun has been seen undergoing such a flash.
But the latest measurements peg SAO 244567’s original mass at about the same as the sun. So what’s going on?
Reindl and her team have an explanation. The dramatic heating, which made it blaze blue-hot, might be due to what is known as a helium-shell flash event: a brief ignition of helium outside the stellar core, causing it to expand and cool again. Reindl calls this “the born-again scenario”.
Within the next few hundred years, SAO 244567 will expand back to its giant dimensions, cooling all the while, and turning orange.
Watch an animation of the star’s evolution below: