Stars go through heating, contracting, expanding, and cooling, but these stages of stellar evolution usually play out over millions of years, making it unusual for scientists to see predictions confirmed in their own lifetime. But 2,700 light years from Earth, in the heart of the Stingray Nebula, a small star has allowed astronomers to observe something incredible – a dramatic contracting and heating phase followed by a rapid rebirth into a cooling, expanding star – just as astronomers predicted.

This is the first time astronomers have been able to witness both the heating and cooling phases of a star's transformation over mere decades instead of millennia. And it's the first time they've been able to predict what will happen next, and see it actually happen.

Scientists have been observing the rapidly-changing star, SAO 244567, for the past 45 years. Between 1971 and 2002, the star's surface temperature skyrocketed by almost 40,000° C, while the star shrank from around four times the diameter of our Sun to just a third of its diameter.

It's not unheard of for a star to move quickly through phases of evolution, and it could be easily explained if SAO 244567 had an initial mass of three to four times that of the Sun. But the data shows that it had an original mass similar to our Sun, and low-mass stars of this type usually evolve over much longer timescales. As a result, the reason for the rapid heating of a small star like SAO 244567 has been a mystery.

Nicole Reindle of the University of Leicester, UK, is the lead author of the latest study on the star. In 2014, she and her team put forward a theory that could explain both SAO 244567's low mass as well as its rapid increase in temperature. They suggested the heating was due to a reignition of helium outside the star's core, known by astronomers as a helium shell flash event or late thermal pulse.

Helium flash events usually occur late in the evolution of around 25 percent of low-to-medium-mass stars, usually after the star has entered red giant phase. A simplified explanation goes like this: after all the hydrogen in a star's shell is burnt, it leaves behind helium ash. The helium ash raises the temperature and density of the core until it reignites in an explosive event that causes the star to grow, cool and temporarily brighten.

"The release of nuclear energy by the flash forces the already very compact star to expand back to giant dimensions – the born-again scenario," says Reindl.

For the team's theory to be confirmed, scientists would need to see the star begin to expand and cool again – which is exactly what happened. Observations made recently by the Cosmic Origins Spectrograph (COS) on the Hubble Space Telescope have revealed that the star has started to cool and expand back to giant dimensions, as if it's been reborn into an earlier phase of stellar evolution by the force of the nuclear energy released by the helium shell flash.

"SAO 244567 is one of the rare examples of a star that allows us to witness stellar evolution in real time," explains Reindl. "Over only 20 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."

At this stage, we don't have the evolutionary models to fully explain SAO 244567's behavior. But in being able to observe the star going through both the heating and cooling stages of evolution, we can develop more refined models of the life cycles of stars, and gain a deeper insight into the way the forces of nature work on stellar objects.

The team's paper regarding SAO 244567 is available at (PDF).

Video credit: ESA/Hubble, L. Calçada

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