The odd behavior of a star in the heart of the Stingray Nebula provides tantalizing evidence that we may be seeing, first-hand, its helium-shell flash: an explosive phase of nuclear burning at the end of a star’s life.
Stars and galaxies tend to evolve on timescales that dwarf human experience. The Sun, for example, has changed little over the past 4.5 billion years of its existence. But SAO 244567 — a star nestled in the heart of the Stingray Nebula — is a rare exception to the rule.
SAO 244567 has been behaving oddly over the last few decades, increasing rapidly in temperature and losing an incredible amount of mass. Astronomers speculate that not only are we watching this star evolve in real time, but we may be directly seeing its helium shell flash: an explosive phase of nuclear burning signifying the dramatic end of a Sun-like star’s life.
Because we typically can’t watch stars evolve before our very eyes, we observe large samples of stars at different points in their lives and try to reconstruct a single star’s evolution from the various snapshots. “This is what makes SAO 244567 so special — we can directly see stellar evolution” says lead author Nicole Reindl (Eberhard Karls University, Germany).
The star’s home is the youngest planetary nebula known. These misnamed stellar remnants are created when the outer layers of a dying star blows off and expands into space. So with a size some 0.16 light-years across — one-tenth the size of most known planetary nebula and roughly 130 times larger than our Solar System (as defined by Pluto’s orbit) — it has been expanding outward for a relatively short time.
Observations from as early as 1971 give a temperature of 21,000 Kelvin for the central star. But in 2002, another look gave a temperature as high as 60,000 K. An increase in temperature this high over such a short time period is unheard of. So Reindl and colleagues dug through old images from a range of space- and ground-based telescopes in order to understand this rapidly evolving star.
The team thinks the star’s jump in temperature and huge stellar winds are telltale signs of a helium-shell flash, which so far has remained undetected by observations.
Stars initially draw their energy by fusing hydrogen into helium deep within their cores. But eventually — still many billions of years into the future for our Sun — a star will run out of hydrogen fuel in its core. It then evolves into a cool and bright giant star with a contracted core but an extended atmosphere. (My favorite example of this type of star, a red giant, is Betelgeuse in Orion.)
After slightly messy physics, the star’s contracted core begins to fuse helium into carbon, while a shell above the core continues to fuse hydrogen into helium. This pattern continues for stars of medium to high mass, creating layers of different nuclear burning around the star’s central core.
For lower-mass stars, the helium fusion may turn off due to a lack of fuel, but the layer above still rains down additional helium ash. This additional helium can ignite fusion by chance in a bright helium-shell flash. This then leads to a thermal pulse where the star temporarily appears to brighten and expand.
“A helium-shell flash dumps a huge amount of energy into a star’s atmosphere in a short period of time, which is what causes the rapid evolution,” says coauthor Jeffrey Kruk (NASA Goddard Space Flight Center).
Throughout this rapid evolution, thick clouds of dust and gas cocoon the star during it’s last stage of life. While the flash has already started to die out, ultimately fusion will cease and all that will be left is a cooling ember known as a white dwarf.
N. Reindl et al. “The Rapid Evolution of the Exciting Star of the Stingray Nebula” Astronomy and Astrophysics, 2014