The Stingray Nebula is the youngest known planetary nebula. For half a century astronomers have witnessed its formation, and now they’ve noticed something strange: it’s fading away.
“This is very, very dramatic, and very weird,” said Martín A. Guerrero of the Instituto de Astrofísica de Andalucía in Granada, Spain, one of the co-authors of a new study on the Stingray Nebula. “What we’re witnessing is a nebula’s evolution in real-time. In a span of years, we see variations in the nebula. We have not seen that before with the clarity we get with this view.”
Between 1996 and 2016, successive Hubble images of the Stingray Nebula revealed that it’s getting weaker, dimmer, and less prominent. It’s a shadow of its former self, and getting weaker by the day. As an example, the brightness due to the presence of ionized oxygen dropped by an unprecedented factor of 1,000 in the twenty years between observations.
What’s going on?
The Stingray Nebula in 1996 (left) and 2016 (right). Image credit: NASA, ESA, B. Balick (University of Washington), M. Guerrero (Instituto de Astrofísica de Andalucía), and G. Ramos-Larios (Universidad de Guadalajara)
The astronomer Karl Gordon Henize was the first to categorize the central star of the Stingray Nebula, known as SAO 244567, in 1967 as a bright blue giant. Just four years later astronomers realized that the late-stage star is surrounded by a faint nebula, which they quickly identified as the beginnings of a planetary nebula. The Stingray has the official designation of Hen 3-1357, and is the kind of nebula formed by a star as it nears the end of its life and begins to expel its outer layers into the surrounding system.
Beginning in 1971, SAO 244567 began to skyrocket in temperature, climbing from 40,000 to 108,000 degrees Fahrenheit by the turn of the millennium. And then it reversed course, cooling and dimming in the process.
As the intensity from the central star dropped, the amount of radiation hitting the nebula fell. Like dimming the lights in your room to make it more moody, the Stingray Nebula slowly shut off.
Astronomers suspect that the sudden spike in temperature was due to a helium flash, a critical condition inside a giant star where a shell of unburnt helium fuses in as little as a day, releasing a tremendous amount of pent-up energy. But because stars are so large, it can take years for the changes to be felt on the surface.
But now that the star is returning to normal, the brilliance of the Stingray is fading. Presumably this happens all the time throughout the universe, but this time astronomers were lucky enough to catch the process in action.