The Stingray Nebula is the youngest known planetary nebula. Astronomers have witnessed its creation for half a century, and now they have noticed something strange: it's fading.
"It's very, very dramatic and very strange," 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 are observing is the development of a fog in real time. In a few years we will see variations in the fog. We have never seen it with the clarity that we get with this view."
Between 1996 and 2016, successive Hubble images of the Stingray Nebula showed it to become fainter, darker, and less noticeable. It is a shadow of its former self and is getting weaker by the day. For example, due to the presence of ionized oxygen, brightness decreased by an unprecedented factor of 1,000 in the twenty years between observations.
The stingray nebula in 1996 (left) and 2016 (right). Photo credit: NASA, ESA, B. Balick (University of Washington), M. Guerrero (Institute for Astrofísica de Andalusia) 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, as a bright blue giant in 1967. Just four years later, astronomers found that the late-stage star was surrounded by a faint nebula that they quickly identified as the beginnings of a planetary nebula. The stingray has the official name Hen 3-1357 and is a type of nebula that is formed by a star as it nears the end of its life and begins to expel its outer layers into the surrounding system.
Starting in 1971, the temperature of SAO 244567 skyrocketed and rose from 40,000 to 108,000 degrees Fahrenheit around the turn of the millennium. And then it reversed course, cooling and darkening in the process.
As the intensity of the central star decreased, the amount of radiation that hit the nebula fell. As if the lights in your room were dimmed to make it more moody, the Stingray mist slowly turned off.
Astronomers suspect the sudden rise in temperature was due to a flash of helium, a critical condition in a giant star in which a shell of unburned helium melts in just one day, releasing a tremendous amount of pent-up energy. But because the stars are so big, it can take years for the changes to become noticeable on the surface.
But now that the star is back to normal, the stingray's brilliance is fading. Presumably, this happens all over the universe all the time, but this time astronomers were fortunate enough to see the process in action.