Hubble has revealed the drastic fading of a nebula, capturing it going through unprecedented changes over a 20-year period. When it comes to objects of the cosmos, we’re more used to thinking of changes in terms of millions, if not billions, of years, not decades.
The nebula Hen 3-1357, better known as the Stingray nebula, was already an exciting discovery. Hailed as the youngest known planetary nebula in 1998 when Hubble first peeked inside its central star’s final stages of life, now it’s been caught doing something that even the Hubble team are calling “weird”.
Photographed by Hubble in 1996, the nebula’s bright blue tendrils and filaments of gas at its center meant it popped against its dark dramatic background, its wavy edges giving it its stingray moniker. Now archival data shows that by 2016, the nebula had dimmed and its wavy edges had almost disappeared.
“This is very, very dramatic, and very weird,” said Hubble member Martín A. Guerrero in a statement. “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.”
The dimming is down to changes in the light emitted by the glowing hydrogen, nitrogen, and oxygen being emitted by the dying star at its center. According to researchers, the oxygen emission dropped by a factor of nearly 1,000 over the two decades.
“Changes in nebulae have been seen before, but what we have here are changes in the fundamental structure of the nebula,” said Bruce Balick, leader of the new research due to be published in The Astrophysical Journal. "In most studies, the nebula usually gets bigger. Here, it’s fundamentally changing its shape and getting fainter, and doing so on an unprecedented time scale. Moreover, to our surprise, it’s not growing any larger. Indeed, the once-bright inner elliptical ring seems to be shrinking as it fades.”
The culprit appears to be the Stingray nebula's central star, SAO 244567, which experienced a rapid rise in temperature, from less than 22,000°C (40,000°F) to 60,000°C (108,000°F), according to a 2016 study based on observations of the nebula between 1971 and 2002. After the star expanded, causing the temperature to drop, the star went through a cooling phase, emitting less ionizing radiation. Hubble effectively caught before and after shots.
Now that the star is cooling, it's likely returning to its early stage of stellar evolution that it was experiencing before its temperature jump. It's hard to know what's in store for this nebula with it being so young, but the researchers estimate that at its current rate of fading, it may be barely detectable in just two or three decades.
Serendipitously, Hubble was in the right place at the right time to capture the rapid changes which, in cosmic terms, occurred in just the blink of an eye.