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The rise and rapid fall of the Stingray Nebula
Astronomy tends to have a timeless feel to it. Stars take millions of years to be born, a given galaxy will look exactly the same today as it did at the height of the Maya empire, even the constellations in the sky look pretty much the same as they did when Indigenous Australians started weaving their stories about them so very long ago in human terms.
Sometimes, though, things happen on a much faster timescale. The Stingray Nebula can lay claim to two such rapid events: It probably only started glowing about 40 years ago, and it faded substantially over just twenty years of that time!
The Stingray Nebula — technically called Henize 3-1357 — is what we call a planetary nebula, gas ejected by a star similar to the Sun as it dies. When such a star runs out of nuclear fuel in its core to fuse, the outer layers swell up and the star becomes a red giant. It then starts to shed the gas making up those outer layers, blowing a wind of material into space.
A lot of gas is blown off this way, so much so that deeper and deeper layers of the star get exposed. Eventually what remains is the star's core, a hot, dense white dwarf, which blasts out ultraviolet light and causes the surrounding gas to glow. The shapes that gas forms can be very strange as winds blown later catch up with and slam into slower material ejected earlier, for example. Magnetic fields, stellar spin, binary companions, and more can all sculpt the nebula into fantastic shapes.
The Stingray Nebula surrounds the star SAO 244567, what's called an asymptotic giant branch star. This means that it's already been a red giant for a while, and is well on its way to shedding its final outer layers. It obviously has been for some time, since the nebula exists, and given measurements of the gas itself (how it's expanding away from the star) it's likely SAO 244567 has been belching out gas for a thousand years or so.
In the late 1980s or early '90s something changed, though. The star started getting hotter... a lot hotter. It went from about 21,000° C in 1980 (much hotter than the Sun's 5500°) to a face-melting 60,000° by 2002. At the same time it shrank in size — this was determined by carefully measuring spectra taken of its light. Astronomers think that it had what's called a late thermal pulse, where gas under incredible pressure in a shell around its core underwent furious rates of fusion, heating the star up.
This extra light zapped the gas already blown out by the star, lighting it up, causing it glow literally like a neon sign. That is, electrons in the gas atoms jumped up in energy levels, then emitted light when they dropped back down. That's when it would've first been visible from Earth, making it the youngest planetary nebula ever seen.
But then the star started to settle down. By 2015 the temperature had dropped to 50,000° and the star swelled back up again.
What all this means to the nebula is that between 1996 and 2016 the gas faded considerably.
As you can see, it looks a lot different in the after shot than the earlier one. To be clear, the structure of the nebula is probably almost exactly the same; that hasn't changed much at all. What has changed is that it's faded, and in some places it's faded a lot.
For example, the outer lobes forming that broad and thick X are gone in the later image. Those have squiggly filaments of gas in them that gave the Stingray its name, but now they're too faint to see. Both images are color-coded the same way; blue light is from oxygen, green from hydrogen, and red from nitrogen. When hit by UV light they respond differently. For example, oxygen fades rapidly, even more so where the gas is dense. The lobes and inner region are heavy with oxygen and have faded the most, in some places by a factor of 900.
This fading does allow astronomers to probe the structure and nature of the nebula; it's possible to determine things like the gas density and temperature, and what exactly it's doing. Sometimes, for example, some gas glows as it slams into other gas, but in this case it looks like everything in the nebula depends on the star to glow.
So what's the fate of this nebula? It may very well continue to fade, even so much Hubble won't be able to see it. However, once the outer layers are completely blown off and the white dwarf is revealed, it will likely light up once again. That may not be for decades or even centuries, though.
Our own Sun will likely go through similar paroxysms when it's in a similar stage, about 7 billion or so years from now. Most likely our future planetary nebula it won't have such a cool shape with all that structure, but it may very well get brighter and dimmer over time, until the Sun blows off its envelope and becomes a white dwarf.
When we study planetary nebulae like the Stingray we learn more about or own distant future. And it amazes me to think that after 11+ billion years of life, the Sun will do this as well, possibly changing significantly over just a few years after all those eons.
Will future astronomers half a galaxy away oooo and ahhhh over its behavior? Who can say. But it's nice to know that even in stellar death there is awe and beauty.