How images are made
The Fading Stingray Nebula: Processing Hubble Images 20 Years Apart
Most image processing tries to show as much as possible. For the Stingray Nebula, showing less was the only way to tell the story.
Two pictures of the same small nebula, taken twenty years apart, show something astronomers almost never see: a planetary nebula changing within a human lifetime. Between the 1996 and 2016 Hubble observations of Hen 3-1357, the Stingray Nebula, the bright blue shells near its center all but vanished and the wavy edges that gave it its name nearly disappeared. The press image released on December 3, 2020 had to make that fading obvious, and that requirement pulled against the instinct of every image processor.
What a planetary nebula is
The name is a historical accident. Early telescopes could not resolve fine detail, and these round, softly glowing objects looked a little like planets in the eyepiece. They have nothing to do with planets. They form around stars of intermediate mass, roughly one to eight times the mass of the Sun, late in life. In the red-giant phase such a star sheds its outer layers into space. Once enough material has gone, the hot core is laid bare, and its intense ultraviolet light strips electrons from the gas around it, which then glows.
The central star, SAO 244567, has a strange history of its own. Observations from 1971 to 2002 showed its temperature climbing to almost ten times that of the Sun's surface, possibly after a brief flash of helium fusion outside its core, after which it began to cool again. Over roughly the same decades, the nebula itself dimmed. Oxygen fell hardest: its emission is now almost 1,000 times fainter.
Two cameras, one comparison
Comparing brightness across two decades is harder than it looks. The 1996 data came from the Planetary Camera chip of the Wide Field and Planetary Camera 2; the 2016 data came from Wide Field Camera 3. Both sets were taken through filters for oxygen, hydrogen-alpha and nitrogen, but on detectors with different sensitivities and different instrumental quirks. Any artifact left in one set and not the other could masquerade as a change in the nebula.
The research team calibrated both datasets carefully to remove instrumental effects before measuring the flux, the brightness of each part of the nebula. Those same calibrated files became the starting point for the press image. A useful check is built into the field: the stars around the nebula, including a distant companion of the central star, stayed constant in brightness while the nebula faded.
The stretch that would have hidden the result
Raw astronomical data hold a far greater range of brightness than a screen can display. One standard goal of processing is to get the most information out of that range: brighten the faint outer wisps while keeping detail in the bright core. Doing so compresses the dynamic range and lowers the contrast between the brightest and darkest parts, an effect sometimes called flattening the image.
The difference is easy to see on a single frame. The same 1996 exposure, shown once with a plain linear scaling and once after a stretch, looks like two different nebulae: the stretched version brings up faint outer structure that the linear version leaves close to black. That is a change in presentation only, and it can look deceptively like the change in the nebula that the study measured.
For most nebulae, flattening is exactly right. For the Stingray it was dangerous. Compress the range too much and both epochs end up looking similarly bright, which buries the very thing the paper reported. The scaling therefore had to be identical for the 1996 and 2016 data, and gentler than usual. Faint structure in the outskirts was interesting to look at but was not the result, so it was allowed to stay faint.
The image went through several versions in discussion with the science team:
- The first applied too much dynamic range compression. Both panels looked flat, and the drop in brightness was muddied.
- The second balanced the brightness levels better but leaned toward small-scale structure, which drew the eye away from the change between the two frames.
- The third, used for the release, kept enough internal structure to show the shells and edges while making the dimming plain at a glance.
This is the point at which processing moves from quantitative to subjective. Calibration and cleaning follow fixed rules, covered in more depth in Image Artifacts Explained. Color balance, tone and composition are judgments, and here those judgments were tied directly to a measurement. The colors themselves follow the usual chromatic order described in Are the Colors Real?.
Questions readers ask
Why is the Stingray Nebula fading?
The release does not give a final answer. The central star, SAO 244567, heated up sharply between 1971 and 2002, possibly because of a brief helium flash, and then began cooling. Researchers link the fading to the behavior of that star but, at the time of the release, could only speculate about what comes next.
How much did the Stingray Nebula dim?
The changes were seen in the light of nitrogen, hydrogen and oxygen. Oxygen emission fell the most, by a factor of nearly 1,000 between 1996 and 2016, and the bright blue inner shells all but disappeared from the images.
Were the two images processed the same way?
Yes. Both started from data calibrated by the research team, and the same brightness scaling was applied to each. Treating them differently could have exaggerated or hidden the change, which was the main finding of the study.
Is the Stingray Nebula related to planets?
No. Planetary nebulae got the name because they looked round like planets in early telescopes. They are shells of gas cast off by aging stars of about one to eight solar masses and lit by the exposed hot core.