(The above image features only optical and narrowband data, for the inclusion of WISE and Spitzer data, please see below.)
This article was composed in partner with Andreas Buzan, Alyssa Manus, and Ruby McGhee. Please see the credits for more information.
About the Image
Overview
We observed the Medusa nebula which is a planetary nebulae characterized by old red giant stars that eject ionized gas creating the nebula’s luminescent shell. The faint hot blue core is called a white dwarf made of exposed carbon and oxygen which ionizes the surrounding previously ejected gas. Free electrons collide and excite the elements: HI, OIII, and SII and then they de-excite and emit narrowband emission lines. Our images consist of LRGB, narrowband (NB) and incorporated mid-infrared (MIR) archival layers.
For our observations, we used the telescopes PROMPT5 and PROMPT7. For PROMPT 7, we used the filters SII, OIII and Halpha, with an exposure length of 600 seconds per filter. For PROMPT5, we used the filters Lum, B, V, and R, with the following observing times for each filter:
| Filter | Number of Exposures | Exposure Length (sec) | Total Exposure Time (sec) |
| Luminance | 10 | 32 | 320 |
| B (blue) | 5 | 133 | 665 |
| V (green) | 5 | 114 | 570 |
| R (red) | 5 | 81 | 405 |
Infrared Data
We first opened all of the files in Afterglow. After checking to make sure our R, V, B, Lum, and narrowband exposures (Halpha, OIII, SII) came out clear, we aligned all of our images using WCS mode. Next, we began making our stacks for the R, V, B, and Lum filters. We used chauvenet rejection with a high and low of 1 and enabled Propagate Mask. Then, we group all of the stacks, as well as the Halpha, OIII, and SII layers, into one image. We then used SkyView Virtual Telescope to gather Wise 12 and Wise 22 data. We colored the Wise 12 data with the “heat” color map and Wise 22 with the cool “color” map. Afterwards, we used IRSA (Infrared Science Archive) to gather Spitzer data. We collected observations from the IRAC 4 telescope, which gathers wavelengths at the 8 micron level, which is similar to the Wise 12 data collecting hot gas information, and colored it with the “balmer” map. We aligned and cropped these images the exact same way as with the optical filters except for changing the blend mode to lighten so they add to the already-existing optical layers instead of partially replacing them.
We decided to color the Wise 22 images a blue color because it represented the cold gas in the nebula. Wise 12 and IRAC were colored red and orange respectively so that they could illustrate the warmth of the region. IRAC was made intentionally dimmer than Wise because the Wise data measured a wavelength that covered more of the heat of the outer regions. The IRAC was included in a similar color to better highlight the more discrete inner structures of the hot gas that aren’t as visible in the Wise data.
Physical Processes
The Medusa Nebula is a planetary nebula that is the result of a red giant star reaching the limit of its combustion and shedding its outer layers of gas. The visible structure is the warm clouds of shed gas, mostly hydrogen and oxygen, that were expelled when the core of the red giant star overcame the forces of gravity holding it together and exploded*. The clouds of dust are warm, because as they were expelled, the atoms were bumping into each other and generating heat in the form of friction. In the middle of the nebula is a cool spot where the red giant was: having blown away most of the gas around where the star was, there is very little gas left near where the core of the star was that could actually retain thermal energy.
We reason that the shape of the nebula arises from an imbalance of densities in the star when it exploded. Consider that an ideal star with even density would explode and create a nebula of a spherical shape. Continuing to assume that the cool spot is the relative center of the nebula, we can see that many of the dust clouds are off to one side. This may be because there was more mass on one side of the star, and when the star exploded, the mass was expelled at a slower rate than the areas of lower mass that would have traveled away more quickly. Assuming the energy exploded evenly from the core, this makes sense, because the areas of higher density would require more energy to be expelled.
*Two of the authors had much debate about what actually constitutes an explosion from a physical perspective and whether the death of a red giant star constitutes an explosion or merely a rapid expansion. Consider the position of Author 1: let’s define an explosion to be a positive feedback loop of potential energy being converted to kinetic, thermal, or some other kind of energy that in turn causes more conversion of potential energy. When a red giant dies, the pressure in the core is able to overcome the force of gravity holding the star together, and as such, the star ejects its outer layers until the core tears itself apart. Theoretically, if the star is ejecting its outer layers, then it is becoming less massive, therefore the force of gravity is less able to hold the star together and the internal pressure would be able to eject the layers more rapidly: this would be the positive feedback loop of our definition where the potential energy is the internal pressure that was being contained by the outer layers of the star via gravity. This would satisfy our definition of an explosion.
Now consider the position of Author 2: an explosion would mean that the force that ejects the material comes directly from what is formed in some reaction inside the star, not by what caused that reaction in the first place. Since the outside of the star is being expelled by an imbalance of pressure caused by the gravity of the outer layers of the star pushing on the interior, then even though there is heat and energy being created, both as the star burns and through friction, the star cannot be considered exploding, but simply rapidly and powerfully changing states between pressurized and unpressurized. This creates a powerful expansion, not explosion.
The authors welcome the continued discussion of this subject, as they gained no valuable insight from their discourse.
Credits
Thank you to Andreas Buzan for writing the section on how the infrared data was collected and making his case that the death of a red giant is an expansion and not an explosion. Andreas maintains his own blog at tarheels.live/blogandreas/.
As well, thank you to Alyssa Manus for writing the section how the infrared data was collected. She also runs her own blog at alyssacmanus.wixsite.com/astrophotography.
And as well, thank you to Ruby McGhee for writing the overview of how the image was taken. Her blog can be found at tarheels.live/rubymcghee/.
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