Capturing a New Type of Supernova

You may have recently heard in science news about a new kind of supernova explosion which had been theorized to exist, but had never been seen before—until now. A team of researchers has published a paper in Nature Astronomy about SN 2018zd, which may fall into this unique flavor of supernovae known as “electron-capture,” where the electrons of oxygen, neon, and magnesium atoms are crushed into the nuclei of these atoms by extreme gravity in the core of the star, triggering the collapse of the star. In such a scenario, the star can no longer counteract the force of gravity and collapses in on itself, creating a shockwave that tears the star apart in a cataclysmic explosion.

This electron-capture mechanism is fundamentally different from the well-known iron core-collapse supernova in which a star has used up all of its nuclear fuel, fusing new elements to the iron which builds in the core until gravity causes the star to collapse on itself. This result is exciting in that: it confirms an existing theory with observational data; it may provide insight into the origins of the supernova that created the Crab Nebula in 1054 AD; and it sheds new light on the life-cycle of stars. To help the science team publicize this result, I pulled together data from disparate sources to help tell the story of SN 2018zd in imagery. 

Incomplete Coverage

SN 2018zd’s host galaxy, NGC 2146, has only been partially observed by Hubble in the past. Adding to the challenge, Hubble did not observe SN 2018zd directly until about a year after the supernova explosion. In that time, the light from the explosion had dimmed significantly, and although it is clearly visible in the Hubble image, it does not exactly convey the idea of a stellar explosion so bright that it outshines an entire host galaxy. Clearly, I was going to have think outside the box to tell this story through imagery. 

Left: NGC 2146 from the Digitized Sky Survey. Right: The same image with the existing Hubble fields of view overlaid, demonstrating the incomplete coverage of this galaxy with Hubble. The smaller red square corresponds to Hubble observations taken after SN 2018zd was discovered.

In addition to the available Hubble data, the science team also has high-quality ground-based optical data from the Las Cumbres Observatory which was taken at nearly the peak of SN 2018zd’s brightness. The supernova is clearly visible as one of the brightest objects in the image. However, the ground-based data suffers from the distortion of Earth’s atmosphere blurring the data. This effectively reduces the resolution of the image, making it difficult to seamlessly blend Hubble and ground-based data together in a cohesive, clean image. 

Las Cumbres Observatory color image of NGC 2146 and SN 2018zd at its peak brightness.

Seeing the Light

Although there is very little color information in the Hubble data—the galaxy had only been observed with two very different filters—it is possible to pull color information from a lower-resolution image while preserving the detail and resolution of the Hubble data by adding it to the ground-based image as a luminosity layer. We have used this technique in the past to great effect. The luminosity, or lightness of an image, provides a very strong visual cue of the resolution of an image. Our vision system is very good at blending lower-resolution color information with higher-resolution lightness to give the perception of an overall higher-resolution image.

Left: The Hubble F814W data combined from two separate observations. Right: The Las Cumbres image with this Hubble image overlaid in luminosity (the image has been stretched to emphasize the edges of the HST image).

In the case of NGC 2146, the Las Cumbres data provide detailed color information that covers similar wavelength ranges as the Hubble red (F814W) data. When combined, we perceive Hubble-quality resolution with rich color, in one almost seamless image. The final problem would be how to render the supernova as the brightest source in the image. 

This animation shows the advantage gained by introducing the Hubble F814W image as a luminosity layer on top of the color information provided by the Las Cumbres data. The bright red regions seen towards the top edge come from regions of star formation within NGC 2146. The Hubble F658N data highlight these regions very well and were also included in the final composite.

A Bright Idea

As I mentioned at the start of this post, the direct Hubble observations of SN 2018zd are rather lackluster in terms of visual impact, but the Las Cumbres data show a dramatic point source. I experimented with using STScI’s Tiny Tim PSF generator to produce the signature look of a bright star with the exact luminosity of SN 2018zd and the exact same configuration of instruments used in the Hubble observations.

Top: Hubble’s view of SN 2018zd almost a year after the outburst. Middle: The Las Cumbres image taken near-peak brightness. Bottom: The Hubble image with a synthetically generated bright star point-spread function as a stand-in for SN 2018zd.

This approach, however, crosses an ethical line in that it introduces an element to the image that is not derived from the actual data. As close as it is to the real thing, it’s just not what was actually observed. This is different from the use of this tool to generate stars for a 3D fly-through animation. In those cases, we have no choice but to make artistic decisions about how best to represent an object based on the available data. In this case, however, the image needs to tell the story with real data.

A compromise was achieved by blending the Las Cumbres observation of the supernova into the Hubble image, as seen at the top of this page. It produces a very large, blobby (yes, that’s a technical term), star compared to Hubble’s usual pinpoint sharpness, but it effectively demonstrates that this star in its final death throes outshined an entire galaxy! 

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