As the largest nebula in the night sky, I should have predicted this wide field mosaic of the Gum Nebula would became my largest astrophotography project by far, with 25 panels and 75 hours of total integration.

I find it fascinating that such a large object wasn’t discovered until the early 1950’s, when it was catalogued by Colin Gum, as part of his PhD thesis. It was hiding in plain sight all along, concealed only by is very low surface brightness. Amazingly, the Gum Nebula is over 35 degrees across! It is the roughly circular red shell that spans across the whole galactic plane in my image.

The Gum 12a nebula (as it was originally catalogued), now colloquially referred to as just the “Gum Nebula” was one of 85 nebulas identified in Colin Gum’s PhD thesis: “A Study of Diffuse Southern H-alpha Nebulae” which was submitted in June 1954. His thesis was not published until 1955 however, as it was initially rejected by two different supervisors. Richard Woolley, the director of the Mount Stromlo Observatory, where Gum was a graduate student, was the first rejection. The second by Harry Plaskett of Oxford. Fortunately, Gum persevered and his thesis was final accepted (enthusiastically this time!) by Ben Gascoigne of Mount Stromlo Observatory. Gum was awarded the first PhD of the newly formed Department of Astronomy at the Australian National University, which had recently began operating the Mount Stromlo Observatory.

Colin Gum had secured his name in astronomy history, but at a significant personnel cost. The discovery of the southern sky’s nebulae had been a race with astronomers cataloguing the same areas from the Harvard Boydon Observatory in South Africa. The stress was suspected as the cause of nervous breakdown Gum suffered at the time. One irony was that Gum’s rival in South Africa was Bart Bok, who would go on to replace Woolley as director of Mount Stromlo. Bok would quickly commission an expansion of Gum’s initial catalogue, by Alex Rodgers. This would become the RCW Catalogue that is still commonly referenced today.

The discovery of the Gum Nebula was just one of a huge list of astronomical achievements by Gum. The following link provides summary of Gum’s life and career more succinctly than I https://science.nrao.edu/about/publications/pawsey/NRAO%20ONLINE.58.2.Colin.Gum.Biographical%20Sketch.pdf

For an interesting historical comparison, below is the first photo of the Gum Nebula, as taken by Colin Gum in 1954 at Mount Stromlo Observatory. The original image was also a mosaic, shot on a 100mm Schmidt telescope with a very wide 11 degree field of view, on circular 22mm film stock punched out of convention roll film. Each panel has an exposure time varying from 20 minutes to 2 hours.



Note that this image caption refers to the old galactic coordinate system, not the new system used today. Curiously, you can read in the above link that Colin Gum also played a key role in the establishment of the current system in 1958.

Given I had so much thinking time during the capture and processing stages of my photo, I couldn’t help but draw some parallels between to Colin Gum’s original image.

The first is the location. Whilst I have no association with Mount Stromlo Observatory, I do live nearby, so my photo is shoot from the same exact longitude, where this area of the sky passes directly through the zenith.

The second is the use of a mosaic with very similar image scales. Gum used a 100mm focal length with a 22mm sensor, similar to the 135mm focal length I used on a full 35mm sensor. Gum’s telescope had a larger aperture (100mm vs 67.5mm), but I had the advantage of running three of my lenses simultaneously.

Overall though, the only fundamental equipment differences are the very low quantum efficiencies of the film used in 1954, which would be orders of magnitude lower than the CMOS sensor in the Canon 6D’s I used.

Below is another slightly later image of the Gum Nebula from 1960, also taken at the Mount Stromlo Observatory, forming part of the original RCW (Rodgers, Campbell & Whiteoak) catalogue.



Note the “supernova” arrow is pointing to the Vela Supernova remnant (SNR), which is not resolved in Gum’s original image. The Vela SNR is a separate object to the Gum nebula, despite the fact its positioned centrally within Gum 12a. The Gum Nebula was originally identified as a large Hydrogen II region ionized by local stars. The most recent evidence however points to it also being a very old SNR too, formed by a supernova around 1 million years ago. The Vela SNR (the thin red and blue filaments in the uploaded image) by comparison is from a supernova event only 11,000 years ago.

The Gum Nebula and the Vela SNR are just two of the jewels of the Southern Milky Way. Given the resolution of the mosaic, I would suggest viewing at full size image (its 200+MP originally, and downsized here on Astrobin to 125MP to stop the uploader crashing) and exploring it at a decent magnification. I literally lost count of the number of galaxies I noticed whilst editing it. There are some very nice ones visible away from the galactic plane, especially on the right side of the image. Enjoy!