In ancient mythology, Queen Cassiopeia was condemned for her pride, her throne bound forever to the turning heavens. What the myth does not tell is that within her constellation lies a remnant of violence far older than any human story. One of Cassiopeia’s brightest jewels was truly lost—not in legend, but in a stellar cataclysm. Torn apart by a supernova, the star’s remains still drift through her celestial realm: a shattered diadem glowing in oxygen blue.

After a half-thousand hours of [OIII] light gathered from the void, the Sapphire Diadem reveals itself—not as a symbol of vanity, but as a memory of transformation: where a queen’s pride became a remnant, and a star’s death became beauty.

The Project: An SFRO Collaboration

Today, we (six astrophotographers from Starfront Observatories) are excited to present our most ambitious project to date.

The idea behind this image was to collect as much data as possible on a rarely imaged supernova remnant (SNR) in order to reveal details never seen before in the vicinity of the SNR. The project initially began with Joydeep and Max in September, while Julien had already started imaging this target in early August. Once they realized they were working on the same object, they decided to combine their efforts, despite using different focal lengths.

Over the course of October, Jamie , Nick, and Jonathan joined the project, contributing additional data. In the end, this collaboration allowed us to accumulate more than 1,000 hours of total exposure on this remarkable SNR (85% of the best subs were selected for the final LRGBHO stacks reaching 1020h).

Figure 1: Field of View of the image and location in Cassiopeia constellation.

Located in the constellation Cassiopeia, between the well-known NGC 7822 (the Question Mark Nebula) and the Heart and Soul Nebulae (Figure 1), this SNR evokes the shape of a diadem, marked by a pronounced blue hue due to the strong presence of [O III] emission. Together, we decided to name this image “The Sapphire Diadem of Cassiopeia.”

SNR G126.2+1.6: History and Observations

SNR G126.2+1.6 is an exceptionally large and faint Galactic supernova remnant located in the Milky Way plane, in the region of the sky usually associated with Cassiopeia. Spanning more than one degree on the sky — over twice the apparent diameter of the full Moon — it is nevertheless extremely difficult to image due to its very low surface brightness. This combination of large angular size and dim intensity makes it a challenging but rewarding target for deep, wide-field astrophotography.

It was first detected and classified as a supernova remnant by Reich et al. (1979) based on radio observations carried out with the Effelsberg 100-m radio telescope. Reich et al. first reported extended non-thermal radio emission in this region, suggesting the presence of a previously unrecognized supernova remnant. Optical confirmation followed in the early 1980s (Figure 2), when Blair et al. (1980) and Fesen et al. (1983) detected faint optical filaments with emission-line ratios characteristic of shock-excited gas. These discoveries firmly established G126.2+1.6 as a true supernova remnant rather than a diffuse H II region.

Figure 2 : overlay of first optical image of SNR 126.2+01.6 from “The discovery of optical emission from the SNR G126.2 + 1.6” by Blair et al. (DOI:10.1086/158494)

From an imaging perspective, SNR G126.2+1.6 is best described as a highly evolved, fragmented shell-type remnant. Its structure is not a clean, continuous arc but rather a network of extremely faint filaments and diffuse patches, shaped by interactions between the ancient shock front and variations in the surrounding interstellar medium. In narrowband images, the remnant is most often revealed in Hα, with localized regions also showing emission in [O III], indicating areas where the shock encounters slightly denser material.

Radio observations show a clear non-thermal synchrotron spectrum, a signature of relativistic electrons accelerated by the original supernova shock. A detailed multi-frequency radio study by Tian & Leahy (2006) refined the physical interpretation of the remnant and remains one of the key modern references on this object. Their analysis also used associated neutral hydrogen (HI) emission to estimate the remnant’s distance.

Distance estimates remain uncertain, but the most widely cited value places SNR G126.2+1.6 at approximately 5–6 kiloparsecs (around 18,000 light-years), based on the HI kinematic analysis of Tian & Leahy (2006). At this distance, the remnant’s true physical diameter would be on the order of 100 parsecs, making it one of the largest known supernova remnants in the Galaxy. For astrophotographers, this enormous physical size explains both its vast apparent extent and its extremely low surface brightness: the energy of the original explosion has been spread thin over a very large volume.

Dynamical models suggest an age of roughly 200,000 years or more, placing G126.2+1.6 among the oldest known remnants. At this advanced stage, the shock has slowed dramatically and the remnant is in the process of fading into the surrounding interstellar medium. This late evolutionary phase is rarely captured in optical images, which adds to the scientific and aesthetic interest of deep observations of this object.

Imaging SNR G126.2+1.6 is therefore less about capturing dramatic arcs and more about revealing subtle, ancient structures at the limit of detectability. Long total integration times, careful background control, and sensitive narrowband processing are essential to bring out its delicate filaments. What emerges is not a bright showcase object, but a quiet and elusive relic — the fading imprint of a stellar explosion that occurred hundreds of thousands of years ago, now only barely visible as it dissolves back into the fabric of the Milky Way.

RGB_3-ezgif.com-optimize.gif GIF : RGB-RGB OIII - RGB Halpha - RGB Halpha OIII

References

Reich, W., E. Kallas, and R. Steube. "G126. 2+ 1.6, a supernova remnant near the X-ray transient 4U 0115+ 63." Astronomy and Astrophysics, vol. 78, no. 2, Sept. 1979, p. L13, L14. 78 (1979): L13. [Link]
Blair, William P., Robert P. Kirshner, T. R. Gull, D. L. Sawyer, and R. A. R. Parker. “The discovery of optical emission from the SNR G 126.2+ 1.6”. No. NASA-TM-80708. 1980. [Link]
Fesen, Robert A., Theodore R. Gull, and Dean A. Ketelsen. "Deep forbidden O III interference filter imagery of the supernova remnants G65. 3+ 5.7, G126. 2+ 1.6, CTA 1, and VRO 42.05. 01." Astrophysical Journal Supplement Series (ISSN 0067-0049), vol. 51, April 1983, p. 337-344. 51 (1983): 337-344. [Link]
Tian, Wen-Wu, and Denis A. Leahy. "Radio observations and spectral index study of the SNR G126. 2+ 1.6." Astronomy & Astrophysics 447, no. 1 (2006): 205-211. [Link]

New Discovery: Cassiopeia’s Opal (BKDWMAH-1)

The team spent countless hours inspecting the subs, partial stacks, and processed results from the 1000+ hours of data, to ensure that details in the raw captures were accurately preserved in the processed result. During this process, Joydeep noticed an unusual phenomenon and brought it to the attention of the team – surrounding the star IRAS 01342+6358 was an unknown object of a shell morphology with strong emissions in Oiii, and some emissions in Ha. On behalf of the team, Julien contacted Pascal Le Dû to determine whether this object (not yet reported in the HASH PN database) could be a new planetary nebula (PN). After several email exchanges and providing the requested data, he told us:

“…This oval ring-shaped morphological signal (bipolar?) is interesting.
It is located in a complex area already classified as an H II region, but it could correspond to a PN…”
“…I think this [O III] ring is related to the prominent star at its center (IRAS 01342+6358), but it is also possible that it is associated with a white dwarf that is not visible in the current images because it is too faint or obscured by another star.
Nevertheless, this signal deserves to be recorded.”

Excited by this incredible news, we decided to name the catalogue object with the initials of all contributors, by order of contributed observation time: Biswas-Kuster-De Winter-Moon-Ambrose-Hill, i.e. BKDWMAH-1, and proposed the simple canonical name of “Cassiopeia’s Opal”.

HOO_BKDWMAH_1_ZOOM1 copie.jpg Figure 3 : Location of BKDWAH 1 in the global field of view and zoom of the interesting area. A different process has been applied to better highlight the [OIII] signal. (Coordinates : RA 01 37 41 DEC +64 14 22)

Additional Objects Of Interest

Besides the stars dominating the field, numerous additional objects become apparent. Among them, two open clusters are clearly visible: NGC 559 and NGC 609.

NGC 559 is a relatively rich open cluster located in the constellation Cassiopeia, approximately 7,200 light-years (2.2 kpc) away. It has an apparent diameter of about 7–8 arcminutes and contains several dozen stars, some of which are bright, blue main-sequence stars. Its estimated age is around 225 million years, making it a moderately young cluster where some of the most massive stars have already evolved off the main sequence. NGC 559 is a popular target for astrophotographers because its concentrated star field stands out nicely against the dense Milky Way background, providing contrast with surrounding nebulosity and faint stellar fields.

NGC 609, also in Cassiopeia, is a smaller and less conspicuous open cluster located at a similar distance of roughly 4,000–4,200 light-years (1.2–1.3 kpc). Its angular size is about 5–6 arcminutes, and it contains fewer stars than NGC 559, with a mix of blue and yellow main-sequence stars. Its estimated age is slightly younger, around 100–120 million years, so it still hosts relatively bright and hot stars. Though more modest in appearance, NGC 609 adds depth to the field and helps illustrate the diversity of star-forming episodes in this region of the Milky Way.

Figure 4 : NGC 559 and NGC 609 respectively

The "Príncipes de Asturias" nebula, a quadrupolar planetary nebula identified from the IPHAS survey in 2006 [Mampaso et al.],[HASH PN DB] can also be observed in this FOV

Figure 5 : “Príncipes de Asturias » Nebula”

Finally, we note the presence of an object with morphology resembling a Herbig–Haro object; however, no catalogued HH object is known at this location

A Herbig–Haro object is a small, bright patch of glowing gas formed when jets of material ejected by a very young star collide at high speed with the surrounding interstellar gas and dust. These collisions create shock waves that heat the gas, making it shine in specific emission lines (such as hydrogen). However, we suspect that the clouds hide the young star that could be responsible of this phenomenon.

Figure 6: A suspected Herbig-Haro object

Joydeep Biswas, Max Kuster, Julien De Winter, Jamie Moon, Nick Ambrose and Jonathan Hill

We collectively discussed the processing of the final image, and critiqued all aspects of the image. Through the process, we produced several individual versions, which we list below, and the leads for each revision:

-Jonathan Hill : Version M