Monoceros Loop Supernova Remnant

The 'Monoceros Loop' is an example of a supernova remnant (SNR) (Reference 1)--ie. the remains of a star that exploded at the end of its life. Located in the constellation Monoceros, the remnant is quite faint and consists of some filamentary structures and some very diffuse structures when photographed in Hydrogen-alpha. Even still, the faint ring is difficult to see, and inverting the image helps to enhance it.

This faint supernova shell borders on two very bright HII regions, the Rosette Nebula (Sh 2-275, lower left) and the Cone Nebula (Sh 2-273, right). While all three objects appear to be in close proximity from our perspective, it is important to remember that the objects exist in three dimensional space, and may not be in the same plane relative to us. Reference 1 examines this question, and presents distances to the objects as follows: the star cluster NGC 2264 lying in the center of the Cone Nebula is estimated as ~ 800 pc away, while the star cluster at the center of the Rosette Nebula is at a distance of ~ 1800 pc; the distance of the SNR is approximately 1600 pc away. Reference 1 therefore concludes that the SNR is likely in contact with the Rosette Nebula, but not the Cone Nebula. Reference 2 quotes a diameter and estimated age for the SNR as 106 pc and 150,000 years old.

This image is an on-going project. I learned of its existance in early 2021, and started a mosaic, but as a string of cloudy weather set in, and this area of the sky started migrating toward the horizon (and into a neighbors tree!) in the early evening, I realized I wouldn't be able to finish it this season. I did, however, have pieces of it, as well as an old DSLR/Ha image of the region, that I have tried to piece together here. I hope to improve on the image as weather permits.....

The image above was taken through narrowband Hydrogen-alpha filters, which isolates the wavelength emitted by excited hydrogen atoms that form the nebula. The Monoceros Supernova Remnant (SNR) is a very faint ring structure lying between the Rosette Nebula (left, Sh 2-275) and the Cone Nebula (right, Sh 2-273), and its extent is roughly encompassed by the large central white circle in the "roll-over" annotated version of the image above. The faint ring-like structure is easier to make out if the image is inverted and stretched to emphasize the fainter nebulosity, as shown below. The most evident part of the ring is located at the bottom of the indicated circle, where the filaments, which represent shocks, can be seen.

The constellation Monoceros (the Unicorn) lies to the East of famous Orion, and just South of Gemini the twins. The constellation is located just north of the Milky Way's winter belt and along the celestial equator. The view at left shows the basic form of the constellation, with the approximate outline of the image above shown as a dotted rectangle.