ONE TEAM, 3559 FRAMES, 260 HOURS OF EXPOSURE TIME, TELESCOPES ON THREE CONTINENTS.
and one mission - not just to explore a supernova remnant previously undiscovered by science, but to shoot it in an ambitious collaborative project.
We recommend viewing the image at 100% in order not to miss the numerous fine details. 



Artistic illustration of the Nereides, sea nymphs in Greek mythology



THE NEREIDES NEBULA — A FITTING NAME
In Greek mythology, the Nereides are sea nymphs and the daughters of the sea god Nereus. They are known for their beauty and their connection to the sea.

SNR G107.5-5.2, with its many dynamic and delicate bluish filaments, appeared to us like the mythical sea nymphs dancing exuberantly in the water, and the connection to the constellation of Cassiopeia is also obvious: 
Thus Cassiopeia was a queen known for her exaggerated beauty. She was the wife of King Cepheus and the mother of Andromeda. Cassiopeia once boasted that she and her daughter were more beautiful than the Nereides. This angered the sea nymphs and as punishment they sent the monster Cetus to destroy the kingdom.



Separate channels



HOW WAS THE SNR DISCOVERED?
As is so often the case in life, it's the things you don't expect that make the strongest impact - such as the discovery of the Nereides Nebula. Our team member Bray published a spectacular widefield image of the Cassiopeia and Cepheus region in July 2022. Virtually invisible on the color image, the [OIII] excerpt showed a conspicuous structure at the bottom of the image, but its shape appeared rather unusual. Investigations quickly showed that there could be other possible structures, especially in H-alpha, which could possibly complete the mysterious [OIII] object to a much larger one. So two separate test images were taken by Bray Falls and our European team to determine the full dimensions of the structure and to rule out possible sources of error. Both images showed H-alpha structures as well as extremely filigree OIII filaments.



In this widefield image of Bray Falls, the Nereides Nebula was discovered in 2022. image


Now the important work begins - on the one hand the creation of an image to show the newly discovered nebula in all its beauty, but also the scientific part, consisting of analysis and research. The most important task was to verify this structure as a supernova remnant unknown to science. To do this, we searched all tables and databases for SNRs that matched the coordinates and size of the Nereides Nebula. None of the entries even closely matched our object. Professor Robert Fesen from Dartmouth College (NH, USA) also confirmed that there appears to be no entry for this SNR.





The biggest surprise, however, was the full extent of this colossal structure. The Nereides Nebula has a maximum diameter of 3 degrees in the night sky, which corresponds to 180 arcminutes and thus 6 times the diameter of the full moon. Coincidentally, there is a famous supernova remnant with the same dimensions - namely the Veil Nebula, or Cygnus Loop. This makes the Nereides Nebula probably the largest and one of the most impressive supernova remnants ever discovered by amateur astronomers.







WHY HAS SUCH A ENORMOUS SUPERNOVA REMNANT REMAINED UNDETECTED UNTIL NOW?
Most known supernova remnants are not discovered visually, but through sky surveys, especially radio surveys. There are a huge number of these radio surveys, as only a certain frequency range is detected per survey. Even if a supernova remnant is completely obscured by cosmic dust and gases, the radio emissions penetrate these obstacles and reveal the debris of the explosions of once gigantic stars. 
One area in the constellation Cassiopeia, however, has been scanned by surprisingly few of these radiosurveys, and the particular frequency range in which the Nereides Nebula likely emits is a blind spot. So this titan, which has the dimensions of the Veil Nebula in the constellation Cygnus, has remained unnoticed until now.
But shouldn't the supernova remnant have appeared in some amateur image, since it is located in such a prominent constellation like Cassiopeia?
Even though the Nereides Nebula appears extremely bright in our image, it took almost 260 hours of exposure time at very dark locations on three continents to make the fine details visible. Without correspondingly narrow-band filters and a special digital process to isolate the lines for [OIII] and H-alpha, the nebula remains virtually invisible.



IS THE CREATED IMAGE ALREADY PROOF OF THE SUPERNOVA REMNANT?
Yes and no. A colorful image is of course only of limited value for science. More important are the resulting raw data, from which a lot of information can be obtained. In the study of supernova remnants, evidence in the visible spectrum is just as important as evidence in the wavelengths of radio and X-ray radiation.
If a supernova remnant candidate already features very thin and delicate filaments in images and shows an almost round and closed shape, then it is already considered a probable supernova remnant. If the filaments then combine in individual elements such as [OIII], SII and H-alpha to form a single structure, there is little doubt about the nature of the structure. However, only science can provide the final proof together with essential information.


The complete [OIII] ring of the Nereides Nebula (negative image)


WHAT HAPPENS NEXT?
The next step is for Professor Robert Fesen of Dartmouth College (NH, USA) to take over the scientific leadership of further research of SNR G107.5-5.2.
The aim is to produce a spectrum, which is needed to isolate the components of the nebula. In addition, we learn the velocity of the gases and can thus determine the expansion rate, which allows us to draw conclusions about the age of the structure.
Furthermore, high-resolution spectra of stars within the periphery of G107.5-5.2. can be used to estimate the distance.



DATA ACQUISITION
The data acquired in Morocco was collected remotely by Yann Sainty at 2 observatories: High Atlas Observatory (https://astromaroc.com/hao/) and Atlaskies (https://atlaskies.com/), located in Oukaimeden in the Moroccan Atlas Mountains.
The data from France were acquired at 4 different locations by Nicolas Martino, Richard Galli and Yann Sainty.
Data from the United States was acquired by Bray Falls in the Sierra Nevada Mountains at the Sierra Remote Observatories (https://www.sierra-remote.com/).



THE TEAM
Bray Falls (US): discoverer, data acquisition
Marcel Drechsler (GER): Co-discoverer, image processing, imaging planning, research, analysis
Yann Sainty (FR): Imaging planning, data acquisition, data compilation, data preparation
Nicolas Martino (FR): Data acquisition, data preparation
Richard Galli (FR): Data acquisition
Professor Robert Fesen (US): Scientific leadership



ABOUT ARTIFICIAL INTELLIGENCE
With an image like this, you might quickly think that artificial intelligence has helped out a little. Yes, indeed - we use various AI applications for our image processing. However, we take great care to ensure that the AI does not artificially add any details to the image. We use AIs to remove noise from the data, to sharpen slightly and to correct distorted stars. After using an AI, we check all details against the raw image for accuracy. All other processing techniques are traditional in nature and are implemented in Photoshop and PixInsight. An important reason for such a clean signal is the processing techniques developed by the team, such as a "dynamic level-weighted layered subtraction processing" (DLWLSP) to isolate the [OIII] and H-alpha line more effectively.

The following comparison shows the [OIII] raw data* cleaned with DLWLSP on the left and the processed data (incl. RGB and H-alpha) on the right. These are image segments with an edge length of 350 pixels each, which corresponds to 100% resolution.


* The raw stack was stretched non-linearly, gradients were removed, cleaned up with DLWLSP and the remaining star artifacts were removed.


***


AFTERNOTE
In fact, the first trace we had of the Nereides Nebula goes back to 2020.
In that year, Andreas Zirke photographed the two PN candidates Strottner-Drechsler 97 and 98.
In the OIII data of this project, a part of the Nereides Nebula was visible for the first time.


In the small picture above left you can see the large PN candidate StDr 98, diagonally above the smaller PN candidate StDr 97.
Image: Andreas Zirke and Marcel Drechsler, 2020
​​In detail: a sharp arc-shaped OIII filament can be clearly seen in this OIII extract.