26 October 2016 | Stardeath in Cygnus


Credit: Martin Rusterholz | Click image to enlarge and here for full-size (5MB)

Image Data
  • Location: France
  • Camera: Apogee CG16M
  • Optics: Takahashi FSQ-106EDX III
  • Mount: ASA DDM85
  • Filters: Astrodon Gen 2, 3nm/5nm Narrowband
  • Exposure: HaOIIIRGB 400:520:210:210:210 min

Supernova remnants are formed from the supernova explosions of large stars and approximately 300 are currently known in the Milky Way. The constellation of Cygnus is home to one of the most well known ones, the Veil Nebula. There are however many more obscure and lesser known examples of supernova remnants in Cygnus (as well as other constellations).

One of these less well known examples is G65.3+5.7, a predominantly OIII shell of ionized gas ejected by the explosion of a star many thousands of years ago. The full 4×3 degree shell (equivalent to the area of 8×6 full moons in the sky) barely fits in its entirety in this 3.5 degree widefield image. The name derives from a naming scheme based on the galactic coordinate system, which is used for various supernova remnants, planetary nebulae and HII regions.

Discovered in an OIII survey of the Milky Way in 1977 by the astronomer Ted Gull, parts of it had been catalogued previously by Stewart Sharpless in his catalogue of HII regions and emission nebulae. The brightest part of G65.3+5.7 (also catalogued as G65.2+5.7) is its southern rim, which is catalogued separately as Sh2-91, while some of the brighter filaments visible near the top right corner are catalogued as Sh2-94. An additional one, Sh2-96 is situated to the north of Sh2-94 and is visible as a red filamentary region. After the publication of the Sharpless catalogue, the astronomer Sidney van den Bergh postulated in 1960 that Sh2-91 and Sh2-94 might form part of a previously unknown supernova remnant, which was confirmed by OIII images taken later due to its prominent oxygen emission.

Despite being a catalogue of emission nebulae, some of the entries in the Sharpless catalogue correspond to a wide panoply of object types including supernova remnants, planetary nebulae, Wolf Rayet nebulae, dust clouds and even galaxies! The confusion is due to the selection critera for the compiling of the Sharpless catalogue being based on the appearance of a particular object on red photographic plates taken at the Palomar Observatory. Considering the Sharpless catalogue was based on photographic plates when it was published in 1959, it is an excellent source of imaging targets for amateur astrophotographers in both the northern and southern hemispheres as some of the Sharpless nebulae are included in the RCW catalogue.

After its discovery in 1977, G65.3+5.7 was confirmed to be a definite supernova remnant in 1979 after various radio and x-ray observations were performed. It is one of the largest supernova remnants in the Milky Way and consists of a lovely tapestry of interlocking filaments. At an estimated distance of approximately 3250 light years, the full span of the whole shell is 228 light years with an estimated age of 20,000 years. While it is bright in x-rays, it has an unusually low surface brightness in radio observations. Professional studies have concluded that G65.3+5.7 might be the oldest example of a thermal composite (mixed morphology) type supernova remnant. Another type of composite remnant are plerionic composite supernova remnants.

Its faintness and large diameter make it a perfect but challenging target for astrophotographers with widefield setups. Despite its photographic discovery, it is possible to visually observe parts of it from a dark sky site with an OIII filter. It can be found north of the famous double star Albireo.

Other objects of interest in this image include the bright star Phi Cygni north of Sh2-91 (the southern rim of G65.3+5.7) as well as the tiny 30 arcsecond planetary nebula Kronberger 15, which can be found somewhere near the bottom left corner of the full resolution image. Another fairly hidden and inconspicuous object is the red “star” visible to the north of Sh2-91 in the full frame image, this is Campbell’s Hydrogen Star and despite its name is a planetary nebula.

Further examples of obscure supernova remnants in the constellation of Cygnus include CTB 80, W63, G70.5+1.9 and HB 21. In fact, the entire sky is full of lesser known supernova remnants that are worthy of long exposure astrophotography with more being discovered as time goes on!

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