NGC 6960+, Cirrus Nebula

Object Description

NGC 6960+
NGC 6960+: The entire Cirrus Nebula extends over a range of about 3°; TS Triplet APO 90, Reducer Photoline 0.79 (490mm / f5.44), SBIG ST-8300; 22L x 300sec 1×1, 21R, 21G, 21B 2×2 300sec; Bernese Highlands; © 2018 Bernhard Blank, Dragan Mihajlovic

The Cirrus Nebula is probably one of the most fascinating, largest and most beautiful nebulae in the northern sky and has therefore been given numerous names: «Bridal Veil Nebula» or just «Veil Nebula» and «Cygnus Loop». The name «Filamentary Nebula» is often used only for the eastern part of NGC 6992 and the name «Network Nebula» or «Lacework Nebula» for the western part of NGC 6960. But sometimes one also uses these names for the entire nebula.

The Cirrus Nebula was discovered visually by William Herschel in 1784 with his 18.7 inch reflecting telescope. For a long time the true nature of these wisps of nebula was unknown and they were given different numbers in the «New General Catalogue», as well as in the «Index Catalogue». It seems that the numbering was later was abandoned, as many seemingly independent wisps of the nebula are no longer labeled.

The visually visible portions of the Cirrus Nebula appear to coalesce into an expanding gas cloud of about 3° apparent diameter. Determining the exact distance is difficult. One finds values from 1300 to 2500 light-years, probably it is close to 1500 light-years. At that distance, the entire nebula is about 70 light-years across.

This supernova remnant is expanding at 0.06 arc seconds annually into a region of interstellar gas and dust, pushing interstellar material before it. The sky inside this arc appears visibly clearer and has more fine stars than outside, which is evident in the region near the bright star 52 Cygni. It is estimated that the supernova probably occurred between 30,000 and 40,000 years ago.

The Cirrus Nebula, like all other known supernova remnants, is a source of radio emission, but is much fainter than the known Crab Nebula in Taurus, which is only 900 years old. Radio emission from the Cirrus Nebula was first discovered by D. Walsh and R.H. Brown at the Jodrell Bank Observatory in England.

Cirrusnebel
Cirrusnebel: The Pickering Triangle in the upper centre of the image. Bottom right NGC 6960 at 52 Cygni; Celestron RASA 11" f/2.22; ZWO ASI6200 Pro; Tentlingen; © 2020 Peter Kocher
Cirrusnebel
Cirrusnebel: The western part NGC 6960 near 52 Cygni; Celestron RASA 11" f/2.22; ZWO ASI6200 Pro; Tentlingen; © 2020 Peter Kocher

The reason for the glow of the nebula still seems to be a mystery. Spectral analyzes showed that the light is fluorescent and that the individual gas plumes have different temperatures of 10'000 to 60'000 Kelvin. However, neither the star energizing the nebula has been identified, nor does a star near the center of the arc show any post-novaan characteristics. The post-nova is thought to be a close companion of a star near the apparent center of the Cirrus Nebula.

The western portion of the Cirrus Nebula, NGC 6960, appears to be drifting past the 4.3 mag star 52 Cygni, but the star is known as a foreground object and has no association with the Cirrus Nebula.

Cirrus Nebula
Cirrus Nebula: NGC 6960 near 52 Cygni; 500 mm Cassegrain 3625 mm f/7.2; SBIG STL11K; 90+15+15+15 min LRGB; Bernese Highlands; © 2011 Radek Chromik
Cirrus Nebula
Cirrus Nebula: Supernova remnant in Cygnus; 500 mm Cassegrain 3625 mm f/7.2; SBIG STL11K; 90+15+15+15 min LRGB; Bernese Highlands; © 2011 Radek Chromik
Cirrus Nebula
Cirrus Nebula: Supernova remnant in Cygnus; 500 mm Cassegrain 3625 mm f/7.2; SBIG STL11K; 90+30+30+30 min LRGB; Bernese Highlands; © 2011 Radek Chromik
Cirrus Nebula
Cirrus Nebula: Eastern part (NGC 6992, NGC 6995); 500 mm Cassegrain 3625 mm f/7.2; SBIG STL11K; 90+30+30+30 min LRGB; Bernese Highlands; © 2015 Radek Chromik

This fiber structure is the special feature of this nebula and could not be fully explained. The thickness of such a typical fiber varies from 500 to 700 AU up to 2000 AU, or about 1 to 5 arc seconds. It's possible that these narrow filaments are actually thin luminous surfaces that we're looking at from the side. Such a surface, viewed from the front, would appear invisible to us because of its weak surface brightness. The expansion of the cloud into the interstellar medium possibly contributes to the filament formation, which resembles a large shock front phenomenon. Electromagnetic forces must also be considered. [4, 110, 115, 169]

«Revised New General Catalogue and Index Catalogue», «Historically Corrected New General Catalogue», Dr. Wolfgang Steinicke, 2021 [277]
NameRADecTypeBmagVmagDimDreyer DescriptionIdentification
NGC 6960 20 45 42.0+30 43 00SNR7.070.00 × 6.0!! pB, cL, eiF, κ Cygni invLBN 191, Veil nebula
NGC 6974 20 51 04.0+31 49 54SNRNeb *, neby cE pfknot in Veil nebula
NGC 6979 20 50 30.0+32 01 36SNR7.00 × 3.0vF, S, iE, sev F st f nrpart of Veil nebula
NGC 6992 20 56 18.0+31 44 30SNR7.060.00 × 8.0!!, eF, eL, eE, eiF, bifurcatedCED 182B, Veil nebula
NGC 6995 20 57 10.0+31 14 00SNR7.012.00 × 12.0F, eL, neb & st in groupsCED 182C, Veil nebula
IC 1340 20 56 08.0+31 02 54SNR25.00 × 20.0Possibly conn with h 2093part of Veil nebula

How to find the Supernova Remnant?

Chart NGC 6960
Chart created using SkySafari 6 Pro and STScI Digitized Sky Survey. [149, 160]
Cirrusnebel
Cirrusnebel: Ausschnitt von etwa 5.25° aus dem STScI Digitized Sky Survey. [147]

The Cirrus Nebula is very easy to find. Point the telescope at the star 52 Cygni, which is located at the eastern wing of the constellation Cygnus. The star is 4.2 mag bright and still easily visible with naked eye. There lies the western part of the Cirrus Nebula NGC 6960, which passes directly nearby the star. Starting from there, the entire Cirrus Nebula can be explored along the nebulae to the east. At the beginning it is recommended to use an eyepiece with the largest possible field of view to get an overview. An O-III filter greatly increases the contrast between the nebula and the sky background. For some parts, it is worth magnifying higher to see more structure in the nebulae.

References

4«Burnham's Celestial Handbook: An Observer's Guide to the Universe Beyond the Solar System» by Robert Burnham; Dover Publications, Inc.; Voume I: ISBN 0-486-23567-X; Volume II: ISBN 0-486-23568-8; Volume III: ISBN 0-486-23673-0
110The Veil Nebula by Roger N. Clark; S&T 10/94, p.104
115Hubble Telescope Probes Shocks in Cygnus; S&T 5/95, p.11
147Aladin Lite; aladin.u-strasbg.fr/AladinLite (2020-12-23)
149SkySafari 6 Pro, Simulation Curriculum; skysafariastronomy.com
160The STScI Digitized Sky Survey; archive.stsci.edu/cgi-bin/dss_form
169Cygnus Loop; Press Release No. STScI-PRC95-11; oposite.stsci.edu/pubinfo/pr/95/11.html
277«Historische Deep-Sky Kataloge» von Dr. Wolfgang Steinicke; klima-luft.de/steinicke (2021-02-17)