The Cygnus Loop

The Cygnus Loop
All Images Copyright Steve Cannistra

Please click here for a labeled version.
Please click here for a medium-size view (35%).

Size: About 3 degrees; 90 light years across
Distance: Approximately 1470 light years
RA: 20h 51m 27s (J2000)
Dec: 31 degrees 07' 28" (J2000)

The famous Cygnus Loop, also known as the Veil Nebula Complex, is shown above in this 18 hour image consisting of Ha, OIII, and RGB data collected over 4 nights.  The loop is created by an expanding shockwave resulting from a supernova that exploded approximately 8,000 years ago.  The shockwave slams into surrounding dust in the interstellar medium, ionizing gases such as hydrogen, oxygen, and sulfur, which emit specific wavelengths of radiation during electron recapture.  Thus, in the above image you can see radiation from singly ionized hydrogen (HII, Hydrogen Alpha, 656.3 nm, shown in red) as well as doubly ionized oxygen (OIII, 496nm and 501nm, shown in teal).  The result is a beautiful, lacelike structure that has fascinated astronomers since its original discovery in 1784 by William Herschel.  Parts of the Cygnus Loop visible in the above image include the Eastern Veil (NGC 6992) on the left, the Western Veil (Witch's Broom) Nebula (NGC 6960) on the right, and Fleming's Triangular Wisp (Pickering's Triangle, right of center).  The filamentous structure of the nebula results from the fact that we are viewing the thin shockwave fronts edge-on.

A supernova results from a massive star (at least 8 solar masses) that has used up most of its elements through fusion, converting hydrogen to helium, oxygen, silicon, sulfur, calcium, gold, and eventually iron.  Iron is not a viable fuel for fusion (it requires too much energy), meaning that this element represents the end of the line for a dying star.  Without the outward force of fusion-induced radiation, the star collapses under its own gravity, resulting in a massive explosion and release of radiation, some of which we see as visible light.  Based upon the current rate of expansion of the Cygnus Loop, and accounting for the expected slowing of this expansion rate over time as the shockwave interacts with the interstellar medium, the supernova explosion that created this nebula is estimated to have occurred around 8,000 years ago, just before the beginning of recorded human history.  Those humans gazing upwards at the time must have seen the sudden appearance of a very bright "star" that waned in intensity over several weeks to months. 
The stellar remnant of a supernova explosion is a neutron star or a black hole (depending upon the size of the residual star).  If the residual star is between 1.44 and 3 solar masses, it forms a neutron star.  If it is greater than 3 solar masses, it forms a black hole.  Whether a neutron star or a black hole, the remnant of the supernova explosion that created the Cygnus Loop has not yet been identified.  The bright star overlying the Western Veil (left hand side of the image) is 52 Cygni, a foreground star that is not related to the nebula.

Controversy existed for many years regarding the distance to this nebula, which was originally thought to be in the range of 2,500 light years.  More recent data using the Far Ultraviolet Spectroscopic Explorer (FUSE) has identified a star (KPD2055+311) that emits strongly in UV light, located behind the Cygnus Loop at a distance of 1860 light years.  Thus, 1860 light years represents an upper limit to the distance of the Cygnus Loop.  This supports data from Blair et al using the Hubble Telescope, which placed the Cygnus Loop at a distance of 1470 light years, now accepted as the most reliable estimate.  Additional information about this interesting region may be found here.

Photographic Details:
Dates of image acquistion:  July 18, August 19, 20, and 21, 2020
Scope:  Takahashi FSQ106 at f5 on the Takahashi NJP Mount
Autoguider:  SBIG ST-402 with 60mm guidescope, focal length 227mm
Camera:  Apogee U16M at -20C, with 7 position 50mm square filter wheel (Apogee FW50-7S)
Filters:  Baader Ha, Baader RGB filters
Exposures:  Total exposure 18 hours
Post-processing:  Calibrated, aligned and stacked in Maxim, followed by DDP in ImagesPlus (IP).  Further processing in Photoshop CS (16 bit format)

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