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Supernova remnant in the constellation of Cygnus

The Cygnus Loop (radio source W78, or Sharpless 103) is a large supernova remnant (SNR) in the constellation Cygnus, an emission nebula measuring nearly 3° across. Some arcs of the loop, known collectively as the Veil Nebula or Cirrus Nebula, emit in the visible electromagnetic range. including the "Western Veil" or "Witch's Broom", the "Eastern Veil", and Pickering's Triangle.

NGC 6960, the Western Veil, is the western part of the remnant, also known as the "Witch's Broom", located at J2000 RA Dec NGC 6995 is located farther south at J2000 RA Dec , and IC 1340 even farther south at J2000 RA Dec .

Also known as Pickering's Wedge, or Pickering's Triangular Wisp, this segment of relatively faint nebulosity was discovered photographically in 1904 by Williamina Fleming at Harvard Observatory, where Edward Charles Pickering was director at the time. The Triangle is brightest along the northern side of the loop, though photographs show the nebulosity extending into the central area as well.

These two objects are generally identified today (as by the NGC/IC Project and Uranometria) with two brighter knots of nebulosity in a cloud at the northern edge of the loop, to the east of the northern edge of Pickering's Triangle. NGC 6979 was reported by William Herschel, and while the coordinates he recorded for Veil objects were somewhat imprecise,

The identifier NGC 6979 is sometimes taken to refer to Pickering's Triangle, but the Triangle is probably not what Herschel saw or what the Catalogue intended for this entry: it was discovered only photographically, after the Catalogue was published, and long after Herschel's observation. NGC 6974 was reported by Lord Rosse, but the position he gave lies in an empty region inside the main loop. It was assumed that he recorded the position incorrectly, and the New General Catalogue gives Rosse's object as the other knot in the northern cloud, located at J2000 RA Dec , one degree north of Rosse's position. (This position is farther east than NGC 6979, even though NGC objects are generally ordered by increasing RA.) These filaments in the north central area are sometimes known as the "carrot". The spectrum at 34.5 MHz of the region associated with NGC 6974 ranges straight over the entire frequency range 25 to 5000 MHz.

The southeastern knot is located at J2000 RA Dec on the southeastern rim of the Cygnus Loop. The knot has been identified as an encounter between the blast wave from the supernova and a small isolated cloud. The knot is a prominent X ray feature, consisting of a number of filaments correlated with visual line emission. However, in 1999, William Blair, assuming that the shock wave should be expanding at the same rate in all directions, compared the angular expansion along the sides of the bubble (visible in Hubble Space Telescope images) with direct line of sight measurements of the radial expansion towards the Earth and concluded that the actual size of the bubble was about 40% smaller than the conventional value, leading to a distance of about 1470 ly.

The brightest far ultraviolet sources of the Cygnus Loop occur in the north east edge of the remnant. The first flight of the High Resolution Emission Line Spectrometer (HIRELS), a wide field, far ultraviolet nebular spectrometer, tuned to OVI emission lines, was launched aboard a Nike Black Brant from White Sands Missile Range to observe the Cygnus Loop, the first observed galactic OVI emission line source.

The X ray source Cygnus X 5 coincides with SNR G074.0 08.6 (the Cygnus Loop), located at J2000 RA Dec , observed by Uhuru at 4U 2046+31. This source also has catalogue numbers 1E 2049.4+3050, 1H 2050+310, and 1M 2051+309, having been observed by the Einstein Observatory, HEAO 1, and OSO 7, respectively. The Cygnus Loop is a strong source of soft X rays. The center of the supernova shell as determined from X ray data lies at J1950 RA Dec A characteristic thermal temperature averaged over the loop from X ray spectral data is Tx = 2.9 ± 1.5 x 106 K. a value that puts the expected remnant firmly within neutron star boundaries. However, despite many searches, no compact stellar remnant had been confidently identified since the identification of the supernova remnant. A noted anomaly is that in X rays, the nebula appears perfectly spherical aside from a "blowout region" to the south. Searches for a compact stellar remnant have been largely concentrated here, as the hole may have been caused by the violent ejection of a neutron star. A detailed 2012 study of the blowout region identified a possible pulsar wind nebula, as well as a point like source within it. Although at almost exactly the same position as a known Seyfert galaxy, the slight offset combined with a lack of a radio counterpart makes the point like source probably unrelated to the galaxy. Whether the feature is a pulsar wind nebula, and if so whether it is related to the Cygnus Loop, is still unknown for certain. If it is indeed the compact stellar remnant of the supernova, the neutron star would have to have been ejected from the center of the nebula at a speed of roughly 1,850, depending on the precise age and distance of the remnant.

In the novel Mindbridge by Joe Haldeman, the Cygnus Loop is the remains of the home star of an omnipotent, immortal race that ultimately decided to destroy itself.