Cygnus Loop

The Triangle is brightest along the northern side of the loop, though photographs show the nebulosity extending into the central area as well.

NGC 6979 was reported by William Herschel, and while the coordinates he recorded for Veil objects were somewhat imprecise,[7] his position for this one is tolerably close to the knot at J2000 RA 20h 50m 27.9s Dec +32° 01′ 33″.

NGC 6974 was reported by Lord Rosse, but the position he gave lies in an empty region inside the main loop.

[12] The knot is a prominent X-ray feature, consisting of a number of filaments correlated with visual line emission.

[12] The presence of a reverse shock is evidence that the knot represents an early stage of a blast wave encountering a large cloud.

[13][14] 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.

[13][14] A larger revised value of 540 pc (1760 ly) appeared to be corroborated by Blair's later discovery, via the Far Ultraviolet Spectroscopic Explorer (FUSE), of a star seemingly behind the Veil.

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.

[22] A detailed 2012 study of the blowout region identified a possible pulsar wind nebula, as well as a point-like source within it.

[22] 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.