Тhe 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. Due to this fact different parts of the Cygnus Loop were gatalogued as different NGC and IC objects. Mouseover will show the catalog numbers of this parts. Several components have separate names and identifiers.
NGC 6960, the Western Veil, is the western part of the remnant, also known as the "Witch's Broom", located at J2000 RA 20h 45m 58.1s Dec +30° 35′ 43″.[3] As the western-most NGC object in the nebula (first in right ascension), its number is sometimes used as an NGC identifier for the nebula as a whole.
NGC 6992, NGC 6995, and IC 1340
These three luminous areas make up the Eastern Veil. NGC 6992 is an HI shell located along the north-eastern edge of the loop. NGC 6995 is located farther south and IC 1340 even farther south.
NGC 6974 and NGC 6979
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, his position for this one is close to the knot.
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, one degree north of Rosse's position. These filaments in the north-central area are sometimes known as the "carrot".
Southeastern knot
The southeastern knot is located 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. By combining visual and X-ray data, it can be shown that the southeastern knot is an indentation on the surface of the blast wave, not a small cloud but the tip of a larger cloud. The presence of a reverse shock is evidence that the knot represents an early stage of a blast wave encountering a large cloud. 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. (William Blair, 1999) A more recent investigation of the Cygnus Loop's distance using Gaia parallax measurements of several stars seen toward the Cygnus Loop has led to a more accurate distance estimate. One of these stars, a 9.6 magnitude B8 star (BD+31 4224) located near the remnant's northwestern rim shows evidence of interactions of its stellar wind with the Cygnus Loop's shock wave, thereby indicating it is located actually inside the remnant. This star's Gaia estimated distance of around 770 pc, along with two other stars both at 735 pc which exhibit spectral features indicating they must lie behind the remnant, leads to new distance of 735 pc or around 2400 light-years. This new distance, surprisingly close to the value estimated some 60 yrs ago by Minkowski, means the Cygnus Loop is physically some 40 pc (130 ly) in diameter and has an age of about 21000 years. |
