cloud of heated and ionized gas and dust
The nebula was discovered on 1784 September 5 by William Herschel. He described the western end of the nebula as "Extended; passes thro' 52 Cygni... near 2 degree in length", and described the eastern end as "Branching nebulosity... The following part divides into several streams uniting again towards the south."
When finely resolved, some parts of the image appear to be rope-like filaments. The standard explanation is that the shock waves are so thin, less than one part in 50,000 of the radius, that the shell is visible only when viewed exactly edge-on, giving the shell the appearance of a filament. Given a distance of 1470 Light Years, this gives the radius of the entire nebula as 38.5 Light Years (total width, 77 Light Years). At 1/50,000th of the radius, this places the thickness of each filament at around 4 billion miles, or roughly the distance to Pluto. Undulations in the surface of the shell lead to multiple filamentary images, which appear to be intertwined.