Pickering's Triangle is a small part of the Veil Nebula. The Veil Nebula is a cloud of heated and ionized gas and dust in the constellation Cygnus.
The source supernova was a star 20 times more massive than the Sun, which exploded around 8,000 years ago. The remnants have since expanded to cover an area of the sky roughly 3 degrees in diameter (about 6 times the diameter of the full Moon). The distance to the nebula is not precisely known, but data suggests a distance of about 1,470 light-years.
The analysis of the emissions from the nebula indicate the presence of Sulphur (Sii), Hydrogen (Ha) and Oxygen (Oiii). In this image, these wavelengths have been mapped to the Red, Green and Blue channels respectively; this is commonly known as the Hubble Palette (SHO). Almost all of the light from this nebula is emitted in the Oxygen wavelength, giving a predominantly blue colour. The deep orange areas are a mixture of Sulphur and Hydrogen.
Some parts of the image appear to be rope-like filaments. The standard explanation is that the shock waves are so thin that the shell is visible only when viewed exactly edge-on, giving the shell the appearance of a filament.
Even though the nebula has a relatively bright integrated magnitude of 7, it is spread over so large an area that the surface brightness is quite low, so the nebula is notorious among astronomers for being difficult to see. However, apparently it can be seen clearly in a telescope using an Oiii filter (a filter isolating the wavelength of light from doubly ionized oxygen).
This image is 16x180secs each Sii, Ha and Oiii, filters with Darks, Flats and Bias. Processed in PixInsight and finished in Photoshop.