(See description below)
Veil (or Cygnus Loop) Nebula
These wisps of gas are all that remain visible of a Milky Way star. About 15,000 years ago that star exploded in a supernova leaving the Veil Nebula. At the time, the expanding cloud was likely as bright as a crescent Moon toward the constellation of Cygnus, visible for weeks to people living at the dawn of recorded history. The supernova remnant lies about 1400 light-years away and covers over five times the size of the full Moon. The bright wisp at the bottom is known as the Witch's Broom Nebula and can be seen with a small telescope. The Veil Nebula is also known as the Cygnus Loop. It gives off strong emission in the X-ray spectrum
The image above was taken by Mikael Leif. Since the Veil Nebula resides in Cygnus, the view is so cluttered with stars that deep exposures fail to convey the true beauty of the nebula. Therefore he used narrowband H-alpha, SII and OIII filters to maximize the nebulosity relative to the star field. However, when trying to reveal the weakest nebula regions, the resulting image still became quite cluttered with stars. To supress the star field even further he did exposures through continuum filters so that the stellar continuum present in the narrowband images could be subtracted. The process of continuum subtraction is simple in principle, but can be more complicated in practice (here's an outline of the process he developed). In the final image the stellar flux has been reduced to ~25% of that in the narrowband images and just 1% of that present if ordinary broadband filters had been used, thereby letting the Veil shine brightly in its own light.