The Veil Nebula, also known as the Cygnus Loop, is a cloud of heated and ionized gas and dust, a large but relatively faint supernova remnant. The source supernova exploded some 6,000 years ago, and the remnants have since expanded outward like ripples in a pond to cover an area roughly about 6 times the diameter, or 36 times the area, of the full moon; a radius of 50 light years.
The Nebula’s distance from our Solar System is about 1,470 light-years, and it is positioned a few degrees to the south of the star Epsilon Cygni, also known as Gienah, at the right wing of the constellation Cygnus or Swan. William Herschel discovered the nebula in September 1784.
The name, the Veil Nebula, generally refers to all the visible structure of the remnant. The structure is so large that several NGC numbers were assigned to various arcs of the nebula. The brighter segments of the nebula have the New General Catalogue designations NGC 6960, 6974, 6979, 6992, and 6995. Different regions of the Veil Nebula have different nicknames. The Western Veil Nebula is also known popularly as the Witch’s Broom (see the cover photo), while the Eastern Veil is sometimes called the Network Nebula.
(2) Pickering’s Triangle (or Pickering’s Triangular Wisp or Pickering’s Wedge), brightest at the north central edge of the loop occupies the central region. It is much fainter, and has no NGC number (though 6979 is occasionally used to refer to it). NGC 6974 and NGC 6979 are luminous knots, an X-ray source, in a fainter patch of nebulosity on the northern rim between NGC 6992 and Pickering’s Triangle. Williamina Fleming discovered these knots photographically in 1904.
The intertwined rope-like filaments of gas resulted from the enormous amounts of energy released as the fast-moving debris from the supernova explosion ploughs into its surroundings and creates shock fronts. These shocks, driven by debris moving at 600,000 kilometres per hour, heat the gas to millions of degrees. It is the subsequent cooling of this material that produces the brilliantly coloured glows. Even though the nebula has a relatively bright-integrated magnitude of 7.0, it is spread over so large an area that the surface brightness is quite low, so the nebula is notorious among astronomers as being difficult to see. This portion of the Veil Nebula is located in a magnificent part of the Veil known as the Witch’s Broom Nebula. The bright blue star – 52 Cygni is unrelated to the supernova explosion. The different colours indicate emission from different kinds of atoms excited by the shock: blue shows oxygen, green shows sulphur, and red shows hydrogen.
Why are astronomers interested in studying supernovae and their remnants? Supernovae are extremely important for understanding our own Milky Way. Although only a few stars per century in our Galaxy will end their lives in this spectacular way, these explosions are responsible for making all chemical elements heavier than iron in the Universe. Many elements, such as copper, mercury, gold, iodine and lead that we see around us here on Earth today were forged in these violent events eons ago. The expanding shells of supernova remnants were mixed with other material in the Milky Way and became the raw material for new generations of stars and planets. The Veil Nebula is a prototypical middle-aged supernova remnant, and is an ideal laboratory for studying the physics of supernova remnants: it is fairly nearby, has a large angular size and has a relatively small amount of foreground extinction.
Sources: Constellation Guide, Hubble, NASA, Wikipedia.