| Diffuse nebula | |
|---|---|
| supernova remnant | |
 Western Veil nebula | |
| Observation data: J2000.0epoch | |
| Right ascension | 20h 45m 38.0s[1] |
| Declination | +30° 42′ 30″[1] |
| Distance | 2400[2] ly |
| Apparent magnitude (V) | 7.0 |
| Apparent dimensions (V) | 3degrees (diameter) |
| Constellation | Cygnus |
| Physical characteristics | |
| Radius | 50-65[citation needed] ly |
| Designations | NGC 6960,[1] 6992,[1] 6995,[1] 6974, and 6979, IC 1340, Cygnus Loop, Cirrus Nebula,[1] Filamentary Nebula,[1] Witch's Broom Nebula (NGC 6960),[3]Caldwell 33/34 |
| See also:Lists of nebulae | |
TheVeil Nebula is a cloud of heated andionized gas and dust in the constellationCygnus.[4]
It constitutes the visible portions of theCygnus Loop,[5] asupernova remnant, many portions of which have acquired their own individual names and catalogue identifiers. The sourcesupernova was a star 20 times more massive than theSun which exploded between 10,000 and 20,000 years ago.[2] At the time of the explosion, the supernova would have appeared brighter thanVenus in the sky, and visible in the daytime.[6] The remnants have since expanded to cover an area of the sky roughly 3 degrees in diameter (about 6 times the diameter, and 36 times the area, of the full Moon).[4] While previous distance estimates have ranged from 1200 to 5800light-years, a recent determination of 2400 light-years is based on direct astrometric measurements.[2] (The distance estimates affect also the estimates of size and age.)
TheHubble Space Telescope captured several images of thenebula. The analysis of theemissions from the nebula indicates the presence ofoxygen,sulfur, andhydrogen.[7] The Cygnus Loop is also a strong emitter ofradio waves andx-rays.[8]
In modern usage, the namesVeil Nebula,Cirrus Nebula, andFilamentary Nebula generally refer to all the visible structure of the remnant, or even to the entire loop itself. The structure is so large that several NGC numbers were assigned to various arcs of the nebula.[9] There are three main visual components:
NGC 6974 andNGC 6979 are luminous knots in a fainter patch of nebulosity on the northern rim between NGC 6992 and Pickering's Triangle.[12][13]
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The nebula was discovered on 5 September 1784 byWilliam 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."[14]
When finely resolved, some parts of the nebula 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,[15] that the shell is visible only when viewed exactly edge-on, giving the shell the appearance of a filament. At the estimated distance of 2400 light-years, the nebula has a radius of 65 light-years (a diameter of 130 light-years). The thickness of each filament is1⁄50,000th of the radius, or about 4 billion miles, roughly the distance from Earth to Pluto. Undulations in the surface of the shell lead to multiple filamentary images, which appear to be intertwined.
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 as being difficult to see. However, an observer can see the nebula clearly in a telescope using an O-IIIastronomical filter (isolating thewavelength of light fromdoubly ionized oxygen), as almost all light from this nebula is emitted at this wavelength. An 8-inch (200 mm) telescope equipped with an O-III filter shows the delicate lacework apparent in photographs. Smaller telescopes with an O-III filter can show the nebula as well, and some[who?] argue that it can be seen without any optical aid except anO-III filter held up to the eye.[citation needed]
The brighter segments of the nebula have theNew General Catalogue designations NGC 6960, 6974, 6979, 6992, and 6995. The easiest segment to find is 6960, which runs behind52 Cygni, a star that can be seen with the naked eye. NGC 6992 and 6995 are objects on the eastern side of the loop which are also relatively easy to see. NGC 6974 and NGC 6979 are visible as knots in an area of nebulosity along the northern rim. Pickering's Triangle is much fainter and has no NGC number (though 6979 is occasionally used to refer to it). It was discovered photographically in 1904 byWilliamina Fleming (after theNew General Catalogue was published), but credit went toEdward Charles Pickering, the director of her observatory, as was the custom of the day.
The Veil Nebula is expanding at a velocity of about 1.5 million kilometers per hour. Using images taken by theHubble Space Telescope between 1997 and 2015, the expansion of the Veil Nebula has been directly observed.[16][17]
