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Veil Nebula

Coordinates: Sky map 20h 45m 38s, +30° 42′ 30″
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(Redirected from NGC 6995)
Veil Nebula
Diffuse nebula
supernova remnant
Western Veil nebula
Observation data: J2000.0 epoch
Right ascension20h 45m 38.0s[1]
Declination+30° 42′ 30″[1]
Distance2400[2] ly
Apparent magnitude (V)7.0
Apparent dimensions (V)3 degrees (diameter)
ConstellationCygnus
Physical characteristics
Radius50-65[citation needed] ly
DesignationsNGC 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

The Veil Nebula is a cloud of heated and ionized gas and dust in the constellation Cygnus.[4]

It constitutes the visible portions of the Cygnus Loop,[5] a supernova remnant, many portions of which have acquired their own individual names and catalogue identifiers. The source supernova was a star 20 times more massive than the Sun which exploded between 10,000 and 20,000 years ago.[2] At the time of the explosion, the supernova would have appeared brighter than Venus 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 5800 light-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.)

The Hubble Space Telescope captured several images of the nebula. The analysis of the emissions from the nebula indicates the presence of oxygen, sulfur, and hydrogen.[7] The Cygnus Loop is also a strong emitter of radio waves and x-rays.[8]

Components

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NASA photograph of the Cygnus Loop in ultraviolet light, with labels for well-known features. (25 November 2012)

In modern usage, the names Veil Nebula, Cirrus Nebula, and Filamentary 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:

  • The Western Veil (also known as Caldwell 34), consisting of NGC 6960 (the "Witch's Broom",[10] Lacework Nebula,[8] "Filamentary Nebula"[10]) near the foreground star 52 Cygni;
  • The Eastern Veil (also known as Caldwell 33), whose brightest area is NGC 6992, trailing off farther south into NGC 6995 (together with NGC 6992 also known as "Network Nebula"[11]) and IC 1340; and
  • Pickering's Triangle (or Pickering's Triangular Wisp), brightest at the north central edge of the loop, but visible in photographs continuing toward the central area of the loop.

NGC 6974 and NGC 6979 are luminous knots in a fainter patch of nebulosity on the northern rim between NGC 6992 and Pickering's Triangle.[12][13]

  • Eastern Veil Nebula
    Eastern Veil Nebula
  • Pickering's Triangle
  • Western Veil Nebula
    Western Veil Nebula

Observation

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George Willis Ritchey image of what he called the Great Nebula in Cygnus (In modern times the Veil Nebula); taken with the two-foot reflecting telescope with 3 hours exposure at the Yerkes Observatory in 1901.

The nebula was discovered on 5 September 1784 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."[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 is 1⁄50,000th 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-III astronomical filter (isolating the wavelength of light from doubly 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 an O-III filter held up to the eye.[citation needed]

The brighter segments of the nebula have the New General Catalogue designations NGC 6960, 6974, 6979, 6992, and 6995. The easiest segment to find is 6960, which runs behind 52 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 by Williamina Fleming (after the New General Catalogue was published), but credit went to Edward 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 the Hubble Space Telescope between 1997 and 2015, the expansion of the Veil Nebula has been directly observed.[16][17]

See also

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References

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  1. ^ a b c d e f g "NGC 6960". SIMBAD. Centre de données astronomiques de Strasbourg. Retrieved 2007-01-02.
  2. ^ a b c Fesen, Robert A.; Weil, Kathryn E.; Cisneros, Ignacio A.; Blair, William P.; Raymond, John C. (2018). "The Cygnus Loop's distance, properties, and environment-driven morphology". Monthly Notices of the Royal Astronomical Society. 481 (2): 1786–1798. arXiv:1809.01713. Bibcode:2018MNRAS.481.1786F. doi:10.1093/mnras/sty2370. S2CID 119000958.
  3. ^ Nemiroff, R.; Bonnell, J., eds. (1 January 2007). "NGC 6960: The Witch's Broom Nebula". Astronomy Picture of the Day. NASA. Retrieved 2007-01-02.
  4. ^ a b Loff, Sarah (24 September 2015). "Veil Nebula Supernova Remnant". NASA. Retrieved 6 September 2018.
  5. ^ Burnham, Robert (1978). Burnham's Celestial Handbook. New York: Dover. p. 800–811. ISBN 978-0-486-23568-4.
  6. ^ King, Bob (5 September 2018). "Explore the Veil Nebula". Sky & Telescope. Retrieved 12 November 2020.
  7. ^ "Pickering's Triangle in the Veil". NASA. Astronomy Picture of the Day. 17 September 2015. Retrieved 6 September 2018.
  8. ^ a b "Cygnus Loop". Encyclopaedia Britannica. Retrieved 6 September 2018.
  9. ^ Tirion; Rappaport; Lovi (1991) [1987]. Uranometria 2000. Vol. 1. Richmond, VA: William–Bell, Inc. p. 120. ISBN 978-0-943396-14-9.
  10. ^ a b Trusock, Tom. "§"The Veil"". Cloudy Nights Telescope Reviews. Small Wonders: Cygnus ...
  11. ^ Frommert, Hartmut. "NGC 6960, 6979, 6992, 6995: Veil Nebula". spider.seds.org.
  12. ^ "NGC/IC Project". Results for NGC 6974. Retrieved 2010-12-03.
  13. ^ "NGC/IC Project". Results for NGC 6979. Retrieved 2010-12-03.
  14. ^ Bratton, Mark (2011). The Complete Guide to the Herschel Objects. Cambridge University Press. ISBN 9781107376854 – via Google Books.
  15. ^ Blair, William. "Cygnus Loop HST Photo Release". William Blair Homepage at Johns Hopkins University. Archived from the original on 2014-07-28. Retrieved 2010-11-29.
  16. ^ "Revisiting the Veil Nebula". phys.org. ESA/Hubble Information Centre. 24 September 2015. Retrieved 26 June 2020.
  17. ^ "Moving filaments of the Veil Nebula". spacetelescope.org. NASA. 24 September 2015. Retrieved 26 June 2020.


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