North America Nebula
The North America Nebula (NGC 7000 or Caldwell 20) is an emission nebula in the constellation Cygnus, close to Deneb (the tail of the swan and its brightest star). The shape of the nebula resembles that of the continent of North America, complete with a prominent Gulf of Mexico.
|H II region|
|Observation data: J2000.0 epoch|
|Right ascension||20h 59m 17.1s|
|Declination||+44° 31′ 44″|
|Distance||2,590 ± 80 ly (795 ± 25 pc)|
|Apparent magnitude (V)||4|
|Apparent dimensions (V)||120 × 100 arcmin|
|Designations||NGC 7000, Sharpless 117, Caldwell 20|
On October 24, 1786, William Herschel observing from Slough, England, noted a “faint milky nebulosity scattered over this space, in some places pretty bright.”  The most prominent region was catalogued by his son John Herschel on August 21, 1829. It was listed in the New General Catalogue as NGC 7000, where it is described as a "faint, most extremely large, diffuse nebulosity.” 
In his study of nebulae on the Palomar Sky Survey plates in 1959, American astronomer Stewart Sharpless realised that the North America Nebula is part of the same interstellar cloud of ionized hydrogen (H II region) as the Pelican Nebula, separated by a dark band of dust, and listed the two nebulae together in his second list of 313 bright nebulae as Sh2-117. American astronomer Beverly T. Lynds catalogued the obscuring dust cloud as L935 in her 1962 compilation of dark nebulae. Dutch radio astronomer Gart Westerhout detected the HII region Sh2-117 as a strong radio emitter, 3° across, and it appears as W80 in his 1958 catalogue of radio sources in the band of the Milky Way. 
The North America Nebula covers a region more than ten times the area of the full moon, but its surface brightness is low, so normally it cannot be seen with the unaided eye. Binoculars and telescopes with large fields of view (approximately 3°) will show it as a foggy patch of light under sufficiently dark skies. However, using a UHC filter, which filters out some unwanted wavelengths of light, it can be seen without magnification under dark skies. Its shape and reddish color (from the hydrogen Hα emission line) show up only in photographs of the area. 
At optical wavelengths, the North America Nebula and the Pelican Nebula (IC 5070) appear distinct as they are separated by the silhouette of the dark band of interstellar dust L935. The dark cloud is however transparent to radio waves and infrared radiation, and these wavelengths reveal the central regions of Sh2-117 that are not visible to an ordinary telescope, including many highly luminous stars. 
Distance and sizeEdit
The distance to the North America has long been controversial, because there are few precision methods for determining how far away an HII region lies. Until 2020, most astronomers accepted a value of 2000 light years, though estimates ranged from 1500 to 3000 light years.
But in 2020, this nebula's distance was pinned down with unprecedented accuracy, after the Gaia astrometry satellite measured the precise distances to 395 stars lying within the HII region. The data show that the North America and Pelican nebulae lie 2,590 light years away (795±25 parsecs). The whole HII region Sh2-117 is then 140 light years across, and the North America Nebula stretches 90 light years north to south. 
HII regions shine because their hydrogen gas is ionised by the ultraviolet radiation from a hot star. In 1922, Edwin Hubble proposed that Deneb may be responsible for lighting up the North America Nebula, but it soon became apparent that it is not hot enough: Deneb has a surface temperature of 8,500° K, while the nebula’s spectrum shows it is being heated by a star hotter than 30,000° K. In addition, Deneb is well away from the middle of the complete North America/Pelican nebula complex (Sh2-117), and by 1958 George Herbig realised that the ionizing star had to lie behind the central dark cloud L935. In 2004, European astronomers Fernando Comerón and Anna Pasquali searched for the ionizing star behind L935 at infrared wavelengths, using data from the 2MASS survey, and then made detailed observations of likely suspects with the 2.2 m telescope at the Calar Alto Observatory in Spain. One star, catalogued J205551.3+435225, fulfilled all the criteria. Lying right in the centre of Sh2-117, with a temperature of over 40,000° K, it is almost certainly the ionising star for the North America and Pelican Nebulae. 
Later observations have revealed J205551.3+435225 is a spectral type O3.5 star, with another hot star (type O8) in orbit. J205551.3+435225 lies just off the “Florida coast” of the North America Nebula, so it has been more conveniently nicknamed the Bajamar Star ("Islas de Bajamar," meaning "low-tide islands" in Spanish, was the original name of the Bahamas because many of them are only easily seen from a ship during low tide). 
Although the light from the Bajamar Star is dimmed by 9.6 magnitudes (almost 10,000 times) by the dark cloud L935, it is faintly visible at optical wavelengths, at magnitude 13.2. If we saw this star undimmed, it would shine at magnitude 3.6, almost as bright as Albireo, the star marking the swan's head. 
- "NASA/IPAC Extragalactic Database". Results for NGC 7000. Retrieved 2006-10-17.
- Kuhn, Michael A.; Hillenbrand, Lynne A.; Carpenter, John M.; Menendez, Angel Rodrigo Avelar (2020). "The Formation of a Stellar Association in the NGC 7000/IC 5070 Complex: Results from Kinematic Analysis of Stars and Gas" (PDF). The Astrophysical Journal. 899 (2): 128–167. doi:10.3847/1538-4357/aba19a. Retrieved 2 September 2020.
- Frommert, Hartmut; Kronberg, Christine (2020). "NGC 7000". SEDS. Retrieved 26 August 2020.
- Latusseck, Arndt (2008). "William Herschel's fifty-two fields of extensive diffused nebulosity - a revision". Retrieved 26 August 2020.
- Seligman, Courtney (2020). "NGC 7000, The North America Nebula". Celestial Atlas. Retrieved 26 August 2020.
- French, Sue (2004). "Navigating North America" (PDF). Sky & Telescope. Sky Publishing Corp. Retrieved 26 August 2020.
- Jardine, Kevin. "Sh 2-117". Galaxy Map. Retrieved 26 August 2020.
- Wager, Sara (2016). "The Cygnus Wall of Star Formation". Astronomy Picture of the Day. NASA. Retrieved 2 September 2020.
- Rebull, Luisa (2011). "Changing Face of the North America Nebula". Spitzer Space Telescope. NASA-JPL. Retrieved 26 August 2020.
- Reipurth, Bo (2008). "Star Formation and Young Clusters in Cygnus" (PDF). Astronomical Society of the Pacific. Archived from the original (PDF) on 2019-03-08. Retrieved 2 September 2020.
- Comerón, Fernando; Pasquali, Anna (2005). "Discovery of the star that ionizes the North America and Pelican nebulae". Centro Astronómico Hispano-Alemán Newsletter. Centro Astronómico Hispano-Alemán. Retrieved 26 August 2020.
- Apellániz1, J. Maíz; Sota2, A. (2016). "The galactic O-star spectroscopic survey (GOSS). III. 142 additional O-type systems". The Astrophysical Journal. IOP Publishing for the American Astronomical Society. Retrieved 26 August 2020.
- Media related to North America Nebula at Wikimedia Commons
- The North America Nebula (NGC 7000) at the astro-photography site of Mr. T. Yoshida.
- NASA APOD: The North America and Pelican Nebulae (June 30, 2009)
- NASA APOD: The North America Nebula (May 1, 2000)
- starpointing.com – Central part of the North America Nebula: The Great Wall
- Creative Commons North America Nebula Data North America Nebula - Creative Commons data Download & editing guide
- North America Nebula on WikiSky: DSS2, SDSS, GALEX, IRAS, Hydrogen α, X-Ray, Astrophoto, Sky Map, Articles and images