| Emission nebula | |
|---|---|
| H II region | |
 North America Nebula in false color narrowband | |
| Observation data: J2000.0epoch | |
| Right ascension | 20h 59m 17.1s[1] |
| Declination | +44° 31′ 44″[1] |
| Distance | 2,590 ± 80 ly (795 ± 25[2] pc) |
| Apparent magnitude (V) | 4 |
| Apparent dimensions (V) | 120 × 100arcmin[3] |
| Constellation | Cygnus |
| Physical characteristics | |
| Radius | 45[4] ly |
| Designations | NGC 7000,Sharpless 117,Caldwell 20 |
| See also:Lists of nebulae | |
TheNorth America Nebula (NGC 7000 orCaldwell 20) is anemission nebula in theconstellationCygnus, close toDeneb (the tail of the swan and its brighteststar) in thenight sky. It is named because its shape resemblesNorth America.
On October 24, 1786,William Herschel observing fromSlough, England, noted a “faint milky nebulosity scattered over this space, in some places pretty bright.”[5]The most prominent region was catalogued by his sonJohn Herschel on August 21, 1829. It was listed in theNew General Catalogue as NGC 7000, where it is described as a "faint, most extremely large, diffuse nebulosity.”[6]
In 1890, the pioneering GermanastrophotographerMax Wolf noticed this nebula's characteristic shape on a long-exposure photograph, and dubbed it the North America Nebula.[7]
In his study of nebulae on thePalomar Sky Survey plates in 1959, American astronomerStewart Sharpless realised that the North America Nebula is part of the sameinterstellar cloud ofionizedhydrogen (H II region) as thePelican 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 astronomerGart Westerhout detected the HII region Sh2-117 as a strong radio emitter, 3° across, and it appears as W80 in his 1958 catalogue ofradio sources in the band of theMilky Way.[8]
The North America Nebula covers a region more than ten times the area of the fullmoon, but its surface brightness is low, so normally it cannot be seen with the unaided eye.Binoculars andtelescopes with largefields of view (approximately 3°) will show it as a foggy patch of light under sufficiently dark skies. However, using aUHC filter, which filters out some unwantedwavelengths of light, it can be seen without magnification under dark skies. Its shape and reddish color (from thehydrogen Hαemission line) show up only in photographs of the area.[7]
The portion of the nebula resembling Mexico and Central America is known as the Cygnus Wall. This region exhibits the most concentratedstar formation.[9]
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 ofinterstellar 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.[10]
The distances to the North America and Pelican nebulae were controversial, because there are few precise methods for determining how far away an HII region lies. Until 2020, most astronomers accepted a value of 2,000 light years, though estimates ranged from 1,500 to 3,000 light years.[11] But in 2020, theGaia astrometry spacecraft measured the distances to 395 stars lying within the HII region, giving the North America and Pelican nebulae a distance of 2,590 light years (795±25 parsecs). The entire HII region Sh2-117 is estimated to be 140 light years across, and the North America nebula stretches 90 light years north to south.[2]
HII regions shine because their hydrogen gas is ionised by theultraviolet 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 1958George 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 the2MASS survey, and then made detailed observations of likely suspects with the 2.2 m telescope at theCalar 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.[12]
Later observations have revealed J205551.3+435225 is aspectral 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 theBahamas because many of them are only easily seen from a ship during low tide).[13]
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 asAlbireo, the star marking the swan's head.[12]
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Media related toNorth America Nebula at Wikimedia Commons
