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North America Nebula
North America Nebula
North America Nebula
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North America Nebula
Emission nebula
H II region
Observation data: J2000.0 epoch
Right ascension20h 59m 17.1s[1]
Declination+44° 31′ 44″[1]
Distance2,590 ± 80 ly   (795 ± 25[2] pc)
Apparent magnitude (V)4
Apparent dimensions (V)120 × 100 arcmin[3]
ConstellationCygnus
Physical characteristics
Radius45 [4] ly
DesignationsNGC 7000, Sharpless 117, Caldwell 20
See also: Lists of nebulae

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) in the night sky. It is named because its shape resembles North America.

History

[edit]

On October 24, 1786, William Herschel observing from Slough, England, noted a “faint milky nebulosity scattered over this space, in some places pretty bright.” [5] 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.” [6]

In 1890, the pioneering German astrophotographer Max 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 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.[8]

General information

[edit]
Enhanced view of the "Mexico and Central America" Cygnus Wall of the North America Nebula

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.[7]

The portion of the nebula resembling Mexico and Central America is known as the Cygnus Wall. This region exhibits the most concentrated star 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 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.[10]

Distance and size

[edit]
On the left image side are the bright North America Nebula (left bright part) with Sadr region (right bright part) in the Cygnus X region, visually interrupted by the Cygnus rift, of the constellation Cygnus, in this X-ray image.

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, the Gaia 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]

Ionising star

[edit]

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.[12]

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).[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 as Albireo, the star marking the swan's head.[12]

See also

[edit]

References

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[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia

North America Nebula

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The North America Nebula, also known as NGC 7000 or Caldwell 20, is a large located in the constellation Cygnus, approximately 2,600 light-years from , renowned for its distinctive shape that outlines the continent of in visible light images. Discovered by astronomer on October 24, 1786, it covers an apparent area of about 120 by 100 arcminutes in the sky—roughly four times the diameter of the —and is ionized by the embedded massive O5V star J205551.25+435224.6, whose radiation causes the surrounding hydrogen gas to glow with a characteristic red hue. This nebula, with a physical extent of around 100 light-years across, serves as an active stellar nursery where young stars are forming amid dense clouds of gas and dust. As part of a broader star-forming complex in Cygnus, the North America Nebula is closely associated with the adjacent (IC 5070), separated by a dark dust lane known as Lynds 935, which obscures the view of the central ionizing star from optical telescopes. The nebula's glow is primarily from ionized hydrogen (), with additional contributions from and oxygen emissions visible in narrowband imaging, highlighting intricate structures like the prominent "Cygnus Wall"—a 20-light-year-long ridge of dense gas undergoing intense . Although its of 4 to 5 makes it faintly visible to the under dark skies, binoculars or small telescopes reveal its full extent best during late summer in the , near the bright star . Observations from space telescopes like Spitzer and Hubble have uncovered embedded protostars and young clusters, underscoring its role in the early stages of within the Milky Way's .

Overview and Location

General Description

The North America Nebula, also known as NGC 7000 or Caldwell 20, is an and situated in the constellation Cygnus. It is named for its distinctive shape, which closely resembles the outline of the North American continent as revealed in long-exposure photographs. This nebula spans a large angular size, encompassing more than ten times the area of the full moon, though its low surface brightness makes it challenging to detect without binoculars or a telescope. Its prominent reddish hue stems from the hydrogen-alpha (Hα) emission lines generated by ionized hydrogen within the interstellar gas clouds. The nebula appears in close proximity to the bright star Deneb, the alpha star of Cygnus, enhancing its position within the rich stellar fields of the constellation. Under light-polluted urban skies, its faint glow becomes even harder to discern.

Position and Visibility

The North America Nebula, designated NGC 7000, is situated in the constellation Cygnus at equatorial coordinates of right ascension 20h 58m 47s and declination +44° 20' (J2000 epoch). It lies approximately 3 degrees east of the bright star Deneb (Alpha Cygni), near the eastern end of the prominent Northern Cross asterism formed by the stars of Cygnus. This positioning places the nebula in a rich star field along the Milky Way, making it a notable feature in the summer sky for observers in the Northern Hemisphere. Observationally, the nebula presents challenges due to its low , requiring away from urban light pollution for effective viewing. Under such conditions, it can appear as a faint, hazy patch to the , but or a small with a wide are typically needed to discern its outline. The nebula spans an angular extent of about 2 degrees across its major axis, allowing it to fit within the field of view of standard 7x50 . It is best observed during summer months from northern latitudes, when Cygnus culminates high overhead after dark, maximizing visibility.

Physical Properties

Distance and Dimensions

The North America Nebula lies at a distance of approximately 2,590 ± 80 light-years (795 ± 25 parsecs) from the Solar System, as established by astrometric measurements of young stellar objects within the complex using data from Gaia Data Release 2. This revised distance, derived from parallax inversions adjusted for systematic uncertainties, places the nebula in the Orion Arm of the Milky Way. Prior to these Gaia-based analyses, distance estimates ranged from about 1,600 to 2,200 light-years, relying on photometric distances to embedded stars and kinematic models of associated molecular clouds. The main morphological feature of the nebula, evoking the shape of the North American continent, extends roughly 90 light-years in the north-south direction, calculated from its apparent angular size of 120 by 100 arcminutes at the adopted . The broader , designated Sharpless 2-117 and encompassing the adjacent at a comparable , spans approximately 140 light-years across. This immense scale underscores the nebula's nature as a vast interstellar structure, far exceeding the dimensions of the Solar System—whose farthest reaches, including the , extend only about 2 light-years—resulting in the region's characteristically low when observed from .

Composition and Spectrum

The North America Nebula, designated NGC 7000, is primarily an consisting of ionized gas, with as the next most abundant element, alongside trace amounts of heavier elements such as oxygen and grains. The ionization state arises from photons from embedded hot stars, creating a plasma dominated by protons (H⁺) and electrons, with singly ionized (He⁺) contributing to the overall . Dust, composed of silicates and carbonaceous materials, constitutes a small fraction by but plays a significant role in the nebula's structure and energetics. Spectroscopically, the nebula exhibits strong emission lines characteristic of H II regions, with the dominant (Hα) recombination line at 656.3 nm responsible for its prominent red hue in optical images. Additional forbidden emission lines from ionized oxygen, such as [O III] at 500.7 nm, appear in the more highly ionized zones, providing green contributions to the spectrum and indicating regions of elevated ionization potential. Other notable lines include [S II] at 6716 Å and [N II] at 6583 Å, with intensity ratios such as [S II]/Hα averaging around 0.35, reflecting low-density conditions and sulfur abundance relative to hydrogen of approximately 1.86 × 10⁻⁵. The physical conditions within the nebula align with typical extended H II regions, featuring electron densities on the order of 10–100 cm⁻³, with measurements yielding an average rms density of about 20 cm⁻³ based on radio continuum observations. temperatures range from 8,000 to 10,000 , with models assuming a uniform value of 8,000 to derive density profiles from free-free emission at 4 cm wavelength. Dust grains absorb radiation from the ionizing , creating prominent lanes that delineate the nebula's morphological features, while re-emitting the in the infrared, as observed in mappings showing (PAH) bands at 7.6 and 8.6 μm.

Structure and Components

Morphological Features

The North America Nebula displays an irregular, overall shape reminiscent of the North American continent, spanning approximately 90 light-years across its ionized gas and dust structures. This distinctive morphology includes a prominent indentation in the southwestern region, colloquially termed the "," which appears as a darker, less luminous area bounded by brighter emissions. To the southeast, a narrow, peninsula-like extension evokes the shape of , contributing to the nebula's evocative nomenclature. A standout substructure is the Cygnus Wall, a dense, elongated pillar of interstellar gas and dust forming the nebula's eastern boundary and visually analogous to the outline of and . This feature extends about 20 light-years, with its surface marked by bright, curved rims that delineate the edges of embedded dense regions. Dark nebulae within the wall create contrasting silhouettes, while irregular finger-like protrusions extend outward, adding to the complex, sculpted appearance of the overall form. The nebula's main body further exhibits a division into a brighter, more compact core representing "North America proper" and fainter, diffuse extensions northward and southward, enhancing its asymmetric profile. These extensions blend gradually into surrounding , with the brighter rims along the periphery highlighting internal density variations through enhanced emission contrasts. The Cygnus Wall and adjacent are separated by a shared dark dust , underscoring the interconnected yet distinct morphological boundaries of the complex.

Associated Nebulae and Dust Clouds

The North America Nebula is closely associated with the (IC 5070, also known as Sh2-117), an adjacent that together forms the W80 complex, a shared in the constellation Cygnus. These two nebulae are physically connected but visually separated by the intervening dark cloud L935, which creates the appearance of a division resembling the in outline images. The lies to the southeast of the North America Nebula, contributing to a unified structure of ionized gas and dust within the same star-forming environment. The dust cloud L935 is a prominent that obscures the optical view between the two nebulae, blocking visible light while allowing penetration in wavelengths. This cloud, cataloged as part of the Lynds dark nebulae, exhibits high levels, with maximum values significantly exceeding the approximately 3 magnitudes observed in the North America Nebula itself, and it shows the brightest 13CO emission in the surrounding region. Estimates suggest L935 has a line-of-sight thickness not exceeding 20–30 parsecs (approximately 65–98 light-years), assuming a roughly spherical shape, which underscores its role as a dense foreground barrier. The and Nebulae are embedded within the larger Sh2-117 . The associated nebulae and clouds share a common distance of approximately 800 parsecs (about 2,600 light-years) as determined by recent observations, and an environment driven by nearby OB associations, forming a continuous ionized expanse over approximately 140 light-years across the Sh2-117 .

Ionization and Dynamics

Ionizing Source

The primary ionizing source of the North America Nebula is the Bajamar Star, cataloged as J205551.3+435225, an O3.5 III((f*)) supergiant in a spectroscopic with an O8 companion. This massive star, with a surface exceeding 40,000 , emits intense radiation that excites and ionizes the surrounding interstellar gas, producing the nebula's characteristic emission. Its bolometric is approximately 10^5 solar luminosities, sufficient to create a parsec-scale Strömgren where recombination balances . Embedded within or directly behind the dense L935 dark dust cloud, the Bajamar Star is heavily obscured in optical wavelengths, rendering it invisible to traditional telescopes, but it is detectable in infrared surveys such as and observations from the ROSAT All-Sky Survey, where its emission shows hardness and intensity typical of O-type stars. The star's position aligns closely with the geometric center of the and Pelican nebulae complex, near the brightest ionized regions and radio continuum rims. Spectral analysis, including revealing strong He II absorption lines indicative of its early O subtype, confirms its classification and role as the dominant ionizing agent. The binary nature, with differences of about 100 km/s between components, further supports its youth and high-energy output, consistent with the nebula's illumination pattern.

Star Formation Processes

The North America Nebula hosts active , evidenced by the detection of numerous young stellar objects (YSOs) and protostars through surveys conducted with the Spitzer Space Telescope's and Multiband Imaging Photometer for Spitzer (MIPS). These surveys have identified over 1,600 candidate YSOs across the and nebulae complex, with a significant concentration in the Cygnus Wall region, where dense molecular clumps exhibit elevated levels of embedded protostellar activity. observations further corroborate this, revealing more than 700 sources, many associated with YSOs showing near- excesses indicative of circumstellar disks and accretion. Star formation in the nebula is primarily triggered by the compression of molecular gas in dense cores due to advancing ionization fronts from nearby massive stars and interactions with the expansion of the H II region shell W80. This radiative compression leads to the gravitational collapse of filaments and clumps, particularly along the Cygnus Wall, where pillar-like structures of gas and dust are sculpted and eroded by stellar winds and ultraviolet radiation, giving rise to Herbig-Haro objects as collimated outflows from newly forming stars interact with the surrounding medium. Ionization fronts aid in this erosion process by photoevaporating material from the pillar surfaces. The stellar population includes clusters of intermediate-mass B-type stars embedded within these dense cores, with estimated ages ranging from 1 to 5 million years based on color-magnitude diagrams and activity indices from infrared and data. These young clusters are integral to the broader Cygnus OB2 association, where ongoing contributes to feedback mechanisms, including outflows and expanding H II regions that both trigger additional collapse in adjacent clouds and regulate further star birth by dispersing gas.

History and Modern Observations

Discovery and Naming

The North America Nebula was first observed by the British-German astronomer on October 24, 1786, during his systematic sweeps of the night sky from his observatory in , . Herschel described it as a "faint milky nebulosity scattered over this space, in some places pretty bright," noting its diffuse and extensive nature without resolving it into distinct stars. This observation marked the initial identification of the object as a nebulous feature in the constellation Cygnus, though its full extent was not apparent through visual means at the time. Subsequent cataloging efforts built on Herschel's work, with his son independently observing and documenting the nebula's most prominent region on August 21, 1829, during his own surveys in . John Herschel's notes contributed to the (NGC), compiled by Danish-Irish astronomer J. Louis Emil Dreyer and published in 1888, where the object was formally designated NGC 7000 and described as a "faint, most extremely large, diffuse" . Later, in 1995, British astronomer included it as Caldwell 20 in his of deep-sky objects, aimed at observers seeking non-Messier targets visible from the . The evocative name "North America Nebula" was coined by German astronomer and astrophotographer Max Wolf in , following his pioneering photographic exposures that first revealed the object's striking resemblance to the outline of the North American continent in emission. Wolf's images, taken with early dry-plate , highlighted the nebula's irregular, continent-like morphology, which had eluded visual observers due to its low and vast angular size spanning over 2 degrees. This naming reflected the growing role of in unveiling subtle celestial structures during the late . Throughout the , astronomers such as the Herschels and others characterized the as a large, diffuse glow in the rich Cygnus star fields, often linking it observationally to the surrounding complex of interstellar material in the region. Early accounts emphasized its proximity to the bright star and its challenging visibility even under dark skies, requiring or optical aid to discern its hazy form. The nebula's association with the adjacent was noted in passing but not fully explored until later studies.

Recent Studies and Imaging

In the late 2010s and early 2020s, astrometric data from the mission provided refined distance measurements to the North America and Pelican Nebulae complex, placing it at approximately 800 parsecs, with the main ionizing star (the Bajamar Star) at 745 ± 25 parsecs based on Early Data Release 3 (EDR3) parallaxes. This precision has enabled better mapping of the region's three-dimensional structure and tangential velocities, confirming the Bajamar Star's role in ionizing the nebula while highlighting velocity differences among nearby stars that suggest dynamical interactions. Infrared observations from space telescopes have revealed hidden populations of young stellar objects (YSOs) obscured by dust in the visible spectrum. NASA's Spitzer Space Telescope, in surveys conducted in the mid-2000s and analyzed through the early 2010s, identified over 200 YSOs clustered along dark dust lanes resembling a "river" in the nebula's "Gulf of Mexico" region, with ages around one million years and evidence of ongoing disk evolution. Complementary far-infrared data from the Herschel Space Observatory, particularly from the Herschel Infrared Galactic Plane Survey (Hi-GAL) in the 2010s, detected colder, more embedded protostars, enhancing the census of star formation activity across the complex and constraining the physical properties of these YSOs through spectral energy distributions. These multi-wavelength infrared studies underscore the nebula's role as a prolific site of low- to intermediate-mass star formation behind foreground dust. Key imaging milestones have advanced visualization of fine structures, such as the Cygnus Wall—a prominent ridge of ionized gas and dust pillars spanning about 20 light-years. NASA's captured detailed views of similar pillar-like features in the portion during observations in the early 2000s, highlighting erosion by stellar winds and radiation that sculpts these formations, though ground-based narrowband imaging has more recently emphasized the Wall's intricate details. In 2016, NASA's featured high-resolution images of the Cygnus Wall, showcasing its W-shaped emission profile and embedded Herbig-Haro objects, which drew public and scientific attention to the region's dynamic morphology. Radio and observations have mapped the underlying molecular clouds and hot gas, supporting the identification of the Bajamar Star as an early O-type source. Millimeter-wave surveys using the Combined Array for Research in Millimeter-wave Astronomy (CARMA) in 2017 revealed extensive CO emission tracing dense molecular gas in the and regions, delineating cloud masses exceeding 10^4 solar masses and inflows indicative of . data from , acquired in targeted pointings during the 2000s and 2010s, detected diffuse hot plasma associated with feedback from massive stars, including the Bajamar Star, with temperatures around 10^7 K confirming its central ionizing influence amid the Cygnus X complex. These datasets align with the 2005 spectroscopic confirmation of the Bajamar Star as an O5 V star, ruling out earlier candidates, and later refined to O3.5 III((f*)) giant in subsequent studies. Ongoing multi-wavelength surveys of the broader Cygnus X complex continue to probe feedback effects and potential exoplanetary systems. Hydrodynamic simulations incorporating data and radio observations model how stellar winds from Cygnus OB2 disrupt molecular clouds, regulating efficiency at levels below 10% in the region. NASA's Kepler and TESS missions, monitoring Cygnus fields since , have identified transiting exoplanets around young stars in the complex, providing insights into disk stability amid intense radiation fields, while the WEAVE spectrograph survey targets thousands of massive stars for kinematic studies of feedback-driven dispersal.

References

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