| NGC 6822 | |
|---|---|
 NGC 6822 fromESO's Wide Field Imager at La Silla Observatory | |
| Observation data (J2000epoch) | |
| Constellation | Sagittarius |
| Right ascension | 19h 44m 57.70s[1] |
| Declination | −14° 48′ 12.0″[1] |
| Redshift | −57 ± 2 km/s (−35.4 ± 1.2 mi/s)[1] |
| Distance | 1.63 ± 0.03Mly (500 ± 10kpc)[2][3][4] |
| Apparent magnitude (V) | 9.3[1] |
| Characteristics | |
| Type | IB(s)m[1] |
| Size | ~9,200 ly (2.82 kpc) (estimated)[1] |
| Apparent size (V) | 15.5′ × 13.5′[1] |
| Other designations | |
| Barnard's Galaxy,DDO 209,Caldwell 57,IRAS 19420-1450,IC 4895,MCG -02-50-006,PGC 63616[1] | |
NGC 6822 (also known asBarnard's Galaxy,IC 4895, orCaldwell 57) is abarred irregular galaxy approximately 1.6 millionlight-years away in theconstellationSagittarius. Part of theLocal Group of galaxies, it was discovered byE. E. Barnard in 1884, with a six-inchrefractor telescope. It isthe closest non-satellite galaxy to the Milky Way, but lies just outside itsvirial radius.[5] It is similar in structure and composition to theSmall Magellanic Cloud. It is about 7,000 light-years in diameter.[6]
NGC 6822 was discovered byE. E. Barnard in 1884 using a six-inchrefractor telescope.
Edwin Hubble, in the paperN.G.C. 6822, A Remote Stellar System,[7] identified 15 variable stars (11 of which wereCepheids) of this galaxy. He also surveyed the galaxy's stars distribution down to magnitude 19.4. He provided spectral characteristics, luminosities and dimensions for the five brightest "diffuse nebulae" (giantH II regions) that included theBubble Nebula and theRing Nebula. He also computed theabsolute magnitude of the entire galaxy.
Hubble's detection of eleven Cepheid variable stars was a milestone inastronomy. Utilizing theCepheid Period-Luminosity relationship, Hubble determined a distance of 214kiloparsecs or 698,000 light-years. This was the first system beyond the Magellanic Clouds to have its distance determined. (Hubble continued this process with theAndromeda Galaxy and theTriangulum Galaxy). This distance to the galaxy was way beyondHarlow Shapley's value of 300,000 light-years for the size of theuniverse. In the paper, Hubble concluded the "Great Debate" of 1920 betweenHeber Curtis and Shapley over the scale of the universe and the nature of the "spiral nebula". It soon became evident that all spiral nebulae were in factspiral galaxies far outside our own Milky Way.
An analysis of Hubble's plates bySusan Kayser in 1966 remained the most complete study of this galaxy until 2002.[8]
In 1977,Paul W. Hodge extended the list of known H II regions in Barnard to 16. Today, there are over 150 of these regions catalogued in Barnard's Galaxy.
Observations of the galaxy show stars-forming in the dense cores of giant clouds of molecular hydrogen gas, cold enough to collapse under its own gravity.[9] The distribution of hydrogen gas is disk-shaped, but mysteriously, it is angled at about 60° relative to the stellar distribution.[5] Most of its stars formed within the last 3 to 5 billion years.[5]
NGC 6822 has spent most of its life in relative isolation. However, it likely passed within the virial radius of the Milky Way some 3 to 4 billion years ago, which may be coincident with its increase in star formation.[5]
![Composite image from data by 2.2-meter MPG/ESO and ALMA with star-forming regions identified.[10]](/mt/?noimg=&dark=on&url=https%3a%2f%2fen.wikipedia.org%2fwiki%2fFile%3aALMA_peers_into_the_hearts_of_stellar_nurseries.jpg)
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![JWST NIRCam’s view of NGC 6822 [11]](/mt/?noimg=&dark=on&url=https%3a%2f%2fen.wikipedia.org%2fwiki%2fFile%3aNGC_6822_(potm2309a).jpg)
