NGC 6822
| NGC 6822 | |
|---|---|
 |
|
| Observation data (J2000 epoch) | |
| Constellation | Sagittarius |
| Right ascension | 19h 44m 56.6s |
| Declination | −14° 47′ 21″ |
| Redshift | -57 ± 2 km/s |
| Distance | 1.63 ± 0.03 Mly (500 ± 10 kpc) |
| Apparent magnitude (V) | 9.3 |
| Characteristics | |
| Type | IB(s)m |
| Apparent size (V) | 15′.5 × 13′.5 |
| Other designations | |
| Barnard's Galaxy,IC 4895, DDO 209,PGC 63616,Caldwell 57 |
|
NGC 6822 (also known as Barnard's Galaxy, IC 4895, or Caldwell 57) is a barred irregular galaxy approximately 1.6 million light-years away in the constellation Sagittarius. In the Catalogue of Named Galaxies, it is called Barnardinus Sagittarii, after its discoverer. Part of the Local Group of galaxies, it was discovered by E. E. Barnard in 1884 (hence its name), with a six-inch refractor telescope. It is one of the closer galaxies to the Milky Way. It is similar in structure and composition to the Small Magellanic Cloud. It is about 7,000 light-years in diameter.
Edwin Hubble, in the paper N.G.C. 6822, A Remote Stellar System, identified 15 variable stars (11 of which were Cepheids) 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" (giant H II regions) that included the Bubble Nebula and the Ring Nebula. He also computed the absolute magnitude of the entire galaxy.
Hubble's detection of eleven Cepheid variable stars was a milestone in astronomy. Utilizing the Cepheid Period-Luminosity relationship, Hubble determined a distance of more than 700,000 light-years. This was the first system beyond the Magellanic Clouds to have its distance accurately determined. (Hubble continued this process with the Andromeda Galaxy and the Triangulum Galaxy). His distance to the galaxy was way beyond Harlow Shapley's value of 300,000 light-years for the size of universe. In the paper, Hubble concluded the "Great Debate" of 1920 between Heber 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 fact spiral galaxies far outside our own Milky Way.
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