NGC 6822

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Observation data (J2000 epoch)
NGC 6822

Constellation	Sagittarius
Right ascension	19^h 44^m 56.6^s^[1]
Declination	-14° 47′ 21″^[1]
Redshift	-57 ± 2 km/s^[1]
Distance	1.63 ± 0.03 Mly (500 ± 10 kpc)^[2]^[3]^[4]
Type	IB(s)m^[1]
Apparent dimensions (V)	15′.5 × 13′.5^[1]
Apparent magnitude (V)	9.3^[1]
Other designations
Barnard's Galaxy,^[1] IC 4895,^[1] DDO 209,^[1] PGC 63616^[1]
See also: Galaxy, List of galaxies

NGC 6822 (also known as Barnard's Galaxy or IC 4895) is a barred irregular galaxy approximately 1.6 million light-years away in the constellation Sagittarius. Part of the Local Group of galaxies, it was discovered by E. E. Barnard in 1881 (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.

1 Observational history
2 See also
3 External links
4 Notes
5 References

[edit] Observational history

Edwin Hubble wrote the seminal paper N.G.C. 6822, A Remote Stellar System (Hubble 1925) wherein he identified 15 variable stars (11 of which were Cepheids). 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 nebula" (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 key milestones 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 this 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 way beyond our own Milky Way.

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 cataloged in Barnard's Galaxy.