NGC 6822 (also known as Barnard's Galaxy, IC 4895, or Caldwell 57) is a

barred irregular galaxy A barred irregular galaxy is an irregular version of a barred spiral galaxy. Examples include the Large Magellanic Cloud and NGC 6822. Some barred irregular galaxies (like the Large Magellanic Cloud) may be dwarf spiral galaxies, which have been ...

approximately 1.6 million

light-year A light-year, alternatively spelled light year, is a large unit of length used to express astronomical distances and is equivalent to about 9.46 trillion kilometers (), or 5.88 trillion miles ().One trillion here is taken to be 1012 ...

s away in the

constellation A constellation is an area on the celestial sphere in which a group of visible stars forms Asterism (astronomy), a perceived pattern or outline, typically representing an animal, mythological subject, or inanimate object. The origins of the e ...

Sagittarius. Part of the

Local Group The Local Group is the galaxy group that includes the Milky Way. It has a total diameter of roughly , and a total mass of the order of . It consists of two collections of galaxies in a "dumbbell" shape: the Milky Way and its satellites form ...

of galaxies, it was discovered by

E. E. Barnard Edward Emerson Barnard (December 16, 1857 – February 6, 1923) was an American astronomer. He was commonly known as E. E. Barnard, and was recognized as a gifted observational astronomer. He is best known for his discovery of the high proper mo ...

in 1884 (hence its name), with a six-inch

refractor telescope A refracting telescope (also called a refractor) is a type of optical telescope that uses a lens as its objective to form an image (also referred to a dioptric telescope). The refracting telescope design was originally used in spyglasses and as ...

. It is the closest non-satellite galaxy to the Milky Way, but lies just outside its virial radius. It is similar in structure and composition to the

Small Magellanic Cloud The Small Magellanic Cloud (SMC), or Nubecula Minor, is a dwarf galaxy near the Milky Way. Classified as a dwarf irregular galaxy, the SMC has a D25 isophotal diameter of about , and contains several hundred million stars. It has a total mass of ...

. It is about 7,000 light-years in diameter.

Observational history

NGC 6822 was discovered by

in 1884 using a six-inch

Edwin Hubble Edwin Powell Hubble (November 20, 1889 – September 28, 1953) was an Americans, American astronomer. He played a crucial role in establishing the fields of extragalactic astronomy and observational cosmology. Hubble proved that many objects ...

, in the paper ''N.G.C. 6822, A Remote Stellar System'', identified 15 variable stars (11 of which were

Cepheids A Cepheid variable () is a type of star that Instability strip, pulsates radially, varying in both diameter and temperature and producing changes in brightness with a well-defined stable frequency, period and amplitude. A strong direct period-l ...

) 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 A nebula ('cloud' or 'fog' in Latin; pl. nebulae, nebulæ or nebulas) is a distinct luminescent part of interstellar medium, which can consist of ionized, neutral or molecular hydrogen and also cosmic dust. Nebulae are often star-forming region ...

" (giant

H II region An H II region or HII region is a region of interstellar atomic hydrogen that is ionized. It is typically in a molecular cloud of partially ionized gas in which star formation has recently taken place, with a size ranging from one to hundreds ...

s) that included the

Bubble Nebula NGC 7635, also known as the Bubble Nebula, Sharpless 162, or Caldwell 11, is an H II region emission nebula in the constellation Cassiopeia. It lies close to the open cluster Messier 52. The "bubble" is created by the stellar wind from ...

and the Ring Nebula. He also computed the

absolute magnitude Absolute magnitude () is a measure of the luminosity of a celestial object on an inverse Logarithmic scale, logarithmic Magnitude (astronomy), astronomical magnitude scale. An object's absolute magnitude is defined to be equal to the apparent mag ...

of the entire galaxy. Hubble's detection of eleven Cepheid variable stars was a milestone in

astronomy Astronomy () is a natural science that studies astronomical object, celestial objects and phenomena. It uses mathematics, physics, and chemistry in order to explain their origin and chronology of the Universe, evolution. Objects of interest ...

. Utilizing the Cepheid Period-Luminosity relationship, Hubble determined a distance of . This was the first system beyond the Magellanic Clouds to have its distance determined. (Hubble continued this process with the

Andromeda Galaxy The Andromeda Galaxy (IPA: ), also known as Messier 31, M31, or NGC 224 and originally the Andromeda Nebula, is a barred spiral galaxy with the diameter of about approximately from Earth and the nearest large galaxy to the Milky Way. The gala ...

and the

Triangulum Galaxy The Triangulum Galaxy is a spiral galaxy 2.73 million light-years (ly) from Earth in the constellation Triangulum. It is catalogued as Messier 33 or NGC (''New General Catalogue)'' 598. With the D25 isophotal diameter of , the Triangulum Galax ...

). His distance to the galaxy was way beyond

Harlow Shapley Harlow Shapley (November 2, 1885 – October 20, 1972) was an American scientist, head of the Harvard College Observatory (1921–1952), and political activist during the latter New Deal and Fair Deal. Shapley used Cepheid variable stars to estim ...

's value of 300,000 light-years for the size of the

universe The universe is all of space and time and their contents, including planets, stars, galaxies, and all other forms of matter and energy. The Big Bang theory is the prevailing cosmological description of the development of the universe. Acc ...

. In the paper, Hubble concluded the " Great Debate" of 1920 between

Heber Curtis Heber Doust Curtis (June 27, 1872 – January 9, 1942) was an American astronomer. He participated in 11 expeditions for the study of solar eclipses, and, as an advocate and theorist that additional galaxies existed outside of the Milky Way, wa ...

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 Spiral galaxies form a class of galaxy originally described by Edwin Hubble in his 1936 work ''The Realm of the Nebulae''Susan Kayser in 1966 remained the most complete study of this galaxy until 2002. 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.

Star formation

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. The distribution of hydrogen gas is disk-shaped, but mysteriously, it is angled at about 60° relative to the stellar distribution. Most of its stars formed within the last 3 to 5 billion years. 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. Image:ALMA peers into the hearts of stellar nurseries.jpg, Composite image from data by 2.2-meter MPG/ESO and

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with star-forming regions identified. Image:Glowing Gas Cloud in the Star-Forming Region of Galaxy NGC 6822 (Hubble).jpg, The Hubble-X Star-forming region. Image:Region of NGC 6822.jpg, Closeup of another star-forming region

Notes

Sources

* * * * *

Robert Burnham, Jr Robert Burnham Jr. (June 16, 1931 – March 20, 1993) was an American astronomer, best known for writing the classic three-volume ''Burnham's Celestial Handbook''. He is the discoverer of numerous asteroids including the Mars crossing asteroid ...

, ''Burnham's Celestial Handbook: An observer's guide to the universe beyond the solar system'', vol 3, p. 1559

External links

The Milky Way's Tiny but Tough Galactic Neighbour
—

ESO The European Organisation for Astronomical Research in the Southern Hemisphere, commonly referred to as the European Southern Observatory (ESO), is an intergovernmental research organisation made up of 16 member states for ground-based ast ...

Press Release
NGC 6822: Barnard's Galaxy
*

– NGC 6822 {{NGC70 Barred irregular galaxies Local Group Sagittarius (constellation) 6822 4895 063616 057b 18840817

Observational history

Star formation

See also

Notes

Sources

External links