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 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 (optics), lens as its objective (optics), objective to form an image (also referred to a dioptrics, dioptric telescope). The refracting telescope d ...
NGC 6822 was discovered by E. E. Barnard in 1884 using a six-inch
refractor telescope
A refracting telescope (also called a refractor) is a type of optical telescope that uses a lens (optics), lens as its objective (optics), objective to form an image (also referred to a dioptrics, dioptric telescope). The refracting telescope d ...
.
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
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 ...
absolute magnitude
Absolute magnitude () is a measure of the luminosity of a celestial object on an inverse logarithmic astronomical magnitude scale. An object's absolute magnitude is defined to be equal to the apparent magnitude that the object would have if it ...
of the entire galaxy.
Hubble's detection of eleven Cepheid variable stars was a milestone in
astronomy
Astronomy () is a natural science that studies celestial objects and phenomena. It uses mathematics, physics, and chemistry in order to explain their origin and evolution. Objects of interest include planets, moons, stars, nebulae, g ...
. 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 gal ...
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 Ga ...
). 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. ...
. 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
ALMA
Alma or ALMA may refer to:
Arts and entertainment
* ''Alma'' (film), a 2009 Spanish short animated film
* ''Alma'' (Oswald de Andrade novel), 1922
* ''Alma'' (Le Clézio novel), 2017
* ''Alma'' (play), a 1996 drama by Joshua Sobol about Alma ...
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
See also
*
Bubble Nebula (NGC 6822)
The Bubble Nebula in Barnard's Galaxy has the official designation of Hubble 1925 I as it was the first (Roman numeral 1) object recorded in a paper by . It includes areas of bright H II region, H II emission. It is located north-west of th ...
*
Ring Nebula (NGC 6822)
The Ring Nebula in Barnard's Galaxy has the official designation of Hubble 1925 III as it was the third (Roman numeral 3) object recorded in Edwin Hubble, Hubble's 1925 paper, ''N.G.C. 6822, A Remote Stellar System''. It includes areas of brigh ...
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
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 ...