Changes

[[File:lighterstill.jpg]][[File:Milky_way_galaxy.jpg|right|frame]]

[[File:lighterstill.jpg]][[File:Milky_way_galaxy.jpg|right|frame]]

A '''galaxy''' is a massive, [[gravity|gravitational]]ly bound [[system]] that consists of stars and stellar remnants, an interstellar medium of gas and dust, and an important but poorly understood component tentatively dubbed [http://en.wikipedia.org/wiki/Dark_matter dark matter].[1][2] The name is from the Greek root galaxias [γαλαξίας], meaning "milky," a reference to the [http://en.wikipedia.org/wiki/Milky_Way Milky Way] galaxy. Typical galaxies range from dwarfs with as few as ten million[3] (107) stars up to giants with one trillion[4] (10¹²) stars, all orbiting the galaxy's center of [[mass]]. Galaxies can also contain many multiple star systems, star clusters, and various interstellar clouds. The [[Sun]] is one of the stars in the Milky Way galaxy; the Solar System includes the Earth and all the other objects that orbit the Sun.

A '''galaxy''' is a massive, [[gravity|gravitational]]ly bound [[system]] that consists of stars and stellar remnants, an interstellar medium of gas and dust, and an important but poorly understood component tentatively dubbed [http://en.wikipedia.org/wiki/Dark_matter dark matter]. The name is from the Greek root galaxias [γαλαξίας], meaning "milky," a reference to the [http://en.wikipedia.org/wiki/Milky_Way Milky Way] galaxy. Typical galaxies range from dwarfs with as few as ten million (107) stars up to giants with one trillion (10¹²) stars, all orbiting the galaxy's center of [[mass]]. Galaxies can also contain many multiple star systems, star clusters, and various interstellar clouds. The [[Sun]] is one of the stars in the Milky Way galaxy; the Solar System includes the Earth and all the other objects that orbit the Sun.

Historically, galaxies have been categorized according to their apparent shape (usually referred to as their visual [[morphology]]). A common form is the [http://en.wikipedia.org/wiki/Elliptical_galaxy elliptical galaxy],[5] which has an ellipse-shaped light profile. [http://en.wikipedia.org/wiki/Spiral_galaxy Spiral galaxies] are disk-shaped assemblages with dusty, curving arms. Galaxies with irregular or unusual shapes are known as [http://en.wikipedia.org/wiki/Peculiar_galaxy peculiar galaxies], and typically result from disruption by the gravitational pull of neighboring galaxies. Such interactions between nearby galaxies, which may ultimately result in galaxies merging, may induce episodes of significantly increased star formation, producing what is called a [http://en.wikipedia.org/wiki/Starburst_galaxy starburst galaxy]. Small galaxies that lack a coherent [[structure]] could also be referred to as [http://en.wikipedia.org/wiki/Irregular_galaxy irregular galaxies].[6]

Historically, galaxies have been categorized according to their apparent shape (usually referred to as their visual [[morphology]]). A common form is the [http://en.wikipedia.org/wiki/Elliptical_galaxy elliptical galaxy], which has an ellipse-shaped light profile. [http://en.wikipedia.org/wiki/Spiral_galaxy Spiral galaxies] are disk-shaped assemblages with dusty, curving arms. Galaxies with irregular or unusual shapes are known as [http://en.wikipedia.org/wiki/Peculiar_galaxy peculiar galaxies], and typically result from disruption by the gravitational pull of neighboring galaxies. Such interactions between nearby galaxies, which may ultimately result in galaxies merging, may induce episodes of significantly increased star formation, producing what is called a [http://en.wikipedia.org/wiki/Starburst_galaxy starburst galaxy]. Small galaxies that lack a coherent [[structure]] could also be referred to as [http://en.wikipedia.org/wiki/Irregular_galaxy irregular galaxies].

 

There are probably more than 100 billion (10¹¹) galaxies in the [http://en.wikipedia.org/wiki/Observable_universe observable universe].[7] Most galaxies are 1,000 to 100,000[4] parsecs in diameter and are usually separated by distances on the order of millions of parsecs (or megaparsecs).  Intergalactic space (the space between galaxies) is filled with a tenuous gas of an average density less than one atom per cubic meter. The majority of galaxies are organized into a hierarchy of associations called clusters, which, in turn, can form larger groups called superclusters. These larger structures are generally arranged into sheets and filaments, which surround immense voids in the universe.

Although it is not yet well understood, dark matter appears to account for around 90% of the mass of most galaxies. Observational data suggests that supermassive black holes may exist at the center of many, if not all, galaxies. They are proposed to be the primary cause of active galactic nuclei found at the core of some galaxies. The Milky Way galaxy appears to harbor at least one such object within its nucleus.[10]

Although it is not yet well understood, dark matter appears to account for around 90% of the mass of most galaxies. Observational data suggests that supermassive black holes may exist at the center of many, if not all, galaxies. They are proposed to be the primary cause of active galactic nuclei found at the core of some galaxies. The Milky Way galaxy appears to harbor at least one such object within its nucleus.[10]

==References==

==References==