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A '''galaxy''' is a massive, [[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].[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.

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],[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]