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==Etymology==
The [[word]] '''galaxy''' derives from the [[Greek]] term for our own galaxy, galaxias (γαλαξίας), or kyklos galaktikos, meaning "milky [[circle]]" for its appearance in the sky. In Greek [[mythology]], Zeus places his son born by a [[mortal]] woman, the infant Heracles, on Hera's breast while she is asleep so that the baby will drink her divine milk and will thus become [[immortal]]. Hera wakes up while breastfeeding and then realizes she is nursing an unknown baby: she pushes the baby away and a jet of her milk sprays the night sky, producing the faint band of light known as the [[Milky Way]].
In the astronomical [[literature]], the capitalized word 'Galaxy' is used to refer to our galaxy, the [[Milky Way]], to distinguish it from the billions of other galaxies. The term Milky Way first appeared in the [[English]] [[language]] in a [[poem]] by [http://en.wikipedia.org/wiki/Geoffrey_Chaucer Chaucer].
:: "See yonder, lo, the Galaxyë
:: Which men clepeth the Milky Wey,
::For hit is whyt."
:::—Geoffrey Chaucer. The House of Fame, c. 1380.
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]
When [http://en.wikipedia.org/wiki/William_Herschel William Herschel] constructed his catalog of deep sky objects, he used the name [http://en.wikipedia.org/wiki/Spiral_nebula spiral nebula] for certain objects such as M31. These would later be recognized as immense conglomerations of [[stars]], when the true distance to these objects began to be appreciated, and they would be termed island universes. However, the [[word]] [[Universe]] was understood to mean the entirety of [[existence]], so this expression fell into disuse and the objects instead became known as galaxies.
==Description==
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], 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==
