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The Wonders of Galaxies
   
Abell 1689: A Galaxy Cluster Makes Its Mark (A galaxy cluster at a distance of about 2.3 billion light years from Earth.) by Smithsonian Institution on Flickr

A galaxy is a massive, gravitationally bound system that consists of stars and stellar remnants, an interstellar medium of gas dust, and an important but poorly understood component tentatively dubbed dark matter.
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The Antennae: A pair of colliding galaxies about 60 million light years from Earth. by Smithsonian Institution on Flickr

The name in greek means milky, a reference to Milky Way. 
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The Triangulum by NASA Goddard Photo and Video on Flickr

The galaxy typical range from dwarf  has about ten-million to a hundred-trillion stars in it.
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time tripping by Adam_T4 on Flickr

Galaxies may contain :

-star systems

-star clusters

-interstellar clouds


The sun is one of the stars in the galaxy.

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Hubble image of Arp 194 by thebadastronomer on Flickr

Galaxies have been organized by their apparent shape. A common form is the "eliptical galaxy". Small galaxies that lack a coherent structure could also be referred to as irregular galaxies.

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Arp 87 galaxy interaction from NASA by Derek K. Miller on Flickr

It is known that there is at least 170 billion
galaxies in the observable universe.
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#30. astrodeep200407aecc by rmforall@gmail.com on Flickr

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.
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Black hole wind by thebadastronomer on Flickr

The Greek philosopher Democritus proposed that the bright band on the night sky known as the Milky Way might consist of distant stars. 
Aristotle, however, believed the Milky Way to be caused by "the ignition of the fiery exhalation of some stars which were large, numerous and close together" and that the "ignition takes place in the upper part of the atmosphere, in the region of the world which is continuous with the heavenly motions.
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galaxies and universes by Torley on Flickr

Galaxies come in three main types:

-ellipticals

-spirals

-irregulars

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587741534400086104_w by gwydionwilliams on Flickr

A majority of spiral galaxies have a linear, bar-shaped band of stars that extends outward to either side of the core, then merges into the spiral arm structure.
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587739720308818090_L by gwydionwilliams on Flickr

Despite the prominence of large elliptical and spiral galaxies, most galaxies in the universe appear to be dwarf galaxies.
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587735665305780388 by gwydionwilliams on Flickr

The average separation between galaxies within a cluster is a little over an order of magnitude larger than their diameter.
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587733412056858653 by gwydionwilliams on Flickr

Stars are created within galaxies from a reserve of cold gas that forms into giant molecular clouds. Some galaxies have been observed to form stars at an exceptional rate, known as a starburst.
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Galaxies by quinn.anya on Flickr

A portion of the galaxies we can observe are classified as active.
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