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PostSubject: The Galaxies   Sat Sep 01, 2012 3:36 am

A galaxy is a massive, gravitationally bound system that consists of stars, stellar remnants, an interstellar medium of gas and dust, and an important but poorly understood component tentatively dubbed dark matter. The word galaxy is derived from the Greek galaxias (γαλαξίας), literally "milky", a reference to the Milky Way galaxy. Examples of galaxies range from dwarfs with as few as ten million stars to giants with a hundred trillion stars, each orbiting their galaxy's own center of mass.

Galaxies contain varying amounts of star systems, star clusters and types of interstellar clouds. In between these objects is a sparse interstellar medium of gas, dust, and cosmic rays. 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 thought to be the primary driver of active galactic nuclei found at the core of some galaxies. The Milky Way galaxy appears to harbor at least one such object.

Galaxies have been historically categorized according to their apparent shape; usually referred to as their visual morphology. A common form is the elliptical galaxy, which has an ellipse-shaped light profile. Spiral galaxies are disk-shaped with dusty, curving arms. Those with irregular or unusual shapes are known as irregular galaxies and typically originate from disruption by the gravitational pull of neighboring galaxies. Such interactions between nearby galaxies, which may ultimately result in a merging, sometimes induce significantly increased incidents of star formation leading to starburst galaxies. Smaller galaxies lacking a coherent structure are referred to as irregular galaxies.

There are probably more than 170 billion galaxies in the observable universe, and possibly more in a theoretical wider multiverse. Most are 1,000 to 100,000 parsecs in diameter and usually separated by distances on the order of millions of parsecs. 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 known as groups and clusters, which, in turn usually form larger superclusters. At the largest scale, these associations are generally arranged into sheets and filaments, which are surrounded by immense voids.

There are three known galaxies within the Hexisan Union which have been explored. The Hexisan Galaxy, Milky Way Galaxy and Andromeda Galaxy. Of these, the Hexisan Galaxy is the most cultivated, with advanced territories across the entire sphere. The next most advanced galaxy is the Milky Way, with a mapped out system of stars and planets. Finally, the Andromeda Galaxy is the least explored and is said to be a chaotic galaxy filled with nearly 400 billion stars (more than both other galaxies star count put together).

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