A galaxy is a massive, gravitationally bound system consisting of stars, stellar remnants, an interstellar medium of gas and dust, and an important but poorly understood component called dark matter. The word galaxy is derived from the Greek galaxias (γαλαξίας), literally "milky", a reference to the Milky Way. Examples of galaxies range from dwarfs with as few as ten million (107) stars to giants with a hundred trillion (1014) 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 (1.7 × 1011) galaxies in the observable Universe. Most are 1,000 to 100,000 parsecs in diameter and 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 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.

Read more about Galaxy:  Etymology, Observation History, Types and Morphology, Formation and Evolution, Larger-scale Structures, Multi-wavelength Observation

Other articles related to "galaxy":

Galaxy - Multi-wavelength Observation
... An ultraviolet flare was observed when a star in a distant galaxy was torn apart from the tidal forces of a black hole ...
NGC 6745
6745 (also known as UGC 11391) is an irregular galaxy about 206 million light-years (63.5 mega-parsecs) away in the constellation Lyra ... After passing through the larger galaxy (NGC 6745A), the smaller one (NGC 6745B) is now moving away ... The larger galaxy was probably a spiral galaxy before the collision, but was damaged and now appears peculiar ...
Galaxy 4 - In Print
... Doctor Who book Galaxy Four Series Target novelisations Release number 104 Writer William Emms Publisher Target Books Cover artist Andrew Skilleter ISBN 0-491-03691-4 Release date November 1985 (hardback ... It was correctly titled Galaxy 4 ...
... Group, an international management consulting firm Astronomy Brightest cluster galaxy, the brightest and most massive galaxies in the universe Blue ...
GRB 970228 - Host Galaxy
... He discovered a faint nebular patch at the burst's position, almost certainly a distant galaxy ... Although there was a remote chance that the burst and this galaxy were unrelated, their positional coincidence provided strong evidence that GRBs occur in distant galaxies rather ... The redshift of the galaxy was later determined to be z = 0.695, which corresponds to a distance of approximately 8.123×109 ly ...

Famous quotes containing the word galaxy:

    for it is not so much to know the self
    as to know it as it is known
    by galaxy and cedar cone,
    as if birth had never found it

    and death could never end it:
    Archie Randolph Ammons (b. 1926)