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A galaxy is a massive, gravitationally 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 dark matter. [1][2] 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 (10 7 ) stars [3] to giants with a hundred trillion (10 14 ) stars, [4] 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 . [5] Galaxies have been historically categorized according to their apparent shape; usually referred to as their visual morphology. A common form is the elliptical galaxy, [6] 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 . [7] There are probably more than 170 billion (1.7 × 10 11 ) galaxies in the observable universe , [8][9] and possibly more in a theoretical wider multiverse . [10] Most are 1,000 to 100,000 [11] parsecs in diameter and usually separated by distances on the order of millions of parsecs (or megaparsecs) . [12] 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. [13] The word galaxy derives from the Greek term for our own galaxy, galaxias (γαλαξίας , "milky one"), or kyklos ("circle") galaktikos ("milky" ) [14] 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. [15][16] 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 English term Milky Way can be traced back to a story by Chaucer:
