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A galaxy is a gravitationally bound system of stars, stellar remnants, interstellar gas, dust, and dark matter.〔〔 The word galaxy is derived from the Greek ' (), literally "milky", a reference to the Milky Way. Galaxies range in size from dwarfs with just a few thousand (103) stars to giants with one hundred trillion (1014) stars,〔 each orbiting their galaxy's own center of mass. Galaxies are categorized according to their visual morphology, including elliptical,〔 spiral, and irregular.〔 Many galaxies are thought to have black holes at their active centers. The Milky Way's central black hole, known as Sagittarius A *, has a mass four million times greater than our own Sun.〔 As of July 2015, EGSY8p7 is the oldest and most distant galaxy with a light travel distance of 13.2 billion light-years from Earth, and observed as it existed 570 million years after the Big Bang. Previously, as of May 2015, EGS-zs8-1 was the most distant known galaxy, estimated to have a light travel distance of 13.1 billion light-years away and to have 15% of the mass of the Milky Way.〔 〕 Approximately 170 billion () to 200 billion () galaxies exist in the observable universe.〔 Most of the galaxies are 1,000 to 100,000 parsecs in diameter and usually separated by distances on the order of millions of parsecs (or megaparsecs). The space between galaxies is filled with a tenuous gas with an average density less than one atom per cubic meter. The majority of galaxies are gravitationally organized into associations known as galaxy groups, clusters, and superclusters. At the largest scale, these associations are generally arranged into sheets and filaments that are surrounded by immense voids.〔 ==Etymology== The word ''galaxy'' derives from the Greek term for our own galaxy, ' (', "milky one"), or ' ("milky circle")〔 due to its appearance as a "milky" band of light 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 often used to refer to our galaxy, the Milky Way, to distinguish it from the other galaxies in our universe. The English term ''Milky Way'' can be traced back to a story by Chaucer : When William Herschel assembled his catalog of deep sky objects in 1786, he used the term ''spiral nebula'' for certain objects such as M31. These would later be recognized as conglomerations of stars when the true distance to these objects began to be appreciated, and they would later 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.〔 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「galaxy」の詳細全文を読む スポンサード リンク
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