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Norma Arm
The Norma Arm is a minor spiral arm of the Milky Way extending from and around its central hub region. The inner portion of the Arm is called ''Norma Arm'' in narrow meaning. The outer part of it is identified either with Cygnus Arm, which lies outside the Perseus Arm, or Outer Arm, which is located further away from the center of the Galaxy than Cygnus Arm.〔 Norma Arm has a radius of 15.5 ± 2.8 kpc. It is named for the Norma constellation, through which the Arm as seen from Earth passes.
Like many other galaxies of similar type, the Milky Way consists of a large mass of stars shaped into the form of a relatively flat disc by gravity. The disc is rotating, with the dense central body of stars moving at greater speeds than those towards the rim of the disc. As a result, the pattern of stars within the Galaxy as viewed from directly above or below the disc has formed into a spiral.
Because of localised gravitational variations, the spiral pattern has itself formed several distinct 'spiral arms', where particularly large numbers of stars can be found.〔
==References==
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「Norma Arm」の詳細全文を読む
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