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NGC 6818 (also known as Little Gem Nebula) is a planetary nebula located in the constellation of Sagittarius. It was discovered by William Herschel in 1787. NGC 6818 is located in the constellation of Sagittarius (The Archer), roughly 6000 light-years away from Earth. The glow of the cloud is just over half a light-year across. When stars like the Sun are near end of life, they send their outer layers into space to create glowing clouds of gas, a planetary nebulae. This ejection of mass is uneven, and planetary nebulae can have complex shapes. NGC 6818 shows knotty filament-like structures and distinct layers of material, with a bright and enclosed central bubble surrounded by a larger, more diffuse cloud. Scientists believe that the stellar wind from the central star propels the outflowing material, forming the elongated shape of NGC 6818. As this stellar wind moves through the slower-moving cloud it creates particularly bright spots in the bubble’s outer layers. ==Gallery== Little gem.jpg|Stellar wind from the central star propels the outflowing material.〔(【引用サイトリンク】url=http://www.spacetelescope.org/images/potw1531a/ )〕 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「NGC 6818」の詳細全文を読む スポンサード リンク
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