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NGC 147 (also known as DDO3 or Caldwell 17) is a dwarf spheroidal galaxy about 2.58 Mly away in the constellation Cassiopeia. NGC 147 is a member of the Local group of galaxies and a satellite galaxy of the Andromeda Galaxy (M31). It forms a physical pair with the nearby galaxy NGC 185,〔(【引用サイトリンク】 work=Results for NGC 147 )〕 another remote satellite of M31. It was discovered by John Herschel in September 1829. Visually it is both fainter and slightly larger than NGC 185 (and therefore has a considerably lower surface brightness). This means that NGC 147 is more difficult to see than NGC 185, which is visible in small telescopes. In the ''Webb Society Deep-Sky Observer's Handbook'', the visual appearance of NGC 147 is described as follows:
The membership of NGC 147 in the Local Group was confirmed by Walter Baade in 1944 when he was able to resolve the galaxy into individual stars with the telescope at Mount Wilson near Los Angeles. ==Characteristics== A survey of the brightest asymptotic giant branch (AGB) stars in the area of radius 2′ from the center of NGC 147 shows that the last significant star-forming activity in NGC 147 occurred around 3 Gyr ago. NGC 147 contains a large population of older stars which show a spread in metallicity and age. The metallicity spread suggests that NGC 147 has had chemical enrichment. However, H I has not been seen and the interstellar medium (ISM) mass upper limit is much lower than expected had the material which is emitted from evolving stars been kept in the galaxy. This implies depletion of the ISM.〔 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 147」の詳細全文を読む スポンサード リンク
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