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NCG 5204 is a spiral galaxy located about 14.5 million light-years away from Earth〔The distance from Earth is .〕 in the constellation of Ursa Major and is a member of the M101 Group of galaxies. It has a galaxy morphological classification of SA(s)m and is highly irregular, with only the barest indication of any spiral arm structure.〔 The galaxy's most prominent feature is an extremely powerful X-ray source designated NGC 5204 X-1. This has resulted in the galaxy being the target of several studies due to the strength of the source and its relative proximity to Earth. ==Structure== With a diameter of approximately 6,000 parsecs (19,000 light-years) across its long axis, NGC 5204 is intermediate between smaller dwarf galaxies and larger, more prominent spiral galaxies such as Andromeda. While the galaxy's overall organization is irregular, there is a somewhat indistinct spiral arm structure at one end of the main disc. The presence of this feature has resulted in it being classified as type SA(s)m, also known as a Magellanic spiral, after the Large Magellanic Cloud (LMC), the best known example of this rare type of galaxy.〔 Although its diameter is almost 50% larger than the LMC, NGC 5204 has a much more diffuse distribution of stars and its mass of around is only about 10% of the LMC or 0.1% of the Milky Way. Its luminosity is about . Like most irregular galaxies, NGC 5204 is relatively rich in gas and dust, although it lacks any prominent nebulas or broad areas of vigorous star formation. Despite its relatively diffuse distribution of stars, the galaxy does have several clusters of hot, young stars that are thought to be the location of most of the eleven known X-ray sources.〔〔 The galaxy also appears to have a larger than normal dark matter component as the estimated mass of its visible portion does not adequately explain the observed rotation curve of the individual stars, even very close to its center. Although most spiral galaxies exhibit a rotation curve discrepancy, this normally does not become apparent until much farther away from the nucleus.〔 No supernovas have been observed in the galaxy to date, although three supernova remnants have been identified. A 1997 paper estimated that the galaxy probably has about one supernova every 2000 years.〔 NGC 5204 has usually been classified as a member of the M101 Group of galaxies, but it is not known to have any close companions.〔 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 5204」の詳細全文を読む スポンサード リンク
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