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Cassiopeia A (Cas A) is a supernova remnant (SNR) in the constellation Cassiopeia and the brightest extrasolar radio source in the sky at frequencies above 1 GHz. The supernova occurred approximately away within the Milky Way.〔 The expanding cloud of material left over from the supernova now appears approximately across from Earth's perspective. In wavelengths of visible light, it has been seen with amateur telescopes down to 234mm (9.25 in) with filters.〔Howard Banich. ''A Visual Guide to the Cassiopeia A Supernova Remnant.'' Sky & Telescope, December 2014.〕 It is believed that first light from the stellar explosion reached Earth approximately 300 years ago but there are no historical records of any sightings of the supernova that created the remnant, probably due to interstellar dust absorbing optical wavelength radiation before it reached Earth (although it is possible that it was recorded as a sixth magnitude star 3 Cassiopeiae by John Flamsteed on August 16, 1680). Possible explanations lean toward the idea that the source star was unusually massive and had previously ejected much of its outer layers. These outer layers would have cloaked the star and re-absorbed much of the light released as the inner star collapsed. Cas A was among the first discrete astronomical radio sources found. Its discovery was reported in 1948 by Martin Ryle and Francis Graham-Smith, astronomers at Cambridge, based on observations with the Long Michelson Interferometer. The optical component was first identified in 1950. In 2013, it was reported that phosphorus had been detected in Cassiopeia A, which confirmed that this element is produced in supernovae, with the Phosphorus-Iron ratio up to 100 times higher in material from the supernova remnant than in the Milky Way in general. Cas A is 3C461 in the Third Cambridge Catalogue of Radio Sources and G111.7-2.1 in the Green Catalog of Supernova Remnants. == Earlier discovery== Calculations working back from the currently observed expansion point to an explosion that would have become visible on Earth around 1667. Astronomer William Ashworth and others have suggested that the Astronomer Royal John Flamsteed may have inadvertently observed the supernova on August 16, 1680, when he catalogued a star near its position. Another suggestion from recent cross-disciplinary research is that the supernova was the mythical "noon day star", observed in 1630, that heralded the birth of Charles II, the future monarch of Great Britain. At any rate, no supernova occuring within the Milky Way has been visible to the naked eye from Earth since. 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「Cassiopeia A」の詳細全文を読む スポンサード リンク
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