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A light echo is a phenomenon observed in astronomy. Analogous to an echo of sound, a light echo is produced when a sudden flash or burst of light, such as that observed in novae, is reflected off a source and arrives at the viewer after a considerably longer duration than it otherwise would have taken with a direct path. Because of their geometries, light echoes can produce the illusion of superluminal speeds. ==Examples== The variable star V838 Monocerotis experienced a significant outburst in 2002 as observed by the Hubble Space Telescope. The outburst proved surprising to observers when the object appeared to expand at a rate far exceeding the speed of light as it grew from an apparent visual size of 4 to 7 light years in a matter of months.〔〔(【引用サイトリンク】title=Hubble watches light echo from mysterious erupting star )〕 Light echoes were used to determine the distance to the Cepheid variable RS Puppis to within 1% of its true value. Pierre Kavella at the European Southern Observatory described this measurement as so far "the most accurate distance to a Cepheid". Using light echoes, it is sometimes possible to see the faint reflections of historical supernovae. Astronomers calculate the ellipsoid which has the Earth and a supernova remnant at its focal points in order to locate clouds of dust and gas at its boundary. By analyzing the spectra of reflected light from these nebulae, astronomers can discern chemical signatures of supernovae whose light reached Earth long before the invention of the telescope and compare the explosion with its remnants, which may be centuries or millennia old. One example is the SN 1572 supernova observed on Earth in 1572, where in 2008, faint light-echoes were seen on dust in the northern part of the Milky Way. Light echoes can be identified by comparing photos of gas and dust clouds taken months or years apart and spotting changes in the light rippling across the clouds. If the source of the light is unknown, several such observations can be fitted to an ellipsoid to allow astronomers to pinpoint the origin. Light echoes have been used to study the supernova that produced the supernova remnant Cassiopeia A.〔 The light from Cassiopeia A would have been visible on Earth around 1660, but went unnoticed, probably because dust obscured the direct view. Reflections from different directions allow astronomers to determine if a supernova was asymmetrical and shone more brightly in some directions than in others. The progenitor of Cassiopeia A has been suspected as being asymmetric, and looking at the light echoes of Cassiopeia A allowed for the first detection of supernova asymmetry in 2010. Light echoes have also been observed in connection with supernovae SN 1993J and SN 1987A, the closest supernova in modern times. The first recorded instance of a light echo was 1936,〔 but it was not studied in detail. 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「Light echo」の詳細全文を読む スポンサード リンク
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