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In astronomy, a green star is a white or blue star that appears green due to an optical illusion. There are no truly green stars, because the color of a star is more or less given by a black-body spectrum and this never looks green. However there are a few stars that appear green to some observers. This is usually because of the optical illusion that a red object can make nearby objects look greenish. There are some multiple star systems, such as Antares, with a bright red star where this illusion makes other stars in the system look green. ==Why stars are not green== A star is usually close to being a black body, give or take a few spectral lines, so its color is usually more or less the color of a black body. The color of a blackbody lies on the Planckian locus in the middle of the diagram on the right. As can be seen, this locus happens to pass through red, orange, yellow, white, and light blue areas, and one can indeed see many stars of these colors. On the other hand it does not pass through green, indigo (dark blue) or violet areas, so stars that appear to have these colors are rare and depend on some additional optical effect. The (blackbody) colors of stars are sometimes confused with the colors of the spectrum, as in the textbook mentioned by Feynman in the quote above. The spectral (rainbow) colors are those on the curved part of the boundary of the diagram on the right. As can be seen, the red, orange, yellow and blue rainbow colors happen to be much the same as blackbody colors. However, stars whose peak emission is green light also emit much red and blue light, and the human visual system happens to interpret this mixture of colors as whitish rather than green. So the fact that some spectral colors appear as star colors is more a quirk of human color vision than a property of stars: if one uses an instrument such as a spectroscope that is better at distinguishing wavelengths of light, then all spectral colors look completely different from star colors. All sufficiently hot stars look about the same shade of blue (and not violet as claimed in some popular accounts). The reason for this is that at sufficiently large temperatures (above about 20000 degrees) all blackbody spectra look about the same in visible light, though they can differ lot at shorter wavelengths. Although their maximum output at visible wavelengths is at violet, they put out enough light at other wavelengths to look light blue: the color at the end of the Planckian locus rather than the color at the end of the spectrum. Human color vision is in fact more complicated than suggested by the explanation above, and in particular the perceived color of an object depends not only on the light it emits, but also on the colors of nearby objects. For example, a blue object close to a red object may appear somewhat greenish; this effect accounts for many apparently green stars. Strictly speaking, green stars are very unlikely rather than completely impossible. The spectrum of a star is not quite a black body spectrum but is altered by absorption and emission lines of the elements and compounds in it. It is conceivable that among the 1022 stars in the visible universe there are a few with such unusual structures and chemical compositions that their color is visibly altered from a black-body color, but no examples of this are known. 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「Green star (astronomy)」の詳細全文を読む スポンサード リンク
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