In astronomy, a green star is a white or blueish star that appears greenish in some viewing conditions (see § Psychology below). Under typical viewing conditions, there are no greenish stars, because the color of a star is more or less given by a black-body spectrum. However, there are a few stars that appear greenish to some observers, due to the viewing conditions, for example the optical 'illusion' that a red object can make nearby objects look greenish (and vice versa). Some multiple star systems, such as Antares, have a bright reddish or yellowish star where this contrast makes other stars in the system seem greenish. Psychology of color perception Color is not a fundamental property, it is in the brain of the observer. As such, no star really has any colour at all. Stars emit energy at many different wavelengths, and humans may perceive color in stars. Instead of talking about the 'true' color of stars, we must talk about how a particular object appears to a particular observer, in a particular context. This issue is not mere semantics: Green can be perceived when there is no 'green' light at all - for example due to the simultaneous colour contrast alluded to above, in that a star can look greenish when it is next to a reddish star (likewise, the reddish star will appear more red when next to a non-reddish star). Green percepts can also be created by blocking red light, and by adapting the 'red' photoreceptors (see complementary colors). This is not an 'illusion', this is simply how humans perceive colour. The perception of colour in stars is complicated by the fact that the cone photoreceptors, from which colour percepts are typically generated, are not active during the dim lighting conditions required to see most stars without magnification - that is, in scotopic vision. Further, stars are effectively single points of light at vast distances from the observer's eye; this light is filtered through the atmosphere, through pollution, through the eye and the brain. It is only because starlight is split, scattered, and/or because the observer's eye is always moving, that the light from a star can stimulate multiple photoreceptors in the retina, and therefore generate percepts of colour, relative to the background illumination. Why stars do not look green A star is usually close to being a black body, give or take a few spectral lines,