What visual threshold, usually around magnitude +6.5, determines if a star is registered by the human eye under ideal conditions?
Answer
Apparent magnitude.
The ability of the human eye to perceive a star is determined by its apparent magnitude, which is a measure of how bright the star appears from Earth, taking into account both its intrinsic brightness and its distance. Under absolutely perfect viewing conditions—meaning far from city lights and with exceptionally clear air—the practical limit for the faintness of a star that the eye can resolve is generally set around an apparent magnitude of +6.5. Stars fainter than this threshold, despite being physically present, emit light that is too diffuse or low in intensity to trigger a conscious visual signal.
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