Why a star that looks single to the naked eye, when observed through a telescope, can split into two closely spaced stars, i.e., turn out to be a binary star system?
The resolution of the human eye is approximately 1 – 2 The resolution of a telescope is proportional to the diameter of the objective, and the diameter of the telescope objective is much larger than the diameter of the pupil. Therefore, the angular resolution of the telescope is much higher than the resolution of the human eye, which, with the correct selection of the eyepiece, makes it possible to distinguish (or, as they say, resolve) binary stars.
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