A multiple star is a group of three or more stars that are so close to eachother that to the unaided eye they appear to be a single star. They are held together my their gravity, two of them forming a binary star, while the third star (or sub-group) orbits around the opponents' center of gravity.

If the luminosity or color of the stars in the group vary significantly, they can appear to us as a variable star. These fluctuations in apparent brightness can be used to estimate the size and luminosity of the individual stars in the system.

Multiple and binary stars are very common. Well known examples include Castor (in the constellation Gemini), Mizar (in Ursa Major), Polaris or the Pole Star (in Ursa Minor), and our nearest neighbor in the Milky Way, Alpha Centauri.

Trying to separate close pairs of stars is a good way to test the optics in your telescope.

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