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A multiple star system consists of two or more stars that appear from Earth to be close to one another in the sky. [dubious – discuss] This may result from the stars actually being physically close and gravitationally bound to each other, in which case it is a physical multiple star, or this closeness may be merely apparent, in which case it is an optical multiple star [a] Physical multiple ...
This category is for physical multiple stars, i.e., small groups of three or more close stars gravitationally bound to each other. Optical companions (stars that appear to be close together but are physically unrelated) do not count. Star systems placed here should be categorized by their number of components.
A binary star or binary star system is a system of two stars that are gravitationally bound to and in orbit around each other. Binary stars in the night sky that are seen as a single object to the naked eye are often resolved as separate stars using a telescope , in which case they are called visual binaries .
A multi-star system consists of two or more gravitationally bound stars that orbit each other. The simplest and most common multi-star system is a binary star, but systems of three or more stars exist. For reasons of orbital stability, such multi-star systems are often organized into hierarchical sets of binary stars. [116]
Triple stars should be placed under the subcategory Triple stars. If a group of four or more stars are physically close and gravitationally bound, it is a physical multiple star and should be placed under the subcategory Multiple star systems. Multiple stars should be categorized by their number of components.
This category contains articles about star systems, which are groups of two or more gravitationally-bound stars in a region usually smaller than one light-year. Star clusters (groups of many more stars that occupy a much larger region) are listed in Category:Star clusters.
The stars with the most confirmed planets are the Sun (the Solar System's star) and Kepler-90, with 8 confirmed planets each, followed by TRAPPIST-1 with 7 planets. The 1,033 multiplanetary systems are listed below according to the star's distance from Earth. Proxima Centauri, the closest star to the Solar System, has three planets (b, c and d).
Most stars are currently classified under the Morgan–Keenan (MK) system using the letters O, B, A, F, G, K, and M, a sequence from the hottest (O type) to the coolest (M type). Each letter class is then subdivided using a numeric digit with 0 being hottest and 9 being coolest (e.g., A8, A9, F0, and F1 form a sequence from hotter to cooler).