Astronomical unit
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The astronomical unit is a unit of length defined to be exactly equal to 149,597,870,700 m. Historically, the astronomical unit was conceived as the average Earth-Sun distance, before its modern redefinition in 2012.
Transits of Venus across the face of the Sun were, for a long time, the best method of measuring the astronomical unit, despite the difficulties (here, the so-called "black drop effect") and the rarity of observations.
Star system
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A star system or stellar system is a small number of stars that orbit each other, bound by gravitational attraction. A large group of stars bound by gravitation is generally called a star cluster or galaxy, although, broadly speaking, they are also star systems. Star systems are not to be confused with planetary systems, which include planets and similar bodies.
The Algol three-star system imaged in the near-infrared by the CHARA interferometer with 0.5 mas resolution in 2009. The shape of Algol C is an artifact.[citation needed]
Artist's impression of the orbits of HD 188753, a triple star system.
Star system named DI Cha. While only two stars are apparent, it is actually a quadruple system containing two sets of binary stars.
Sirius A (center), with its white dwarf companion, Sirius B (lower left) taken by the Hubble Space Telescope.