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.
A star is a luminous spheroid of plasma held together by self-gravity. The nearest star to Earth is the Sun. Many other stars are visible to the naked eye at night; their immense distances from Earth make them appear as fixed points of light. The most prominent stars have been categorised into constellations and asterisms, and many of the brightest stars have proper names. Astronomers have assembled star catalogues that identify the known stars and provide standardized stellar designations. The observable universe contains an estimated 1022 to 1024 stars. Only about 4,000 of these stars are visible to the naked eye—all within the Milky Way galaxy.
Image of the Sun, a G-type main-sequence star, the closest to Earth
A star-forming region in the Large Magellanic Cloud
Stars in the night sky
Infrared image from NASA's Spitzer Space Telescope showing hundreds of thousands of stars in the Milky Way galaxy
