Star

A star is an massive astronomical object consisting of a luminous sphere of plasma held together by its own gravity. The nearest star to Earth is the Sun at approximate 151,000,000 kilometres, and the second is Proxima Centauri located 4.24 light-years away from Sun. Many other stars are visible to the naked eye at night, but due to their immense distance from Earth they appear as fixed points of light in the sky, furthermore Sirius A is the brightest star in the night sky with a apparent magnitude of -1.46. The most prominent stars are grouped 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, but most are invisible to the naked eye from Earth, including all individual stars outside our galaxy, the Milky Way.

Stars can form orbital systems with other astronomical objects, as in the case of planetary systems and star systems with two or more stars. When two such stars have a relatively close orbit, their gravitational interaction can have a significant impact on their evolution. Stars can form part of a much larger gravitationally bound structure, such as a star cluster or a galaxy.

Binary Stars
Binary star systems consist of two or more stars that are gravitationally bound, and generally move around each other in stable orbits. When two such stars have a relatively close orbit, their gravitational interaction can have a significant impact on their evolution. For example, a nova occurs when a white dwarf accretes matter from a companion star.