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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 .
This core convection occurs in stars where the CNO cycle contributes more than 20% of the total energy. As the star ages and the core temperature increases, the region occupied by the convection zone slowly shrinks from 20% of the mass down to the inner 8% of the mass. [25] The Sun produces on the order of 1% of its energy from the CNO cycle.
The most common kinds of binary system are binary stars and binary asteroids, but brown dwarfs, planets, neutron stars, black holes and galaxies can also form binaries. A multiple system is similar but consists of three or more objects, for example trinary stars and trinary asteroids .
Nemesis is a hypothetical red dwarf [1] or brown dwarf, [2] originally postulated in 1984 [3] to be orbiting the Sun at a distance of about 95,000 AU (1.5 light-years), [2] somewhat beyond the Oort cloud, to explain a perceived cycle of mass extinctions in the geological record, which seem to occur more often at intervals of 26 million years.
If the Solar System were such a binary system, the outer limits of the resulting circumstellar habitable zone could extend as far as 2.4 AU. [80] [81] With regard to spectral types, Zoltán Balog proposes that O-type stars cannot form planets due to the photoevaporation caused by their strong ultraviolet emissions. [82]
Representative lifetimes of stars as a function of their masses The change in size with time of a Sun-like star Artist's depiction of the life cycle of a Sun-like star, starting as a main-sequence star at lower left then expanding through the subgiant and giant phases, until its outer envelope is expelled to form a planetary nebula at upper right Chart of stellar evolution
Binary star system detected near Sagittarius A*. This image indicates the location of the newly discovered binary star D9, which is orbiting Sagittarius A*, the supermassive black hole at the ...
Star formation theory, as well as accounting for the formation of a single star, must also account for the statistics of binary stars and the initial mass function. Most stars do not form in isolation but as part of a group of stars referred as star clusters or stellar associations. [2]