Among the numerous binary star systems, the tight binary system designates the pair of stars that are located very close to each other. Stars may directly interact with one another in a close binary system.
It is the degree of interaction between the two components is the criterion of "tightness" of a binary stellar system, rather than the distance between them. For example, two red dwarfs with 0.2 solar masses and rotating at a distance of 1 astronomical unit from each other do not constitute a tight system, while two extremely massive stars at the same orbit are a tight binary system (normally the distance between components of a tight binary system is around 1010 m, i.e., dozens of the Sun' radii). Two such dwarfs will exist independently from each other, while the supergiants will actively interact with each other. Thus, the degree of interactions between stars is what counts in the formation of a tight binary system.
Gravitational forces in a system of closely located binary stars strive to stretch each of the stars. If gravity is reasonably strong, a critical moment occurs when the substance from one star starts flowing towards the other star. Two pear-shaped figures around stars whose surfaces present a critical border are known as Rochet cavities. When the more massive star spends nearly all of its hydrogen, it will start swelling and filling its Rochet cavity. The gas will be partially entrapped by another star, and partly dispersed around it, forming a blanket. The star known to astronomy as β Lyra represents such a system. Two stars of this binary system are stretched towards each other by mutual gravity.
Algol stage occurs upon completion of the stage described above, when the star that was initially heavier star becomes much thinner. Algol is a binary system where an orange star of a smaller mass rotates around a massive blue star, but the smaller star passes the evolutionary path quicker that the larger star, and it starts growing in size.
Finally, a compact object is formed within the system: a white dwarf, a neutron star, or a black hole. Accretion of the second star's substance to the surface of the compact object occurs. This results in powerful X-Ray radiation that can be recorded from spacecraft (the Earth's atmosphere does not transmit X-Ray spectrum.) Note that all neutron stars discovered by X-Ray observations were at some point components of tight binary systems.
Material from bulging stars strives towards the dwarf, and increases its surface temperature. Thermonuclear explosion of colossal power takes place in the thin surface layer of gas as a result of this phenomenon. We observe the discharge of accumulated blanket gases (weighing about one hundredth of the Sun's mass) as the birth of a new star. Despite the enormous energy discharge accompanying the explosion, the scattering blanket does not exert any specific influence on neighboring star. Outbreaks sometimes repeat after dozens or hundreds of years. Other explosions occur only once. Observations reveal that about one hundred new stars are annually born in our Galaxy.
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