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Binary pairs have been observed in protostars that have yet to reach the main sequence, suggesting that binaries form in the early stages of star formation. This could be due to the fragmentation of the molecular cloud as the stars first form, allowing for multiple stars to be created in the same system.<ref>http://www.phys.lsu.edu/astro/nap98/bf.final.html</ref>
Binary pairs have been observed in protostars that have yet to reach the main sequence, suggesting that binaries form in the early stages of star formation. This could be due to the fragmentation of the molecular cloud as the stars first form, allowing for multiple stars to be created in the same system.<ref>http://www.phys.lsu.edu/astro/nap98/bf.final.html</ref>
Because most binary stars are of different masses, one will evolve off the main sequence before its companion. In this scenario, several different events may happen. The two stars may remain '''detached''' if the companion is far enough away in distance, and not very gravitationally massive. The two stars will be ''semi-detached'' if the star that has evolved into the giant star is gravitationally close enough to its companion and exceeds its Roche Lobe, accruing mass from its companion. In this situation, much of the giant's mass will transfer to the companion star, actually making it the more massive of the two, despite still being in the main sequence. [[Algol]] is the prime example of this.
Because most binary stars are of different masses, one will evolve off the main sequence before its companion. In this scenario, several different events may happen. The two stars may remain '''detached''' if the companion is far enough away in distance, and not very gravitationally massive. The two stars will be ''semi-detached'' if the star that has evolved into the giant star is gravitationally close enough to its companion and exceeds its [[Roche lobe]], accruing mass from its companion. In this situation, much of the giant's mass will transfer to the companion star, actually making it the more massive of the two, despite still being in the main sequence. [[Algol]] is the prime example of this.
In some situations, the two stars of the binary system are so close that the expanding giant may actually come in atmospheric contact with its companion. This is known as a '''contact binary'''. In this situation, the very close companion may cause the atmosphere of the giant to literally "splash away" leaving a naked core. Meanwhile the companion, due to friction may spiral towards the core of the once giant. The result will be a merged core that becomes a white dwarf.<ref name="psu">http://www.astro.psu.edu/users/rbc/a1/lec16n.html</ref>
In some situations, the two stars of the binary system are so close that the expanding giant may actually come in atmospheric contact with its companion. This is known as a '''contact binary'''. In this situation, the very close companion may cause the atmosphere of the giant to literally "splash away," leaving a naked core. The companion, meanwhile, may spiral towards the core of the once giant from the friction of their atmospheres. The result will be a merged core that becomes a white dwarf.<ref name="psu">http://www.astro.psu.edu/users/rbc/a1/lec16n.html</ref>
===Type I supernova===
===Type I supernova===
If one of the stars on a binary system is a [[white dwarf]] that is close enough to its companion when the companion star exceeds its Roche lobe, the white dwarf will steadily accrete gases from its companion's atmosphere. This accreted gases will build up and compress on the surface of the white dwarf, due to its high gravity at its surface. The gas will then steadily heat up as more material accumulates. The result is that [[hydrogen]] fusion may occur on the surface of the dwarf and through the tremendous release in energy, throw the rest of the collected gas off in a brilliant flash. This extremely bright event is called a [[nova]]. <ref>D. Prialnik, Novae, Encyclopaedia of Astronomy and Astrophysics (2001, p.1846-1856)</ref>
If one of the stars on a binary system is a [[white dwarf]] that is close enough to its companion when the companion star exceeds its Roche lobe, the white dwarf will steadily accrete gases from its companion's atmosphere. This accreted gases will build up and compress on the surface of the white dwarf, due to its high gravity at its surface. The gas will then steadily heat up as more material accumulates. The result is that [[hydrogen]] fusion may occur on the surface of the dwarf and, through the tremendous release in energy, throw the rest of the collected gas off in a brilliant flash. This extremely bright event is called a [[nova]]. <ref>D. Prialnik, Novae, Encyclopaedia of Astronomy and Astrophysics (2001, p.1846-1856)</ref>
The above event will occur as long as the additional mass the white dwarf accreted from its companion doesn't cause the star to exceed the [[Chandrasekhar limit]], which is 1.4 solar masses. If the material buildup on the white dwarf exceeds this limit, the electron degeneracy of the white dwarf itself will no longer be able to hold the star up against gravity. In this scenario a type I [[supernova]] will occur that destroys the star. The most famous example of this is the supernova [[SN 1572]], considered one of the foremost events in astronomy. [[Tycho Brahe]] observed this event extensively.<ref name="psu">http://www.astro.psu.edu/users/rbc/a1/lec16n.html</ref>
The above event will occur as long as the additional mass the white dwarf accreted from its companion doesn't cause the star to exceed the [[Chandrasekhar limit]], which is 1.4 solar masses. If the material buildup on the white dwarf exceeds this limit, the electron degeneracy of the white dwarf itself will no longer be able to hold the star up against gravity. In this scenario a type I [[supernova]] will occur that destroys the star. The most famous example of this is the supernova [[SN 1572]], considered one of the foremost events in astronomy. [[Tycho Brahe]] observed this event extensively.<ref name="psu">http://www.astro.psu.edu/users/rbc/a1/lec16n.html</ref>