http://www.ualberta.ca/~pogosyan/teaching/ASTRO_122/lect13/lecture13.htmlhttp://abyss.uoregon.edu/~js/ast122/lectures/lec10.html
http://csep10.phys.utk.edu/astr162/lect/binaries/eclipsing.html
http://csep10.phys.utk.edu/astr162/lect/binaries/accreting.html
http://csep10.phys.utk.edu/astr162/lect/binaries/visual.html
http://www.sciencedaily.com/releases/2012/10/121015150031.htm

Binary Stars

• Most stars are not isolated in space.
• About one half of all stars which we see are in a double or multiple star system.
• In a multiple star system, the stars all orbit about the common centre-of-mass of the system.
• This is not accidental! The process of star formation tends to cause many stars to be formed close to each other.

Red Explosions: Secret Life of Binary Stars Is Revealed

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Contact Binary Star Envelopes

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A close binary system in which a relatively massive donor star transfers material faster than it can be digested by a less massive accreting star. As a result, the transferred material eventually overflows the latter's Roche lobe, forming a common envelope around both stars. Frictional drag between the common envelope and the stars causes the two stars to spiral inward and the common envelope to be ejected. In this way it is possible for the components of a binary to reduce their separation from many AU (corresponding to an orbital period of tens of years) to a few stellar radii (corresponding to an orbital period of hours). One possible example of a star in the process of ejecting its common envelope is Eta Carinae.