Roche lobes An imaginary surface around a star. Each star in a binary system can be pictured as being surrounded by a tear-shaped zone of gravitational influence, the Roche lobe.
The point at which the Roche lobes of the two stars touch is called the inner Lagrangian or L1 point.
Since the outer gas envelopes of the stars are in contact (overflowing their Roche lobes), they essentially share a common photosphere despite having two distinct nuclear-burning cores. Indeed, Stars B and C are separated by only some 0.
A contact binary is a type of binary star in which both components of the binary fill their Roche lobes. The uppermost part of the stellar atmospheres forms a common envelope that surrounds both stars.
contact binary A binary star system in which both stars have expanded to fill their Roche lobes and the surfaces of the two stars merge. The binary system now consists of two nuclear burning stellar cores surrounded by a continuous common envelope.
The Roche lobes of each component, which in isolation would be spheroidal, are drawn out into cone-like extensions meeting at a point between the two stars known as the Lagrangian point, ...
Common Envelope - A stage in the evolution of a close pair of stars in which matter shed by one of the stars fills the region just outside the Roche lobes of the two stars ...
The first equipotential surface for two massive bodies describing circular orbits around one another which forms a figure eight enclosing the two objects. The Roche lobes are the two lenticular volumes enclosing the two bodies. [H76]
Rods ...
As the red giant expands, the material in its outer layers don't expand out in any direction due to the proximity of the nearby white dwarf, so the material is funneled toward it. This is due to the constraints of the Roche Lobes.
If one of the stars is big enough to transfer mass through the Lagrange point then the system is a semidetached binary. If both stars fill their Roche lobes then the system is known as a contact binary.
 See also: Lobes, Roche lobe, Mass, Dwarf, Solar
  
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