English: Stages in the life of a binary system as a common envelope is formed. The system has mass ratio M1/M2=3. The black line represents the inner critical Roche equipotential. The dashed line represents the rotation axis.
(a) Both stars lie within their Roche lobes, star 1 on the left (mass M1 in red) and star 2 on the right (mass M2 in orange).
(b) Star 1 has grown to nearly fill its Roche lobe.
(c) Star 1 has grown to overfill its Roche lobe and transfer mass to star 2: Roche lobe overflow.
(d) Transferred too fast to be accreted, matter has built up around star 2.
(e) A common envelope, represented schematically by an ellipse, has formed.
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