The del­i­cate bal­ance of grav­ity is key to un­der­stand­ing bi­nary stars

Sky at Night Magazine - - LUNAR IMAGING -

The Roche lobe is the teardrop-shaped re­gion around a star in a bi­nary sys­tem, within which gas is grav­i­ta­tion­ally bound to the star. But at the point be­tween the two stars where the lobes meet – the in­ner La­grangian point (L1) – grav­ity and cen­trifu­gal forces can­cel out and the gas feels no net force. That is un­less one of the stars has ex­panded to the point that it fills its Roche lobe. This hap­pens in cat­a­clysmic vari­able and many X-ray bi­nar­ies. In th­ese cases, the at­mos­phere of the lobe-fill­ing star is pushed be­yond the L1 point and flows to­wards and around the other star. As the es­cap­ing gas stream en­cir­cles the star it even­tu­ally loops around and col­lides with it­self. This causes it to lose en­ergy, spread out and form an ac­cre­tion disc around the com­pan­ion star.

Bi­nary com­pan­ion In­ner La­grangian point (L1) Star Roche lobe Ac­cre­tion disc Gas stream from gi­ant star es­cap­ing through L1 point Gi­ant star which has ex­panded to fill its Roche lobe

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