Roche Lobes
We have noted that there is a unique gravitational potential
energy contour in a binary system that intersects itself in one point called the
inner Lagrange point. The interior of this contour defines two regions, one around each star,
called Roche lobes. The adjacent right figure illustrates.
Mass Accretion
Therefore, our previous discussion of gravitational potentials may be summarized by saying that
mass accretion can occur if
one of the stars expands to fill its Roche lobe, allowing matter to
spill over the inner Lagrange point onto the other star
as shown in this
animation.
(The accretion in this case involves an accretion disk around the second star.
We shall discuss accretion disks shortly.)
Roche Lobes for Algol
The adjacent figure illustrates the Roche lobes for the Algol binary system.
Notice that the K2 subgiant star on the left has expanded to fill
its Roche lobe. As a result, the star is distorted. On the other hand, the B8 main sequence star on
the right is still well within its Roche lobe. Thus, the system is unstable against accretion of
matter from the K2 star onto the B8 star through the inner Lagrange point. There is spectroscopic
evidence for a thin accretion stream flowing through the inner Lagrange point and into the body of the
B8 star. This is illustrated by the dotted red line.