Filling Roche Lobes
We shall see in our discussion of the death of stars that stars expand late in their lives
when they leave the main
sequence. Thus, in the stellar evolution of a close binary system it is common for one or both stars
to fill their
Roche lobes. This gives the possibility of mass transfer. On the other hand, if the stars
are well separated the expanding stars do not fill their Roche lobes and no mass transfer
is possible.