Transcript: How does mass transfer occur in a close binary pair? The Roche surface or lobes define the region of space where gas is bound to one or both stars. If either star becomes a giant the outer layers swell to fill the lobe, and the teardrop shape becomes the actual surface of the star. In the first stage of binary evolution neither star fills its Roche lobes. In the second stage the more massive star becomes a giant first and fills its Roche lobe. Mass then flows at the point between the lobes like sand through an hourglass onto the lower mass star causing it to gain mass. In the third stage the lower mass star becomes a giant and fills its Roche lobe producing a contact binary. In the process of mass transfer the fundamental course of stellar evolution for each star in a binary is altered with interesting consequences.