Astronomy in-class activity: mass transfer in close-pair binaries

Astronomy 10 In-class activity 23 v.07.04.30, Spring Semester 2008
Cuesta College, San Luis Obispo, CA

Astronomy 10 learning goal Q10.2

Students find their assigned groups of three to four students, and work cooperatively on an in-class activity worksheet to summarize the three stages of initial mass transfer in a close-pair (mass transferring) binary star system.Star B is more massive than Star A, due to its being closer to the center of mass, and also because of its larger Roche lobes. As a result, Star B will end its main sequence lifetime earlier than Star A, and become a giant or supergiant.

As a result of Star B expanding and filling its Roche lobe up to the "neck" or "pinch point," hydrogen will then be transferred to Star A. This makes their masses become more equal, making their separation distance decrease, while increasing their orbital speeds. As a result, centrifugal forces increase, such that the size of Star B's lobe shrinks (while it is expanding in its giant/supergiant phase), making the "spillage" from Star B to Star A rapid.

Eventually the masses of Star B and Star A equalize; this is when their separation distance is the smallest, and their lobes are equal in size, but the physical size of Star B (still) fills its (smaller) Roche lobe.

As Star B still transfers hydrogen to Star A, this makes their masses become unequal, making their separation distance increase, while decreasing their orbital speeds. As a result, centrifugal forces decrease, such that the size of Star B's lobe expands. This makes it harder for Star B to "spill over" hydrogen out of its Roche lobe, and the transfer of material slows, and eventually stops when Star B is unable to exceed its much larger Roche lobe.

(Transfer from Star A, when it eventually ends its main sequence lifetime, to Star B will eventually occur, but this discussion is outside the scope of this course.)

Thus with all observations of close-pair binary star systems where mass transfer is taking place, it is more likely to see a less-massive star feeding a more-massive star than vice versa.

Follow-up post: