Consider two stars orbiting around each other in space Of course the binary system itself orbits around the centre of our Galaxy, and so in general the paths along which the two stars move are inter¬twining spirals threading their way among the other stars. For simplicity, suppose that we are travelling through space with the same average speed as the two stars. From our new vantage point, the stars will appear to move continually around each other, with both stars remaining all the time in the same plane. We call this plane the ORBITAL PLANE. This plane in fact moves slowly through space, although for most binary systems it appears to move so slowly (because the stars are so far away) that it may be thought of as fixed. Within the orbital plane there is a point that corresponds to the centre of mass of the binary system. Remember that we are imagining that we are travelling the same course as the binary system. Therefore, from our viewpoint, the centre of mass of the binary system travels an exactly parallel path to our own, and is therefore a fixed point on the orbital plane. The two stars orbit about this point. In general their paths are ellipses with the centre of mass as one focus. At every instant in time, the two stars and the centre of mass all lie in a straight line. The centre of mass is the balance point. If the distances from the centre of mass to the two stars are denoted by RI and r2, and the stars’ masses are M, and M2, then M1r1=M2r2 Therefore the two stars trace out on the orbital plane two similar, but differently sized ellipses, with the lighter star tracing the larger

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