MARK GARLICK / SCIENCE PHOTO LIBRARY MARK GARLICK / SCIENCE PHOTO LIBRARY
Orbits of binary stars. Sequence of diagrams of the position of the stars in a binary star system as they orbit each other. The stars orbit a point known as the barycentre (green cross) or centre of mass. The more massive star (yellow) is closer to the barycentre than the less massive star (blue). Both stars follow elliptical orbits that share a focus (similar to the centre of a circle) at the barycentre. If the orbits are strongly elliptical, as shown here, then the distance between the stars will vary from one extreme (furthest apart, top) to the other (closest, bottom).
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