MARK GARLICK / SCIENCE PHOTO LIBRARY MARK GARLICK / SCIENCE PHOTO LIBRARY
Orbits of binary stars. Diagram showing the mutual orbits (yellow and blue) of two stars making up a binary star system. The stars orbit a point known as the barycentre (green dot) 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. The orbital paths sweep out equal areas in the same time, as shown by the three successive points in the orbits (lower left and upper right) and the radial lines showing the areas. The stars are shown at the apex of their orbits (apastron), when they are moving at their slowest. At the other extreme, half an orbit later at the periastron, they move at their fastest.
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