ECLIPSING VARIABLE STAR

pair of stars revolving about their common centre of mass in an orbit whose plane passes through or very near the Earth. An observer on the Earth thus sees one member of the binary pass periodically over the face of the other and diminish its light through an eclipse. The star Algol was the first recognized as an eclipsing binary, by John Goodricke, in 1782. Several thousand are now known. By comparing the observed duration of the eclipse to the period of the orbit, as determined spectroscopically, astronomers can find the diameters of the stars relative to the size of their orbits, and from this they can calculate the densities of the stars. See also variable star; binary star.