That gravity is caused by the mass of a companion star. So, although the star cannot be seen its presence can be inferred by the visible star's motion. These are known as astrometric binaries . If the motion caused by the companion star is too small to be detected by astrometry, spectroscopy can reveal its presence. The spectra of stars sometimes reveal the presence of two stars, either by containing incompatible spectral lines or displaying movement of the lines caused by the Doppler effect as the stars orbit one another.
Some binary stars appear to be variable stars.
These are the eclipsing binaries and have an orbital plane which is inclined to the Earth so that the stars pass in front and behind of one another, causing eclipses which dim the light output. Another, more violent, variable, known as a nova, is also thought to be caused within binary star systems in which one member is a white dwarf. See also supernova . Binding energy . The energy equivalent of the mass defect of an atomic nucleus.