Eclipsing Binary Star Systems

Eclipsing binaries are binary star systems in which the light varies because the stars pass completely or partially in front of each other on their orbits.

The example show above illustrates schematically the variation in light output expected when a small hot star and cool larger star orbit each other. In this example, the smaller star is hotter and therefore is brighter per unit surface area than the larger star. That is why the dip in light output is larger when the smaller star is eclipsed by the larger star (the primary eclipse) than when the larger star is eclipsed by the smaller (the secondary eclipse).

Light Curves

The variation of the light from a binary star system as a function of time is called the light curve for the system. If the system is an eclipsing binary (or if one or both of the binaries are intrinsically variable stars), the light output from the system varies with time.

The adjacent image shows a light curve measured by the Hipparcos satellite. In different star catalogs this system goes by the designations HIP 53806, HD 95492, or V359 Vel. The spectrum indicates spectral type B9V. The brightness of the star varies between about 7.58 and 7.84 magnitudes, with a period of 4.5350 days. It is classified as an Algol-type eclipsing binary (see the separate discussion of the eclipsing binary Algol elsewhere in this module). The very sharp dips in light output are characteristic of this type of eclipsing variable. Hipparcos found 343 previously unknown eclipsing binaries; this is one of them.

Interpreting Light Curves

It is often possible from the detailed structure of a light curve and general considerations to decide whether the variation is most likely to be because of intrinsic variability (see the later discussion of variable stars in Chapter 21) or because of eclipses. A light curve of the kind displayed above with very sharp dips in light output almost certainly indicates an eclipsing system.