If in a binary star system the orbits are oriented so that one star can pass in front of the other as we view the system, we say that the binary is an eclipsing binary. In some cases, this can lead to significant variations in the light output of the system.

Orbital Orientation in Binary Systems

As we have noted, binary star systems can have different orientations of their orbital planes with respect to our line of sight, as illustrated in the following figure.

Tilt of the orbit of a binary system.

If the angle i is sufficiently close to 90 degrees, the two stars can eclipse each other as they revolve around their common center of mass.

Light Curve for Eclipsing Binaries

In that case, we expect the variation in the light curve illustrated in the following figure.

Schematic of eclipsing binary system

Twice in each orbit, we expect dips in the light output (notice that this is true independent of whether we can distinuish the two stars visually from each other). The duration, detailed shape, and depth of these dips will depend on the details of the orbital inclination to our line of sight, and to the sizes and detailed features of the two stars.

In the preceding illustration the smaller blue star is assumed to emit more light per unit surface area than the larger red star. Thus, the light output is dimmed most when the blue star is eclipsed (this is called the primary eclipse) and is dimmed less when the red star is eclipsed (this is called the secondary eclipse).

Light Curve of Eclipsing Binary Discovered by Hipparcos

The following image shows a light curve measured by the Hipparchos satellite. HIP 53806 (HD 95492, V359 Vel).

This star is of spectral type B9V. It 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. (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.

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