Eclipsing Binaries

Figure 1.  Eclipsing Binary (detached, Algol type)

Figure 1. Eclipsing Binary (detached, Algol type)

I’m Debra Fischer, a Professor of Astronomy at Yale University. Many of you have already discovered some amazing eclipsing binary light curves, and we wanted to provide you with some information. The Figures here show examples that you have put into collections. Some great additional examples are shown in a paper from the Kepler team (Prsá et al. 2010

The Kepler light curves show how the brightness of the star changes with time.  In Figure 1 (APH10135736 = KID 6449358) above, there are two stars orbiting each other.  Similar to transiting planets, these stars cross in front of each other. The light curve shows the brightness level of the star, plotted vs time in days.  Most of the time, both the larger, hotter star and the smaller cooler star yield a combined brightness measurement for the light curve. When the deep dip in brightness (the primary minimum) occurs it’s because the smaller cooler star is eclipsing the hotter star, which contributes most of the light; when the smaller dip (secondary minimum) occurs, it’s because the larger hotter star is eclipsing the smaller star, which contributes less light to the combined brightness. Stars with flat regions punctuated by relatively sharp dips (e.g. Figure 1) are known as Algol binaries.

A key indicator of eclipsing (or transiting planet) light curves is repeatability.

  1. you can count the number of days between the large dips to determine the orbital period (about 5 days) of this binary star system in Figure 1

  2. you can determine how long it takes the stars to cross by the duration of the transit dip (hours for Figure 1)

  3. you know that one star is larger than the other if the transits don’t have equal dips

Notice that the depth of the brightness dips for an eclipsing binary star can be similar to those for a transiting planet. The transit depth tells us the ratio of the size of the transiting (or eclipsing) object relative to the size of the primary star and the smallest stars have diameters that are similar to Jupiter (stars are gas and the increased gravity from the larger mass star compresses the structure).

Figure 2. Contact eclipsing binary stars

Figure 2. Contact eclipsing binary stars

Sometimes binary stars are so close that the surfaces are distorted into an elliptical shape and the light curve between the eclipses is rounded, as in the left image of Figure 2 (APH10039007 = KID 9357275), where the orbital period is a little more than one day. You can see both the primary and secondary transit dip in this light curve. The most bizarre eclipsing binary light curves are those where the stars are even closer together, called over-contact binaries. An example of this is shown in the right image of Figure 2 (APH10102932 = KID 4633285). These stars can be so close together that they share a common envelope. The eclipse depth is variable, the light curve looks irregular, and there can be mass transfer between the stars.