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.

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



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28 responses to “Eclipsing Binaries”

  1. Jaromir says :

    Finally, someone explained how eclipsing binarys look on graphs.

  2. Joe Riley says :

    Professor Fischer,

    I just started “classifying” with the zooniverse web site. I am NOT in school nor have I had any formal schooling in the area of Astronomy.

    As I am enjoying the process of classification I am never sure that I am classifying what I am seeing… “correctly”. Is there a form of feedback from Zooniverse that I could be expecting?


    • debrafischer says :

      Hi Joe! Thanks very much for working on the light curve classification. There are a lot of curves that are difficult to classify. We’re monitoring progress of the crowd (but not individuals) to see how we can help with more examples. This is a different kind of research project and like all research, we don’t have the answers – you’re all figuring this out!

      • Phoenix12 says :

        Eventually we should be able to demonstrate each transit and relation to its star in evolutionary terms according to the laws of physics. So we should be able to build up a graph data base of all the transits according to whether they are giant, dwarf, gaseous or solid etc. and their proximity, mass, energy transfer if relevant, temperature etc. The professional astronomers will be able to quantify the relevant criteria and parametersinvolved and also be able to back up this with spectroscope analysis emanating from the stars. We already know conditions and proximity criteria relevant for existence of life forms. But we will also be able to demonstrate the range of star forms, transits, and other criteria in the same way we are now able to present a graphic presentation of the spectrum

  3. Carl Tanner says :

    Hi Debra,

    I imagine we will find quite a few binary systems in the Kepler data, but that won’t preclude the existence of planets in those systems, as we already have quite a few examples of such systems. Even with contact binaries, we may find planets in orbit about these, but it would be interesting to do the calculations for the stability of such systems. I would also do follow up RV surveys on the Kepler stars, as those that have no transiting planets may still have planetary systems present, but not detectable via the transit method. Something, maybe, for Darwin or GAIA, when they’re launched in the next few years. It may also pay to recheck some existing planetary systems for transits…the anomalies in the light curves of some stars (especially M class) maybe planets orbiting in line of sight and outside the invariant plane of the systems present at these stars, although the usual suspects (starspots etc) cannot be ruled out.

    • debrafischer says :

      Hi Carl ! Great comments! Orbits around binaries can definitely be stable. However, they are more difficult to follow up with Doppler observations. Complications include the large radial velocities (RV’s) of the binary stars and the fact that the planets must be further away to be in stable orbits – this distance decreases the amplitude of the Doppler signal. Matt Giguere (on the science team) earned his MS degree by carrying out a search for Rossiter McLaughlin velocities (serendipitously observed during transit) in all of the stars on our Doppler planet search projects. Finally, including Kepler stars on RV programs would be great fun – unfortunately, most of those stars are faint (compared to nearby stars) and that makes this tough. But, you’re right – there are loads of planets in the field that don’t transit. Based on geometrical arguments and assumptions about the architectures, fewer than 1% will transit (more than 99% won’t transit!).

  4. mutabilitie says :

    Sorry if this is a stupid question, but so far I haven’t seen it answered elsewhere: what is the best way of classifying eclipsing binaries of the ‘shutter effect’-type? So far I’ve been classifying the ones I’ve come across as ‘variable’ and ‘pulsating’, because that struck me as the closest match, but I’m not entirely sure whether that’s right, and obviously I don’t want to generate lots of faulty data which other people will have to correct…

  5. Carl Tanner says :

    Hi Debra. That’s true….the problems of geometric a priori chances for many transiting systems are rather small due to the larger separations of the planets around those that are binary systems. However, they can be just as small around single stars as well, due to those same factors. Even at relatively short distances from their stars (especially M class stars, being small targets to begin with), your chances for finding a transit drop off rather quickly, even if the orbits might be generally favorable!!. I think it will be a lot easier to do RV surveys of those dim stars once GAIA and such are launched, since they will have the capability to do precise RV observations of such stars.

  6. Carl Tanner says :


    Eclipsing binaries will have light curves displaying both a primary (large) and secondary (small) drop in the light curve coming from the system that will occur at regularly intervals, due to the nature of the orbits of the two stars about their mutual centre of gravity. Stars that are variables (which includes your pulsators) can be distinguished from eclipsing binaries by looking closely at the light curves and their shapes. For example, Cepheids, whilst being pretty regular pulsating stars, show a characteristic shape in their light curves which can be easily separated (on the whole) from the shape of the light curves coming from an eclipsing binary. Cepheids have a “sawtooth” like curve with a sharpish rise and then gradual fall over a regular period, the length of which depends on the type of Cepheid and the star in question.

    If you look at other types of variability, then it depends on the type of variability that’s occurring as to what you may classify any stars as. Given the relatively short time spans we are dealing with, most of the variability will be various forms of rapid pulsations (i.e. Delta Scuti type variables, etc), starpots, UV Ceti type flares etc etc. Any giants will most likely be semi-regular or irregular variables and because of their usually rather long pulsation and variability periods you’re not going to be able to see an entire light curve showing clear signs of either, unless you find some of the short term oscillations of their fundamental (basal) periods. Also, most variables will show changes in the spectrum coming from the stars during their variability/pulsations, due to actual physical changes occurring to the stars whilst this is happening. This is not the case with eclipsing binaries, where for the most part the light is coming from stable stars. However, there are exceptions to the usual case, but that’s just something you have to expect as an astronomer :).

  7. mutabilitie says :

    @Carl Tanner
    Thanks for the detailed explanation. Some of it went a little above my head, though, I’m afraid – please bear with me; I’m only a literature postgrad with a passing interest in sciency stuff, so this is all slightly outside my comfort zone…;)
    So if I understand you correctly, if the curve shows a repeating pattern and looks kind of like a loose mesh, I should put the star down as ‘regular’ rather than ‘pulsating’? Or have I managed to miss the point completely?

  8. Carl Tanner says :


    Anytime :). If you were having any troubles with the explanations, just let me know and I’ll help where I can 🙂

    In any case, if you want a good book to read about the subject…one for the average, non science reader, try to get a copy of this book…”David Levy’s Guide to Variable Stars”. It will go a long way to help you out with understanding what variable stars are and how to observe them 🙂

  9. Taryn East says :

    Excellent post. Now I understand. This should probably be added to the tutorial so people can tell the difference between planetary transits and eclipsing binary transits. 🙂

  10. AJ says :

    Do we still mark the transits for binaries? Or click “no” ?

  11. Jonty says :

    Hey folks.

    Regarding classification of eclipsing binaries I’m wondering about how to highlight the transits. One I am currently looking at has 38 transit features and as such is extremely awkward since if I mark them all I can’t click on the finish icon. Should I just be marking one of each repeated transit or is there something else to do which i’m missing?

    Thanks in advance, Jonty

  12. Bogdan says :

    If it’s a binary doesn’t that mean that there are two stars? So both can be seen and studied right? So is there any way to see and confirm the existence of the companion star?

  13. Bogdan says :

    So is there any way to see and confirm the existence of the companion star by telescope?

  14. Chris says :

    Jonty : There’s a known bug that it’s hard to mark more than 18 – we didn’t really anticipate this. If you mark more than 10, the system takes note.

    Bogdan : Depends on how bright it is, and how far away from the primary it is – I’ll try and write more about this in the near future.

  15. mikeformumdad says :

    Hi Debra

    I am new to this, having followed the stargazing program on the BBC, my only nailed on cert, thus far is APH43020557 will an a creditation be given


    • Meg says :

      Hi Mike,
      We save all classifications from logged in users with their user ids, so if you mark a transit we will certainly give you credit for the discovery on the candidates page and if we write a paper on that specific system, we will give you an authorship on the paper if you are the first to identify the transit, or acknowledge you in the paper if you are not the first but one of the other classifiers that helped identify the system.

  16. uckle says :

    Dear all,
    Having read some of the above, and as a beginner myself,I recognise there seems to be a rather large gap between what you as experts understand and take for granted and what we as beginners know.
    I think you may be having difficulties in answering what we need in simple enough terms.
    It is so easy to slip into science-speak and not put into everyday language what the questioner needs to know, at least at the beginning.i.e. something like ‘regular v-shapes= …… ,rope-like structures= ….fuzzy edges== …may be pulsators etc etc
    I would be willing as a complete non-expert, but someone who understands a BIT about it to try to compile a COMPLETE IDIOT’S guide to the patterns.
    Otherwise I PLEAD with you to talk in terms of patterns at first and advanced astronomy later.Thanks

  17. Meg says :

    Hi Uckle,
    Have you checked out our site guide – that’s geared more towards beginners –

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    comments are added- checkbox and from now on whenever a comment is added I recieve 4 emails with the exact
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