Date: May 26, 2012

Title: Encore: Eclipsing Binaries

Podcaster: AAVSO, with Mike Simonsen, Virginia Renehan and Rebecca Turner.

This podcast originally aired on August 7, 2009

Organization: American Association of Variable Star Observers (AAVSO)

Description: Today we will discuss eclipsing binaries, variable stars that are actually two stars in orbit around each other, where one star periodically eclipses the light from its companion. Astronomers are very interested in eclipsing binaries because they help us measure stellar sizes, mass, composition, motion and evolution.

Bio: The AAVSO is an international non-profit organization whose mission is: to observe and analyze variable stars; to collect and archive observations for worldwide access; to forge strong collaborations between amateur and professional astronomers; and to promote scientific research and education using variable star data.

Today’s Sponsor: “This episode of 365 days of Astronomy” is sponsored by – Expanding your horizons in astronomy today. The premier on-demand telescope network, at dark sky sites in Spain, New Mexico and Siding Spring, Australia.


Mike: Hi again, and welcome to the Restless Universe, the podcast of the American Association of Variable Star Observers. You can find us on the web at

I’m Mike Simonsen, and with me today are Virginia Renehan and Rebecca Turner.

Today we are going to talk about eclipsing binary stars. Virginia, why don’t you tell us what makes eclipsing variables change brightness.

Virginia: An eclipsing binary is a pair of stars in which the orbit plane of the two stars is nearly in line with the line of sight of the observer. As the stars orbit each other the components undergo mutual eclipses. So, eclipsing binaries are variable stars, not because the light of the individual components varies, but because of these eclipses.

The light curve of an eclipsing binary is typically characterized by periods of constant light, with periodic drops in intensity.

Rebecca: If one of the stars is larger than the other, one will be obscured by a total eclipse while the other will be obscured by a partial or annular eclipse. We call these the primary and secondary eclipses, respectively.

The period of the orbit of an eclipsing binary can be determined from a study of the light curve. The relative sizes of the individual stars can be determined by observing how quickly the brightness changes as the disc of the near star slides over the disc of the distant star.

Mike: So, astronomers are very interested in eclipsing binaries because they help establish important parameters and limits to stellar sizes, composition, motion and evolution. For example, if an eclipse lasts for thirty minutes total, and the period of the system is six hours, the size of the eclipsing star is one-twelfth the size of its orbit.

Virginia: Yea, but before astronomers can start modeling and doing calculations, someone has to collect the data. Why don’t we start there and then move forward.

Rebecca: Sounds like a good idea. So where can we find information or lists of stars that are eclipsing binaries?

Virginia: Fortunately, eclipsing binaries are not as rare as you might think. A quick search of the International Variable Star Index (VSX) shows over 39,000 known eclipsing variable stars. Other sources of information are the General Catalog of Variable Stars and, of course, from the AAVSO Eclipsing Binary Section’s list of program stars and ephemeris

Mike: Wait, a quick explanation of ephemeris is in order here too I think. An astronomical ephemeris provides data on phenomena of interest to astronomers like eclipses, planetary motions, the phases of the Moon. In the case of eclipsing binaries, an ephemeris is used to predict when the eclipses will happen. What eclipsing binary folks really want to know is the time of minimum (TOM). The exact point in time that is the middle of the eclipse and the faintest part of the light curve.

Rebecca: One of the keys to successful observation of eclipsing binaries is planning. First, you must determine what stars he should attempt to observe. Your choice of stars should certainly be within the limitations of your instrument and at a declination easily reached from your latitude.

The duration of the eclipse may also be a limiting factor. You want to be able to observe the whole eclipse. You don’t want the star to set behind a tree half way through the eclipse!

Information, including maximum and minimum brightness, period, eclipse duration, sky position and other related elements can be found in the publications Virginia mentioned a minute ago.

Virginia: When one of these stars enters its eclipse phase it won’t wait for you to figure out any details you may have failed to attend to earlier. You need to be ready, have your telescope pointed at the right field of stars and have your comparison star charts handy before the eclipse begins. It’s a very good idea to locate the star field a few nights in advance of a scheduled minimum.

All observations should be timed to the nearest minute. So, an accurate time source is essential.

Mike: Although most observations of fainter eclipsers are done with a CCD these days, if you are observing visually, the hardest thing to do is to determine when the eclipse has actually commenced. You look at the star and think, “nope, looks just like it did a minute ago”, and then “nope, no change”, and then the next time you look you can tell it’s fainter and you get all excited and you know you are about to witness an eclipse in real time with your very own eyes.

Never let your expectations influence the outcome of the observation in progress. Each estimate must be independent of all expectations. Record what your eye sees — not what you think the eye should be seeing!

Virginia: One very famous and bright eclipsing binary is Algol, the Demon Star, in the constellation Perseus. Algol fades and re-brightens, like clockwork, every 2.87 days. You can find out which nights and at what times Algol will be in eclipse in Sky and Telescope magazine each month. If you’re lucky, you can see an entire eclipse over the course of an evening. The star fades noticeably from magnitude 2.1 to 3.4 over the course of several hours. Under dark skies you can follow the whole eclipse with the naked eye. In the city, you may need binoculars to witness the whole event, but it’s an easy target.

Rebecca: Epsilon Aurigae is another famous eclipsing binary, but it only has eclipses once every 27 years! By the time this episode airs in August of 2009, the next eclipse of epsilon Aurigae should just be starting. Algol just happens to be one of the stars in the Ten Star Training Tutorial the AAVSO has prepared, to teach people how to make observations of epsilon Aurigae.

Mike: I feel a plug coming here.

Rebecca: Well it does fit within the subject of this podcast quite conveniently. Epsilon Aurigae is a mysterious eclipsing binary that has baffled astronomers for 175 years. As part of the International Year of Astronomy, the AAVSO is carrying out a project called Citizen Sky, whose aim is to study this star and publish the results. This study of epsilon Aurigae will be the largest citizen science project in history, and anyone can participate. If you want to learn more you can find it on the web at

Virginia: One of the interesting things about eps Aur is the theory that the secondary is a dusty disc of material surrounding a pair of stars or something else we can’t imagine. Even stranger, the duration of the eclipse has decreased, but the time of minimum has lengthened over the last five eclipses. Things are changing before our very eyes on human timescales, which is unusual in astronomy.

Mike: Right. Most changes in stars occur on timescales of millions or billions of years. Any change in period or profile of the eclipses in EBs is one of the things astronomers are particularly keen to learn about, because changes in period or amplitude may be the result of stellar evolution right before our very eyes.

Rebecca: When we talk about stellar evolution we are usually referring to the life cycle of a single star. Many factors influence the evolution of a star, but by far the most important of these is the initial mass of the star. Stellar evolution gets even more complicated when a star has a close companion. In some cases mass is transferred from one star to the other, and this changes the evolutionary path both stars follow.

Virginia: One way to classify binary stars is by their distances to each other, relative to their sizes. Detached binaries are binary stars where each component is within its Roche lobe. The stars have no major effect on each other, and essentially evolve separately. Most binaries belong to this class.

Mike: The Roche lobe is the region of space around a star in a binary system in which orbiting material is gravitationally bound to that star. If the star expands past its Roche lobe, then the material outside of the lobe will fall onto its companion. It is an approximately tear-drop shaped region, with the apex of the tear-drop pointing towards the other star.

Virginia: Semidetached binary stars are binary stars where one of the components fills its Roche lobe and the other does not. Gas from the surface of the donor star is transferred to the other, accreting star. This mass transfer dominates the evolution of the system. In many cases, the inflowing gas forms an accretion disc around the accretor.

Rebecca: A contact binary is a type of binary star in which both components of the binary have filled their Roche lobes. The stars orbit around each other inside a common envelope made up of the outer layers of the stellar atmospheres.

Mike: When a binary system contains a compact object such as a white dwarf or neutron star, gas accreted onto the compact object releases gravitational potential energy, causing the gas to become hotter and emit radiation. Cataclysmic variables are examples of these kinds of systems. They also happen to be the topic of our next episode. So tune in next month, when Virginia, Rebecca and I will be discussing white dwarfs, accretions disks and some of the wildest stars in our Restless Universe. Good-bye.

End of podcast:

365 Days of Astronomy
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