With the advent of relatively inexpensive CCD cameras, amateur astronomers with modest-sized telescopes are in an excellent position to contribute valuable scientific data to the astronomical community. One type of object that can be very interesting and useful to observe is the eclipsing binary. And there are a lot of them.
Due to a sometimes fortuitous alignment of the orbital plane of a binary star along or near our line of sight, one or both stars pass directly in front of the other periodically, and this type of object is known as an eclipsing binary.
The brightest eclipsing binary in our sky is Algol (Beta (β) Persei). Known to vary in brightness since antiquity, astute ancient Arab astronomers gave Beta Persei the name “al Ghul” which, loosely translated, means “the Demon Star”. Today, we know that Algol’s brightness variations are caused by a hot blue B8V star (Algol A) going behind and in front of its cooler and less massive but larger K0IV companion (Algol B). Since the two stars orbit each other once every 2.867328 days (they are very close, separated by just a little over 5½ million miles), every 2 days, 20 hours, 48 minutes, and 57 seconds Algol B passes in front of much-brighter Algol A for a few hours, and the single point of light we see from Earth dims by 1.3 magnitudes. This is the primary eclipse. A secondary eclipse also occurs half a period before or after each primary eclipse. When Algol A passes in front of Algol B, the brightness of the point of light we see drops by only 0.05 magnitude. This shallow secondary minimum occurs because Algol B is not nearly as bright as Algol A.
Eclipsing binaries like Algol (which are close enough to each other to form an interacting pair) are interesting subjects for amateur astronomers to monitor. Periods can change, phases can shift, and unexpected events can occur, such as when Dr. Jim Pierce (now Emeritus Professor of Astronomy at Minnesota State University in Mankato) and I were the first to observe ultraviolet flare events from the eclipsing binary V471 Tau at Iowa State University’s Erwin W. Fick Observatory in 1978.
So, how do you know when eclipses will occur, how deep they will be, and how long to monitor the star before, during, and after the event? A great starting point is the Eclipsing Binary Ephemeris Generator by Shawn Dvorak which shows you a number of stars that will be in eclipse and observable from your location on any given night. The Timing Database at Krakow (TIDAK), maintained by Jerzy M. Kreiner at the Mt. Suhora Astronomical Observatory in Poland, is another great source of eclipsing binary information.
A schedule, if you will, of eclipsing binary primary eclipses (like other astronomical events) is called an ephemeris. Eclipsing binary ephemerides look like this one for Algol:
HJD = 2452500.21 + E × 2.867315
Here, HJD is the heliocentric Julian date of minimum light. Julian date is a continuous count of days and fractions thereof elapsed since an arbitrary starting date of noon Universal Time (UT) on January 1, 4713 B.C. The heliocentric Julian date removes the orbital motion of the Earth from the ephemeris calculations, centering the times of events on the Sun rather than the Earth. An event could be observed to occur as much as 8.3 minutes earlier or later than calculated depending on where the Earth is in her orbit relative to the star. The first number in the equation above, in this case 2452500.21, refers to the heliocentric Julian date of some arbitrary starting minimum. The E stands for epoch, simply a consecutive integer count of successive minima, and the second number, in this case 2.867315, refers to the orbital period of the eclipsing binary in days. The Kreiner website takes the chore out of choosing the appropriate value of E for the time you want to observe by calculating the HJDs (and corresponding Earth-based UT dates and times) of the eclipsing binary you choose over the next several days.
You should monitor a star before, during, and after the eclipse, so having a rough of idea of what object you should observe and when does not require you convert heliocentric Julian date to the Julian date at the telescope. However, any event times from data you record at the telescope must be converted to HJD for it to be useful. There is an online tool to do this for you. Of course, you not only need to know the UT date and time of an event, but also the equatorial coordinates (right ascension and declination) of the object you were observing to calculate the heliocentric Julian date.
We’re not even going to get into barycentric Julian date (BJD), or the fact that the distance between the Sun (or the barycenter of the solar system) and the eclipsing binary of interest is growing (radial velocity > 0) or shrinking (radial velocity < 0), and that this means that the period we measure is not exactly the same as the true orbital period of the system. But it is very close.