Observing Project 12C Minima of Algol and Eclipsing Binaries

Algol is the classic example of an eclipsing binary. Its secondary component orbits the primary each 2.867 days. Each time the secondary passes through inferior conjunction with its primary, it creates a deep partial eclipse causing the entire system to fade in total magnitude by about 1.4 magnitudes, or nearly 70%. This is an event that can be observed easily with the unaided eye. Each month the minima times for Algol, or time of maximum eclipse are published by Sky & Telescope and other publications such as the Observer's Handbook. When at its normal brightness,Algol is as bright as is magnitude +2.1 Almach (Gamma Andromedae) to its west, but at its faintest, it fades dimmer than nearby Epsilon Persei (+2.9) on its east.

In addition to the more famous cycle, when the companion is on the far side of Algol, it is partially eclipsed in its own right. Though the primary is much brighter, the eclipse of the secondary also produces a small light drop of about 0.2 magnitudes. You can document the rise and fall of Algol with a 35-mm camera mounted on top of your clock-driven telescope. The times of minimum brightness are easy enough to find. Make a five to ten minute exposure of the area around Algol at minimum and then make an identical exposure the next night. The difference in brightness between the two exposures will startle you when you develop the film. You can also detect the secondary eclipse event on film about 35 hours after the primary eclipse event.

There are other fine examples of eclipsing binaries in the sky that you can observe as well. Beta Lyrae is another such example of two stars that eclipse each other on a regular basis, but have a different type of light curve. Algol's primary star is reasonably spherical, but the two stars of Beta Lyrae are so close together that gravity distorts both components into ellipsoids. As a result of the shape and distance between the two stars, there is not a constant peak magnitude with Beta Lyrae, but rather a continually changing brightness throughout the 12.94-day period. Photograph it the same way as you did with Algol and be sure to follow it every day you can to document its cycle. During the fortnight of Beta Lyrae's cycle, note that there are two distinctly separate minima spaced about 6.48 days apart. Beta Lyrae cycles from magnitude +3.3 down to +4.4, back up to +3.3 then down to about +3.8 before recovering back to +3.3 to end the cycle.

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