Kidspace--Graphing the Eclipse of the "Demon" Star

by Hugh Ross

In the previous issue of Facts & Faith I introduced you to variable stars, stars that change in brightness. I invited you to observe and graph the brightness changes of a famous variable star, Delta Cephei. A $25 RTB gift certificate still awaits the first fifteen-year-old or younger person to send in an accurate graph.

I've heard from some who live in light saturated areas such as Los Angeles and Manhattan that the non-variable stars I recommended as a basis for estimating the brightness of Delta Cephei were too dim to be helpful. Try again with a pair of binoculars, and you should have greater success.

The variable star toward which I point you in this article is so bright that everyone will be able to see it and estimate its brightness without optical aid. This star, first called the "demon" star, then Algol (its catalog name is Beta Persei), equals the brightness of the main stars in the Big Dipper. Ancients called it the "demon" star because once every 2.87 days it fades to about a third of its usual brightness. They thought a demon was trying to steal Algol's light.

What is really happening? Algol has a much fainter companion star orbiting it. Every 2.87 days this fainter star passes between Algol and us, blocking much of Algol's light from our view. It eclipses Algol just the way our moon blocks out the sun's light when it passes between us and the sun.

Algol's maximum brightness is officially designated as 2.2 magnitudes, and its minimum brightness, 3.47 magnitudes. (Remember, the smaller the magnitude number, the brighter the star.) The eclipse of Algol takes about three hours in all, but Algol stays at minimum brightness for just a few minutes.

On most nights Algol will shine steadily at maximum brightness. But at predictable times, it grows dim. Eclipse times for North American observers are given in the table that follows. If you live elsewhere, you will need to adjust for your time zone.

Again I offer a $25 RTB gift certificate (good toward items in our catalog) to the first person aged fifteen or younger who sends in a graph of Algol's eclipse. As always, adults are welcome to participate, just for the fun, not for the prize.

With the help of star charts, find Algol. Look for the second brightest star in the constellation Perseus. Next, compare Algol's brightness to the brightness of other stars (the non-varying ones) nearby and assign a number to Algol's brightness. For example, if Algol's brightness seems about halfway between that of stars labelled 2.9 and 1.9, give Algol a 2.4. Be sure to record the time at which you made your estimate.

To graph Algol's entire eclipse, start estimating and recording its brightness about two hours before the published eclipse time and continue to record your estimates every fifteen minutes until about two hours after that eclipse time. This effort should result in a very neat and complete graph. I look forward to seeing yours. Happy star gazing!

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