Last updated: 16 February 2001

OBSERVING GUIDE: Tracking the Variable Star ALGOL

From: sherrodc@ipa.net (Clay Sherrod)

OBSERVING GUIDE - Using Your AutoStar "Minimum of Algol" to Start You on your Way to Observing Variable Stars

Scrolling, "moding," "entering," and slewing through your AutoStar keypad, you no doubt have come across the prompt under "Event" entitled "Minimum of Algol."

We all know that Algol is a bright variable star (beta Persei) in the constellation of Perseus. Perseus is "circumpolar", or tightly placed relative to the north celestial pole so that it circles nightly, monthly and even yearly in such a way that it never "rises or sets", but rather seems to be "tethered" in a neat little circular path about Polaris, the north star.

This allows its second-brightest star, ALGOL, or "Demon Star" to be visible throughout the course of a night and even throughout the year. Hence, it is an ideal subject to begin your adventure into the observations of VARIABLE STARS: stars that are not fixed in brightness, but rather change as they brighten and dim, usually somewhat predictably.

It is possible that ALL stars are somewhat variable, even our own sun if seen from some remote part of our galaxy, but some are more pronounced in their changes, sometimes varying as much as 10 to 12 magnitudes in a short period of time! Indeed, even a NOVA or SUPERNOVA is a variable star of sorts, rapidly surging in brightness and then steadily fall off to sometimes invisibility.

ALGOL gets its name from the Arabic Al Ra's al Ghul, meaning "the demon's head in reference to early associations of the star to the serpentine-laced head of the Gorgon Medussa; to the ancient Hebrews, the star also had a rather ominous association, "Satan's head."

Although the star's dramatic brightness changes have been known since antiquity, it was not until only 1667 that careful attention (scientifically, not mythologically, at least!) was given to Algol; even then it took ANOTHER 120 years before its nearly-three day cycle was determined accurately!



You obviously do not need an AutoStar to find Algol, nor to find out when it is a minimum (about 3.5 magnitude). Indeed, you can compute it yourself, because the star runs one complete cycle EVERY 2 days 20 hours 49 minutes, or you can find reference to it in Sky and Telescope magazine each month.

Also, you do not need an ETX telescope to observe Algol's brightness changes....you do not need a telescope AT ALL! In fact, monitoring the brightness changes requires that you scan wide areas of the sky for bright stars with which to compare Algol's brightness at any given time (see ASO attached chart for Cassiopeia and Perseus comparison stars). You can keep up with the CYCLE of the star's brightness with AutoStar, but the actual observing is best done with the naked eye.

Algol chart

When comparing magnitudes (start yourself a little chart and plot the star's magnitude yourself over a month-long period - makes a great family or club project), it is advisable to use BINOCULARS that are slightly out-of-focus on both Algol and the comparison star(s) to make small "disks" of light that are easier to compare than "points" of light. With such a rapid variation of only a couple of days, it is wise to monitor Algol as often as possible; write down the time (get from your AutoStar), date and your magnitude determination at every possible outing.



Algol is undoubtedly the best known, and one of the most rapidly-changing variable stars; it is an ideal naked eye variable that will allow you to acclimate yourself into the exciting study of variable stars (pull up www.AAVSO.org for the charts and information available free of charge for variable star observing through the American Association of Variable Star Observers).

The cause of Algol's rapid brightening and subsequent fading are well understood. Algol is TWO stars, not just one, as seen from Earth being aligned as an "eclipsing binary" star in which one star periodically occults, or covers up, the other in their tight orbit around one-another. Hence, with Algol, this system "eclipses" on a very predictable cycle; the MINIMUM BRIGHTNESS (about 3.5) is likely the brightness of the two stars seen aligned toward Earth, while the MAXIMUM BRIGHTNESS results when both stars are "in full view" (even though we cannot resolve the double nature as the stars are too close).

Algol is a relatively close star to our sun, at a distance of only 115 light years. The total luminosity of the double star system is on the order of 140 times that of the sun; when comparing the mass of both stars in the Algol system to the sun, we find that the smaller of the two stars is only .86 times the mass of the sun, while the larger star is 4.5 times more massive.


Observers who have never attempted variable star observing are missing a treat; it is one field in amateur astronomy that you can actually witness CHANGE taking place in an otherwise seemingly "fixed" deep space. Over time, you may want to move up to telescope variable stars that demonstrate wide ranges of variation, sometimes showing a bright star only - weeks later - the star has faded to near-invisibility. A good example and starter-star for ETX telescopes might be SS CYGNI, in the constellation of Cygnus. Log onto the AAVSO website to download the charts for this variable.

REGARDING AAVSO CHARTS: for the ETX and/or LX scopes, always download the "Reversed" charts which provide (thank goodness) star patterns as they appear in the mirror right angle eyepiece of the ETX/LX scopes; this provides direct comparisons. Be sure and get the "A" charts (wide field finder charts) for each star you desire, and THEN get the "B" or "C" charts (higher magnification with fainter comparison stars) for the same star for telescopic viewing.

Enjoy yet another wonderful exploration that provides you with exciting stellar changes and gives you some "purpose" to your telescope after AutoStar has given your grand tour and you have "....seen all there is to see!" Enjoy!

P. Clay Sherrod
Arkansas Sky Observatory
Conway / Petit Jean Mountain

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