A Demon Star for Halloween

by John Hlynialuk
A traditional depiction in mythology of Perseus is shown below. This is
plate 6 in Urania's Mirror by Jehoshaphat Aspin, London, 1824. One of 32 hand-coloured constellation cards, the set depicts most of the familiar constellations as works of art rather than what is seen in modern atlases which strive for accuracy in star position, brightness, type, etc. and provide no fanciful artwork at all. Pity.


There are many interesting stars in Perseus but at this time of year when the ghouls are out, we pick on Algol the Demon Star. Modern astronomers have dug deep and revealed an extra-ordinary story.

The second brightest star in Perseus, Algol, has a name which translates from Arabic as “head of the ogre”, original: “ra’s al ghul”, a creature who stalked graveyards and consumed human flesh. There is also the obvious connection to our word “ghoul.” So from ancient times, Algol has been called the Demon Star.

In the myth associated with the Demon Star, innocent Andromeda is on the verge of being dismembered for being beautiful (it was her mother’s boast that got her into trouble, actually). Our hero, Perseus is returning with the severed blood-soaked head of Medusa (she had a bad hair day) and saves Andromeda from Cetus the Sea Monster by turning the monster into stone. There are (unconfirmed) reports that more than one of the Greek islands are the site of the monstrous body, -check the tourist literature.

By the end of the mythological saga, Perseus, Andromeda, her boastful mother -Queen Cassiopeia and her father, King Cepheus become constellations in the sky, -even Cetus is there a bit further off. The classical depiction of Perseus (in the image above) shows Algol, the Demon Star, as one of Medusa’s eyes. But like only a few other stars in the heavens, Algol actually changes its light output visibly! It’s as if Algol, the Demon Eye of the Medusa, is winking at us. It is certainly a relief that she is 98.2 light years away, well beyond turning-to-stone range.

It is not definitively known if ancient myth-makers noticed Algol’s variation and built it into their stories, but by 1783, English astronomer John Goodricke showed that the variable Algol had a very regular period indeed. This unique discovery gained Goodricke the Royal Society’s Copley Medal, a prize first awarded in 1731 and still presented annually today. Algol’s precise clock-like variations occur over 2 days, 20 hours and 48.9 minutes. During that time, it fades for exactly 10 hours and then brightens back to normal. Repeat 2.86730 days later.

The variability of Algol lies in the fact that it is actually three stars and two orbit each other so that they pass directly in front of each other from our perspective. The combined light output drops during the 10 hour eclipse which gives these stars their type name: eclipsing binaries. Algol was the first variable recognized as such and is one of only a few that can be noticed by the naked eye. It’s like Medusa is continually trying to change us to stone, -with a 10 hour wink- thank goodness the “petrification energy” gets totally diluted over the vast distance.

An ordinary star chart is provided below (Credit IAU and Sky&Tel) to help you locate Algol in Perseus. It is usually easy because it spends most if its time as the second brightest star in Perseus after Mirfak. Algol is usually a magnitude 2.1 star and only drops to magnitude 3.3 every 2.86736 days for about 10 hours. So you need a schedule to tell you when the eclipse will happen during a time when Algol is in the night sky. There are four good chances in October and these are given at the end of this article.

IAU Perseus

The light curve diagram provided here for Algol shows the two dips in its brightness as each of the two stars eclipse each other. The primary eclipse occurs when the larger, cooler, dimmer Algol B partially hides the smaller, hotter, brighter, more massive Algol A. The secondary eclipse is when Algol A hides Algol B.


The times when an eclipse occurs are provided in various publications or online. Google “Algol minima” for some of those. In any case, the next good opportunities to see Algol blink at us are Oct 18 at 2:09 am when Perseus is high in the sky. Then on Oct 21, at 22:58 EDT (about 11 pm) with Perseus still high in the east, and Oct 23 with an eclipse starting at 19:47 EDT (7:47 pm Perseus above NE horizon). The last opportunity in October is Oct 26 with an eclipse starting at 4:36 pm EDT. By mid-eclipse Perseus should be high enough above the NE horizon for Algol to be seen. You can watch the star come back out of eclipse in the wee hours of the morning, around 2:30 am or so.