Sun rises at 6:10am
Sun sets at 5:42pm
Moon phase: 2nd quarter waxing gibbous
Moon sets at 12:45am, Saturday
Moon rises at 2:00pm, Sunday
In the spirit of Halloween and the Day of the Dead, let’s look for the Demon Star in the constellation Perseus the Hero this weekend. Don’t worry, it’s not really demonic! It’s a star with the official name Algol, pronounced AL-gul.
The name Algol comes from the Arabic and means “the head of the demon.” The Greeks and Romans identified this star with the Head of Medusa, a fearful monster with snakes in place of hair. The star has also been called the Ghoul Star.
You can use the constellation Cassiopeia to locate Perseus in the northeast in the evening sky. Look for the sideways “W” of Cassiopeia, Perseus is just below that. The brightest star in Perseus is named is Mirfak. Look to the north-eastern horizon after 6:45 pm to see Perseus rising up. The brightest star is his heart, Mirfak, and the second brightest star is Algol.
While you study this part of the sky, look just to the right of Cassiopeia and find Andromeda. If we have clear skies, you should just be able to make out the Andromeda Galaxy, which looks like a smudge in the sky, until you focus in with binoculars and detect her lovely spiral shape!
In Greek mythology, Andromeda is the daughter of the Ethiopian king Cepheus and his wife Cassiopeia. When Cassiopeia's hubris leads her to boast that Andromeda is more beautiful than the Nereids, Poseidon sends the sea monster Cetus as divine punishment. Andromeda is stripped and chained naked to a rock as a sacrifice to sate the monster, but is saved from death by Perseus. (No doubt Andromeda fluttered her lashes and said breathily, “My Hero!”)
Algol is a very interesting star. It’s known to vary in brightness in a way that’s extremely regular. The cycle lasts exactly two days, 20 hours and 49 minutes. For a few hours during the cycle, Algol’s brightness falls far below normal, then returns to normal. All the while, the star remains visible to the eye.
Algol’s brightness variation is not due to some special quality of a single star. Instead, this is a multiple star system, where one star regularly passes in front of another as seen from our earthly perspective.
Thus Algol is what’s called an “eclipsing variable star.” Thousands of these stars are known, but Algol is perhaps the most famous of this class because its periodic dip in brightness can be seen with the eye alone, and because the cycle is relatively short.
The ancient stargazers had no knowledge of multiple star systems, but possibly they did notice this star’s brightness change. Perhaps the brightness change is why, throughout parts of the ancient world, the star Algol was associated with demons or monsters.
Thank you for keeping up with the Night Sky articles. If you are out later on in the week, each star rises about four minutes earlier each day than written here, and the moon rises 50 minutes later. Night Sky is researched and compiled by Lisa Davis-Burnett. Earthsky.org is a key resource for information and images. Questions or comments? Email This email address is being protected from spambots. You need JavaScript enabled to view it.
