From the September 2012 issue

Dare to stare at the “evil eye”

November 2012: One of the sky's scariest stars puts on a disappearing-reappearing act.
By | Published: September 24, 2012
This month, I’d like you to find the celestial ghoul’s head and watch for its “evil eye” to wink. I’m referring to Algol (Beta [β] Persei), the most enchanting naked-eye variable star in the night sky. To follow its changes, you need nothing more than your unaided eyes. But despite the star’s visibility, catching it at minimum light can be tricky, unless you know when to search it out. First, though, let’s look at a bit of interesting history.

The “ghoul”
In the classical view of the sky, Algol represents the Gorgon Medusa’s severed head, which Perseus clasps firmly in his left hand by the snakes the monster had for hair. In the Iliad, Homer tells us the head is “a ghastly sight, deformed and dreadful.” Others have associated Algol with one of the Gorgon’s eyes, capable of turning an onlooker to stone.

Algol (Beta [β] Persei) puts on one of the most spectacular variable-star shows in the sky. If you want to estimate its changing brightness, compare it to the magnitudes of nearby stars (shown in green). // Astronomy: Roen Kelly
But Algol’s name has a more ancient origin. It comes from the Arabic al Ghul (meaning “the demon”), from which we derive the modern word ghoul. According to the 1786 English translation of William Beckford’s Vathek, an Arabian tale, Ghul describes a “species of monster which was supposed to haunt forests, cemeteries, and other lonely places; and believed not only to tear in pieces the living, but to dig up and devour the dead.”
It’s not hard to envision Algol as a corpse eater if you look through the eyes of ancient skywatchers who imagined the night sky as a celestial graveyard filled with the souls of their ancestors. Seeing 70 percent of Algol’s light suddenly “consumed,” only to have it mysteriously restored, then devoured again days later, must have instilled in them a sense of dread.
“The imagination of a Poe could not have represented a more startling thing — a sun that winks like a gloating demon!” observed Garrett P. Serviss in his 1910 Round the Year with the Stars. “One may easily cultivate an uncanny feeling while watching it.”

Stellar magic
Algol “winks” because it is an eclipsing binary star (a faint K-type star and a bright B-type star), a system whose members pass in front of each other from Earth’s perspective. (It is, in fact, the prototype of this class.) Most of the time, Algol shines at 2nd magnitude, about the same as the brighter stars in the Big Dipper. But once every 2.87 days, the fainter star in the system passes in front of the brighter one; we see Algol begin to fade until its light dips to a minimum magnitude of 3.4. It shines thus for about two hours before the spectacle starts to reverse and Algol’s light returns to what Serviss called “pristine splendor.”

Algol’s November minima
Date Time (UT)
Nov. 1 22h51m
Nov. 4  19h40m
Nov. 7 16h29m
Nov. 10 13h18m
Nov. 13 10h07m
Nov. 16 06h56m
Nov. 19 03h45m
Nov. 22 00h34m
Nov. 24 21h23m
Nov. 27 18h12m
Nov. 30 15h01m
Of course, the challenge is to catch “the Demon” winking because the entire event lasts about 10 hours. The table on this page will help you get started; it shows the times of minimum eclipse throughout November.
The best way to record Algol’s changes is to compare its brightness with other stars of known magnitude that do not vary in brightness. The chart on this page shows Algol and several nearby stars whose magnitudes cover the variable’s range.
Algol’s quirks have given it sinister overtones in mythological interpretations of the sky, where it represents the severed head of the Gorgon Medusa, as in Johannes Hevelius’ Uranographia. // Atlas Coelestis/Johannes Hevelius
When making a magnitude estimate, try keeping the variable and its comparison star along the plane of your eyes. At maximum, Algol’s brightness matches that of magnitude 2.1 Beta Andromedae. At minimum, it’s barely dimmer than magnitude 3.3 Epsilon (ε) Cassiopeiae (the easternmost star in the celestial W).

As always, let me know how you do at