How the 'demon star' got its name

GLENN ROBERTS @chronicleherald Glenn K. Roberts lives in Stratford, P.E.I., and has been an avid amateur astronomer since he was a small child. He welcomes comments from readers at glennkroberts@



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The constellation of Perseus - the Prince is a conspicuous constellation in the late-autumn night sky. It's located almost directly overhead by mid-evening, once the sky has fully darkened. It sits between the constellations of Auriga - the Charioteer (to the left), Andromeda - the Chained Princess (to the right), and Cassiopeia - the Queen (to the upper right) in the northern part of the sky. Sitting amidst the Milky Way, it can sometimes be difficult putting together the figure of the prince; only 22 of the constellation's 65 stars make up the prince's form. ABOUT PERSEUS One of the original 48 constellations established by second century Greek mathematician, geographer, and astronomer Ptolemy (c 100-170 A.D.), Perseus is one of the 88 recognized modern-day constellations. In Greek mythology, Prince Perseus, son of Zeus and a mortal maiden, is the hero demi-god who slew the monstrous Medusa and cut off her head. Medusa was one of three demon-like sisters with hair of living, poisonous snakes, brass hands, and horrifying faces, with the power to turn anyone who met their gaze, or anything they looked at, to turn to stone. Two of the sisters, Stheno (the eldest) and Euryale (the second-eldest) were immortal; Medusa, the youngest, was mortal. ON A MISSION Perseus was sent to slay Medusa by Polydectes, King of Seriphos (an island municipality in the western Aegean Sea), because he wanted to marry Perseus's widowed mother and wanted Perseus out of the way (and, preferably, dead from Medusa's fatal gaze). However, with the help of a number of the Olympian gods – a mirrored shield and enchanted sack from Athena (the Greek goddess of wisdom, warfare, and handicraft); a pair of gold, winged-sandals from Hermes (the wing-footed messenger of the Olympian gods); a gold sword from Hephaestus (the Greek god of artisans, blacksmiths, metallurgy, fire, and volcanoes); and an invisibility helmet (known as "Hades' Helm") from Hades (the Greek god of death and the underworld) – Perseus was able to slay Medusa. Arriving at Medusa's lair via Hermes' sandals, and wearing Hades' Helm, which rendered him invisible, Perseus was able to sneak up on her. Using the mirrored surface of Athena's shield (enabling him to avoid looking directly into her eyes and turning to stone), he back-handed the gold sword of Hephaestus and cut off her head. Still avoiding looking in her deadly eyes, Perseus put her severed head into Athena's magical sack. SAVING ANDROMEDA From the blood that spurted onto the ground when Perseus took off Medusa's head, the winged-horse, Pegasus, sprang forth. Taking advantage of this fortuitous turn of events, Perseus leapt onto the back of Pegasus and headed homeward. On his way home, he rescued Princess Andromeda from the sea monster Cetus that was ravaging the Aethiopian countryside. Princess Andromeda's parents, Queen Cassiopia and King Cepheus, had chained their daughter to a large rock on the shore in hopes that by sacrificing her to the sea monster, as directed by the Delphic oracle, it would leave them and their country alone. Fortunately for Princess Andromeda, Prince Perseus saw her predicament and intervened, showing Medusa's head to the sea monster, which promptly turned to stone and sank beneath the waves. As is to be expected in such tales, Perseus wins Andromeda's heart and hand, and with the blessing of her parents, marries her. Perseus eventually returns home, and, according to one legend, turns King Polydectes to stone with the Medusa head; he would later become the King of Argos. There are a couple of interesting add-ons to this tale. While flying back home on Pegasus (pre-andromeda rescue), Perseus encountered Atlas, the Greek Titan condemned by Zeus to hold up the heavens for eternity. Perhaps taking pity on Atlas, Perseus took Medusa's head from the sack and pointed it at Atlas, immediately transforming him into stone, creating the Atlas Mountains, the mountain range separating the Sahara Desert from the Mediterranean Sea and the Atlantic Ocean. It is said that while Persus was flying over the Red Sea, some of Medusa's blood, dripping from the sack, fell into the water and formed the sea's famous red corals. THE DEMON STAR Within the constellation of Perseus is the asterism of Medusa's head (Caput Medusae), identified by the bright star, Algol (Beta Persei), and three other stars - Omega Persei, Mu Persei, and Rho Persei. Algol (from the Arabic for "ghoul" or "demon") is said to represent one of the eyes of Medusa and is often referred to as "The Demon Star". It is also sometimes called "The Winking Demon Star," as it is a variable star that periodically dims and then brightens again, giving it the appearance of "winking." HOW DOES IT WINK? Algol is a three-star system, consisting of two eclipsing binary stars (where the two stars pass in front of each other from the perspective of an observer on Earth) and a third, dim star. It is the eclipsing stars that gives the star its "winking" nature. A "binary" is a two-star system in which the two stars are gravitationally bound to and in orbit around each other; they differ from "double stars" or "optical doubles," which are merely two stars that appear visually to be near one another, but really have vastly different true distances from Earth. As the larger, fainter and cooler star passes in front of the hot, luminous primary, part of the primary's light is blocked, and Algol's normally constant magnitude of +2.1 (visible to the unaided eye) temporarily dims to +3.4, before slowly returning to the former magnitude. This occurs every 68 hrs, 48 mins, 56 secs (2.86 days) in a roughly six- to 10-hour-long eclipse. VARIABILITY RECORDED Algol was one of the first non-nova variable stars to be discovered. Although the ancient Egyptian calendar of lucky and unlucky days of about 3,200 years ago is believed to be the oldest documented reference to Algol's variability (with, presumably, the unlucky days occurring on those days when Algol's light diminished), it wasn't until 1667 that Italian amateur astronomer Geminiano Montanari (1633-1687) first recorded Algol's variability. It would be another 115 years, in 1782, before the mechanism of Algol's variability would be explained by English amateur astronomer John Goodricke (1764-1786). Today, astronomers know that variable star systems are fairly common throughout the Milky Way Galaxy; the 2022 Revised Version of the New Catalogue of Suspected Variable Stars (GVCS 5.1) lists 58,305 named variable stars. While you're having a look at Algol, slide your binoculars or telescope to the north and look for the beautiful duo of open star clusters NGC 869 and NGC 884, most often referred to as the "Double Cluster in Perseus", a jewel box of stars spilled on black velvet, visible to the unaided eye between Perseus and Cassiopeia. THIS WEEK'S SKY Bright Venus (magnitude -4.3, in Virgo - the Maiden) rises around 3:15 a.m., reaching 32 degrees above the southwest horizon, and then fades as dawn breaks shortly before 7 a.m. Jupiter (mag. -2.9, in Aries - the Ram) becomes accessible when it reaches 12 degrees above the eastern horizon shortly before 5 p.m., reaching its highest point in the evening sky of 56 degrees above the southern horizon by about 10:40 p.m., and remaining observable until it drops below seven degrees above the western horizon around 4:45 a.m. Saturn (mag. +0.8, in Aquarius - the Water-bearer) becomes visible around 5:15 p.m., 28 degrees above the southern horizon, reaching a high point of 30 degrees above the horizon, before disappearing from sight when it drops below 11 degrees above the southwest horizon around 10:20 p.m. Neither Mercury nor Mars are visible this week; Mercury sits just on the western horizon at dusk, and Mars is still too close to the sun. Until next week, clear skies.