Perseus and the Demon Star


Is it any wonder that many avid stargazers detest Daylight Saving Time? When DST ends on Nov. 3, we can get a decent view of the night and still get to bed at a reasonable hour.

Nov. 3! I’m not much of a conspiracy theorist, but does anybody remember when DST was just a summer thing? It seems like over the years Eastern Standard Time has shrunk to just a few months. DST is now the exception, not the rule.

God bless EST. By 7:30, blessed darkness will have fallen. The constellation Perseus rises beautifully in the northeast about halfway up to the top of the sky. The brightest stars of the ancient Greek hero form an upside-down “Y.”

Here, my well-rested stargazing friends, there is much to see.

Start from the brightest star of the constellation, located at the center of the “Y.” The star Mirphak gets its name from the old Arab stargazers, who mapped the sky with great precision. Mirfak is a shortened version of Mirfak al Thurayya, the “Elbow Nearest the Many Little Ones.”

“Little Ones” there are aplenty near Mirfak. Look at the area with binoculars and you will discover the dozens of stars in the “Perseus Association,” which provides one of the best “bino” views in the sky.

Mirfak is relatively distant from Earth for a naked-eye star at something like 620 light-years away. To be so bright, it must be powerful, producing 4,000 times the energy output of our puny daytime star, the sun.

Up and to the right from Mirfak is M34, visible in a small fuzzy patch in binoculars. In a small telescope, it will resolve into a few dozen stars. M34 is a star cluster, a group of gravitationally bound stars traveling together in space.

The real “star” of Perseus is Algol, the Demon’s Head or Demon Star. Find it by looking to the right and slightly down from Mirphak. Observe it over the course of several nights, and you will notice that it varies distinctly in its brightness.

Algol has a long and storied history. From early times, it has represented the head of the snake-haired Medusa, which Perseus holds, newly severed, in his hand. One look at her face turned the unfortunate viewer to stone, so don’t look at Algol too long.

How did such a great hero end up clutching such a monstrosity? Read on, gentle reader.

King Acrisius should have been a happy man. He ruled the bountiful land of Argos. His daughter, Danae, charmed everyone with her intelligence and beauty. But Acrisius was driven nearly insane by a prophecy that foretold his death by a son, yet unborn, of his innocent daughter.

To prevent Danae from marrying, the king locked her in a tower. He vowed that the all-too-human eyes of potential suitors would never again gaze upon her. The unlucky king didn’t figure on immortal eyes, however. Zeus, king of all the gods, easily saw through the tower walls. “Hubba, hubba,” he said in ancient Greek, turned himself into a shower of gold coins, and rained himself down on Danae’s lap through a skylight.

Nine months later, Perseus was born. In anger and fear, Acrisius set his daughter and grandson adrift into the sea in a wooden chest. This was, of course, very bad parenting, but Danae’s desperate pleading must have sounded false. “It was the coins, Dad, the coins, honest.”

The chest came to ground at Seriphos, an island ruled by the vicious Polydectes. Perseus grew to young manhood. All the while, Polydectes kept a lustful eye on Danae. After his wife died, Polydectes asked the comely woman to marry him.

She said no. Polydectes, true to his nature, imprisoned her in his palace and turned her into a love slave.

Polydectes knew that he had to rid himself of Perseus, so he gave him an impossible task — to bring home the head of Medusa, the Gorgon. You were wondering perhaps when we would get to the Gorgon. Well, here she is.

The Gorgons were three sisters with live snakes for hair and a single eye and tooth, which they had to share among themselves. Medusa was so ugly that a single glance at her face turned the unfortunate viewer to stone.

Tough job, but Perseus undertook it, hoping that he could eventually find a way of liberating his mother. Using only the reflection of Medusa in his shield, he lopped off her deadly head and put it in a sack.

Perseus had many adventures on the way home. Among other things, he rescued his future wife, Andromeda, from the clutches of the sea monster Cetus, but that’s another constellation story.

Polydectes was surprised to see Perseus return. He figured that the hero would not survive his encounter with the Gorgon. He was even more surprised, but only for a split second, when Perseus pulled Medusa’s head out of the sack and turned him into a statue.

After many great adventures, Perseus returned with his mother and wife to Argos, the land of his birth. His grandfather, Acrisius, had been driven from power during their absence, and Perseus became king.

Despite his high status, Perseus was a man of the people, often participating in their athletic competitions. At one such contest, he threw a discus with such force that he accidentally killed someone in the crowd. It was thus that the prophecy that began this tale came true. The unfortunate bystander was none other than Acrisius.

After the death of Perseus, he ended up among the stars as a constellation. That lofty status seems to be the reward the Greek and Roman gods gave to their noblest of heroes.

There is no record that I know of that the ancients noticed the strange variability of Algol. But they almost certainly must have noticed its changes in brightness.

Why else would they put the starry version of the hideous Gorgon’s head with her winking eye in Perseus’s outstretched hand?

There the matter remained for nearly two millennia until after the invention of the astronomical telescope.

Sometime before 1672, the Italian astronomer Geminiano Montanari noticed (or perhaps rediscovered) that Algol was not steady in its brightness.

From 1782-83, British astronomer John Goodricke carefully studied the varying brightness of the star.

Goodricke ‘s life is proof enough that the term “disabled” ought to be replaced with “differently-abled,” as some have suggested. When he was five years old, he lost his hearing after a severe bout with scarlet fever. By his 18th birthday, he was already an accomplished amateur astronomer with the ability to distinguish exceedingly minor variations in the brightness of a star.

Because of his extraordinarily acute vision, Goodricke noticed that Algol faded from bright to dark and back again over a period of two days, 20 hours, and 40 minutes. Every 2.87 days, Algol dims abruptly for about ten hours and it did so with clock-like regularity.

Algol was thus the first “variable” star to be discovered and measured accurately.

Goodricke suggested that Algol was really two stars — a small, bright primary star and a dimmer, larger star in orbit around it. As the dimmer star passes in front of the brighter one, the light from the brighter star is filtered through the dimmer one. As a result, the bright star dims abruptly. Such stars came to be called eclipsing binaries.

The greatest astronomer of Goodricke’s age, William Herschel, rejected the idea. He couldn’t see the second star in his telescope. Later, Herschel discovered other stars that had companions in orbit around them. He changed his mind and accepted Goodricke’s findings wholeheartedly, even though Agol’s companion is too faint and too close to Algol to be seen in a telescope.

You can observe the dimming of Algol yourself by using an old stargazer’s trick. Variable-star observers measure the changing brightness of a star by comparing it night after night with the brightness of a star that doesn’t vary.

The perfect star for that purpose is called Delta, down and to the left of Mirfak. Most of the time, Algol will look brighter than Delta. When Algol is in eclipse, it will look about as bright as (or even a tiny bit dimmer than) Delta.

It might take several nights of observing to see the change, but it’s worth the wait. The Demon’s Head has winked at you from the deep recesses of space. Don’t forget to wink back.

When I observe Algol, I can’t help but think of mythic characters like Perseus and Medusa. But it’s hard not to reflect also on the short life of John Goodricke.

Goodricke studied other variable stars, notably a class of stars we now call Cepheid variables. As a result of his body of work, especially his close study of Algol, Goodricke was elected a Fellow of the Royal Society on April 16, 1786, at the unheard of age of 21 years.

However, he never knew that he received the honor. He succumbed to pneumonia just four days later. What was lost to the world when his star dimmed so tragically early, never to be seen again?

As for Perseus, he lives now among the stars, the head of Medusa with its single, winking eye still clutched in his hand. From his lofty perch, he speaks to us from our ancient past, when good deeds, long suffering, and a sympathetic heart never went unrewarded, at least in the stories they told.

By Tom Burns


Tom Burns is the former director of the Perkins Observatory in Delaware.

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