Is it any wonder that most avid stargazers dislike Daylight Saving Time? Now that cursed DST has ended, we can get a decent view of the night and still get to bed at a reasonable hour.
By 7:30 blessed darkness has fallen. The constellation Perseus rises beautifully in the northeast about half way up to the top of the sky. The brightest stars of the ancient Greek hero form an upside-down “Y,” a crude stick-figure representation of a human. To those who claim that the constellations don’t look like the things they are supposed to represent, consider that a stick figure bears only a passing resemblance to a person, yet it is an almost universal representation of the same. So lighten up.
Here within the confines of Perseus, 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. Mirphak is a shortened version of Mirphak al Thurayya, the “Elbow Nearest the Many Little Ones.” Binoculars will show the myriad fainter stars of the Perseus Association, the “many little ones” that give Mirphak its name.
However, the real “star” of Perseus is Algol, the Demon’s Head. Find it by looking to the right and slightly down from Mirphak.
Since ancient times, Algol has been associated with the severed head of the snaked-haired Medusa, one of the hideous Gorgon sisters. Her countenance was so ugly, it was said, that a single glimpse would turn the unfortunate witness to stone.
Perseus was the ancient hero who did the severing. In subsequent adventures, he used the head as a weird weapon. He is often depicted in ornately illustrated, seventeenth-century star maps as holding the head up to inflict rocky rigidity on his unseen opponent.
Sometime before 1672, the Italian astronomer Geminiano Montanari noticed that Algol was not steady in its brightness.
Note that I didn’t say “discovered” above. Astronomers have speculated that the ancients must have noticed Algol’s freakish shifts in brightness. Otherwise, why would they have associated that particular star with the equally freakish Gorgon?
Sadly, there is no direct evidence from the old constellation stories that ancient stargazers ever noticed the shifting brightness of Algol. Still, the suggestion is tantalizing, even more so since many cultures give the star demonic significance.
The Chinese called it Tseih She, the Pile of Corpses. Richard Allen’s Star Names, Their Lore and Meaning cites the Talmud to suggest that ancient Hebrews called it Rosh ha Satan, the head of Satan. According to Allen, they also associated Algol with Lilith, Adam’s demonic first wife.
In any case, Montanari gets credit for the discovery of Algol’s variability in modern times. There were no other stars like it, so the race was on to study it carefully and discover the reason for its shifts in brightness. Algol thus became the first of many “variable” stars to be measured accurately.
From 1782-83, British astronomer John Goodricke carefully studied the changing brightness of the star and noticed that it 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.
Goodricke suggested that Algol consists of two stars, not one. A small, bright primary star has 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 Algol’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, which is marked on most star maps down and to the left of Mirphak. 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 out of the deep recesses of space. Don’t forget to wink back.
Tom Burns is director of the Perkins Observatory in Delaware.