Scare up some Halloween stargazing with Algol
With Halloween coming on Saturday, spooky times are right around the corner. Let’s direct our gaze toward an eerie variable star named Algol. Located in the constellation Perseus, the hero, Algol displays such a regular cycle of dimming and brightening that it caught the attention of the ancients long ago. Some regarded the star as being demonic or possessed due to its odd behavior.
Greek star lore of Algol is centered on its host constellation. Andromeda was chained to a rock as sacrifice to the sea monster Cetus after her mother, Cassiopeia vainly boasted that the maiden’s beauty surpassed the Nereids. Andromeda was just moments from being attacked when Perseus swooped in and using the severed head of Medusa turned Cetus to stone, thereby saving Andromeda from a gruesome fate. In constellation drawings of Perseus, he is often shown holding the Gorgon’s head represented by the star Algol.
The name Algol comes from the Arabic, Ri’B al Ohill meaning the head of the ogre or demon. Another name is al-ghul meaning ‘mischief maker’. This name is thought by some to be the source for our modern day word ‘ghoul’. In his Iliad, Homer described Algol as “the Gorgon’s head, a ghastly sight, deformed and dreadful, and a sight of woe”. The star’s Hebrew name is Rosh ha Sitan translated as ‘Satan’s head’, but also has ties to Lilith, Adam’s original and demonic first wife. In Chinese astronomy Algol is called Tseih She, the 5th star of the mausoleum and translates as ‘piled up corpses’. A more contemporary reference can be found in author Douglas Adams’ The Hitchhiker’s Guide to the Galaxy in which the star is home to the Algolian Suntiger.
Lying about 93 light years distant Algol is, perhaps, the most famous variable star in the sky. In 1782, its period was determined by 18 year old astronomer, John Goodricke who lived in England. A bout with Scarlet Fever left Goodricke deaf/mute, but his visual skill allowed him to document that Algol fades in brightness and recovers every 68.8 hours. Goodricke hypothesized that Algol has an orbiting companion that is cooler and darker. The darker body’s orbit places it between our line of sight and the star every 2.867 days accounting for the drop in brightness. For his work, Goodricke was awarded the Royal Society’s Copely Medal in 1783. The society named him Fellow at its meeting on April 16th, 1786. But sadly, Goodricke died just 4 days later at the age of 21. It is unlikely he knew of the honor.
In 1881, Director of the Harvard College Observatory, Edward Pickering produced evidence suggesting that Algol was an eclipsing binary star system. Pickering’s hypothesis was confirmed in 1889 when German astronomer Hermann Vogel, detected doppler shifts in the spectrum of Algol. His observations revealed a shifting spectral signature due to an orbiting secondary star. As the secondary approaches along our line of sight, its spectrum shifts toward the blue. When receding, the star’s spectrum shifts towards the red. This discovery made Algol the first spectroscopic binary star system ever detected.
Algol is a favorite of veteran stargazers because its variation can be easily seen with the unaided eye. This feature makes it a worthy target for novice observers. The drop in brightness is called the Minima of Algol and is fun to watch. Dates and times of the minima are easy to track down via the internet. Find one that suits your schedule and watch the ‘Demon star’ make a dramatic drop and recovery in brightness.
