Algol: The Demon Star

Last week, I told one of my favorite constellation stories, the legend of Perseus and Andromeda. Well, not only is the story great, but the constellations are pretty cool, too. I already told you how to find the Andromeda Galaxy in Andromeda's constellation. This week, we will find an object in Perseus, called Algol: the Demon Star!

Perseus as imagined by Johannes Hevelius. Hevelius drew his constellation figures from a perspective outside the celestial sphere looking in, which makes them appear backwards from our Earthly view. This image is reversed, so the words are backwards, but the stars are actually how we would see them in the sky. Image Credit: Uranographia

As related last week, Perseus used Medusa's head to turn Cetus the Sea Monster into stone and rescue Andromeda. Medusa's severed head is represented in the stars! (Astronomers have no qualms about imagining strange images in the stars. There is even someone's hair up there). In the wishbone shape of Perseus, the right-hand curved line of stars represents Medusa's head. Algol is the brightest eye in Medusa's head. See image below.

Looking straight up, facing south. Algol is circled. Image Credit: Stellarium

Algol is a fascinating star. Ancient stargazers must have noticed this star acting strangely, because its name has ominous meanings in many languages. Arabic astronomers named it
ra's al-ghūl  which means "head of the ogre" or "head of the demon". (Astronomy is everywhere, comic fans!) The ancient Hebrew name for the star was Rōsh ha Sāṭān or "Satan's head". Chinese astronomers called it 大陵 (Dà Líng), which means "mausoleum". So what's going on with this spooky star? 

Algol's brightness changes regularly. While Algol is usually the second brightest star in Perseus, every three days it dims for a 10 hour period, dropping significantly in brightness. After 10 hours, it returns to its previous brightness. This effect is noticeable to the naked eye, and was surely observed by ancient astronomers. Ancient people might have thought something evil was at work.

Actually, Algol (also called Beta Persei by modern astronomers) is an eclipsing binary star system. The brighter star, Beta Persei A, is regularly eclipsed by the dimmer star, Beta Persei B. It takes about 10 hours for Beta Persei B to pass in front of Beta Persei A, causing the star to look dimmer to an Earth observer. The two stars are only 0.062 astronomical units apart. This is incredibly close, about 5 times closer than Mercury orbits our Sun! In fact, Beta Persei B orbits so close to Beta Persei A that plasma streams between the two stars. 

Although ancient people must have noticed the strange dimming of Algol, it was not officially recorded by astronomers until 1667. The cause of the mysterious dimming was not confirmed until 1889.

This is a series of 40 images (taken with an infrared telescope) put together in a movie. The bright object is Beta Persei A, and the dimmer one is Beta Persei B. You can see how the dimmer star passes in front of the brighter star. Image Credit: Dr. Fabien Baron, Dept. of Astronomy, University of Michigan and Wikipedia Commons

By the way, astronomers find planets around other stars in the Milky Way by noticing a similar effect. They can detect a star dimming when a planet passes in front of it. Using this method, scientists have identified over 4,000 planets orbiting other stars in our Milky Way galaxy! The Kepler Space Telescope has also found over 2,000 eclipsing binary stars, similar to Algol, while looking for planets. 

You can learn more about constellations and astronomy at the Peoria Riverfront Museum's Dome Planetarium. Find our full show schedule and descriptions here. Also, join us for a free event, the first Wednesday of the month, Relax Under the Stars! Our first program is this Wednesday, December 3. Stop by the planetarium from Noon-1 for a short star talk, and then relax under the starry dome, letting your tension float away with the calming music.