Watch "the Demon Star" Brighten
A bright, naked-eye multiple star in the constellation of Perseus, Algol reaches its minimum brightness of magnitude on Friday, October 25. Here are some observing tips.
The “Demon Star” Algol in Perseus (the Hero) is among the most accessible variable stars for beginner skywatchers. Like clockwork, this star’s visual brightness dims noticeably for about 10 hours once every 2 days, 20 hours, and 49 minutes. That happens because a dim companion star orbiting nearly edge-on to Earth crosses in front of the much brighter main star. Once the eclipse begins, the star steadily drops in brightness for five hours, and then it ramps up again during the second five hours - until the eclipse is over.
Algol represents the glowing eye of Medusa the Gorgon, whose severed head Perseus is carrying. That’s why this star has the nickname the “Demon Star”. Perseus is positioned to the upper left of the bright Pleiades star cluster. Another name for Algol is Gorgonea Prima “the First Gorgon”. The star that sits two finger widths to the right of Algol is Gorgonea Tertia “the Third Gorgon”. By the way, Gorgonea Secunda “the Second Gorgon” and Gorgonea Quarta “the Fourth Gorgon” are the stars Pi (π) Perseii and Omega (ω) Perseii. They sit to the upper right and lower right of Algol, respectively. The four Gorgons stars combine to form Medusa’s box-shaped head.
On Friday, October 25 at 8:51 pm EDT, Algol will reach its minimum brightness of magnitude 3.4 (i.e., the smaller, dimmer star will be fully in front of the larger, brighter star). At that time, Algol will be positioned a third of the way up the northeastern sky. By 1:51 am EDT, Algol will be approaching the zenith and will have brightened to its usual magnitude of 2.1.
The easiest way to detect Algol’s variability is to note how bright Algol looks compared to other neighboring stars. When it isn’t dimmed, Algol is brighter than any other nearby star save Mirfak, which sits a fist’s diameter to Algol’s left (or celestial north).
Star Walk 2 will help you identify Algol and other celestial objects in the sky above you.