To find this star, locate Perseus just east of Cassiopeia, below it in this evening’s eastern sky, as shown by the diagram. If you have a particularly dark sky, between Cassiopeia and Perseus you should be able to spot a couple fuzzy blurs known as “h and c (Chi … KYE) Persei” (PER-see-eye). They’re actually eta and chi, but most astronomers say “h” instead of eta for the better rhythm. This “double cluster” is only about 12.8 million years old and embedded in a much larger halo of stars adding up to about 20,000 times the mass of our sun at a distance of 7,500 light years.

The brightest star in Perseus is Mirfak (MEER-fahck), which is part of a 50-to-70-million-year-old cluster of stars that includes many of its neighbors, but not Algol. This region is rich with clusters because it is in the plane of our galaxy, the Milky Way. In Figure I, the equator of the galaxy is shown with the anticenter in Auriga (OH-ree-guh). This is the point opposite the center of our galaxy in Sagittarius that we can admire during the summer.

In most illustrations, Mirfak marks Perseus’ elbow, which is the meaning of its name. In the mnemonic stick figure I use for the constellation, Mirfak is in the middle of the arc of stars. Algol is the third star along a southward branch of stars from Mirfak. The fourth is a much fainter star, Rho Persei.

Some of the constellations actually look like their name, the best example being Leo the lion, but many are simply the projections of our myths onto the sky. Before writing was common, they sky was people’s story book handed on for generations.

FIGURE 2 — Algol’s light curve shows two dips in its brightness as each of the two stars eclipse each other. The primary eclipse occurs when the larger, cooler, dimmer Algol B partially hides the smaller, hotter, brighter, more massive Algol A. The secondary eclipse is when Algol A hides Algol B.

Perseus is the great hero of Greek mythology who slew the gorgon Medusa, whose terrifying gaze petrified those who looked into her eyes. Using the sickle given to him by Hermes, the shield from Athena and the sea nymphs’ Cap of Darkness, he beheaded her and placed her head in a magic bag as Pegasus, the winged horse sprang from her neck. Not finished with his adventures, in our sky, Perseus is riding on Pegasus with Medusa’s head tied to his saddle to rescue Andromeda. Her mother, Cassiopeia, bragged of her beauty, angering Poseidon who demanded she be sacrificed to Cetus, the sea monster. Perseus slew the sea monster and married Andromeda.

When I point out Perseus and Algol to my students, I warn them to not look too long at the Demon Star as it is Medusa’s eye, able even in death to petrify those who stare at it!

But is there a physical reality about this star communicated by the myths and names? Indeed there is. Algol’s brightness drops dramatically every two days, 20 hours, 48 minutes and 57 seconds. At these times, Algol dims from being the second brightest star in Perseus to match its neighbor, Rho. This occurs because Algol is actually three stars orbiting each other in a plane almost parallel to our line of sight. The dimming, though, is due to the two central stars of this system that are separated by 5.8 million miles (Earth is 93 million miles from the Sun). The more massive star of the pair, A in the diagram, is a bright mid-life, blue-white star similar to Alpheratz (AL-fur-oughts) in Andromeda. B, the other, is a larger and dimmer star, more reddish-yellow like Hamal in Aries, that has swollen up due to acquiring mass from A and going into its stellar death throes. When the larger, dimmer star, B, passes between us and brighter A, it blocks most of the light from the system. The entire process takes about 10 hours, of which two hours are spent at the minimum brightness.

This event is so regular, it’s quite easy to view for yourself. The best opportunities in December will be on the 16th at 1:57 a.m., the 18th at 10:48 p.m. and the 21st at 7:35 p.m. These are the times of mid-eclipse, so you have about an hour on either side to see Algol at its dimmest. Sky and Telescope Magazine also offers a “Minima of Algol” web page where you can find these times for any date. You do have to register for the site, but there is no charge.

Don’t be surprised if it makes you find it a bit creepy to see a star dimmer than usual! We’re very used to the starry sky seeming to be unchanging, so witnessing a change — as the ancients did when Algol dimmed and comets appeared — can lead to apprehension, as it did when our forebears named Algol for the demon!

The astronomers of the Adirondack Public Observatory will be happy to focus their telescopes on Algol, the star clusters in the Milky Way, galaxies beyond our own and other wonders from our Roll Off Roof Observatory (RORO) above Little Wolf Pond in Tupper Lake. Currently, we’re open to the public on the first and third Fridays of each month, weather permitting, of course. For updates and notices, check out our website at www.adirondackpublicobservatory.org and our Facebook page. On our public observing days, you can also call the RORO at 518-359-6317 to talk with one of our astronomers. Observing starts about one half-hour past sunset.