Jimmy Westlake's Celestial News column appears monthly in the Steamboat Today.
Find more columns by Westlake here.
Most stars shine with a constant brightness in our sky over the eons of time, however, a few stars do not. These are the variable stars whose light output can change in a matter of minutes or months. Our autumn sky holds two of the most spectacular variable stars known to astronomers and both can be observed with nothing more than your naked eyes.
First is Mira, also known as Omicron Ceti. Mira’s variability was suspected by astronomers throughout the centuries, all the way back to the Greek astronomer Hipparchus in 134 BC. But it was an amateur astronomer, David Fabricius, who in 1596 confirmed that Mira is a variable star. Soon after, the great astronomer Johann Hevelius coined the name Mira for this star in 1662, a name that translates as “the Wonderful Star.”
Modern astronomers recognize Mira as the prototype long-period, red-giant variable star. At its faintest, Mira fades to magnitude 10, far below naked-eye visibility, but at its brightest, it can shine as a second magnitude star, similar to the stars that form Orion’s Belt or the Big Dipper. At maximum light, Mira is nearly 1,600 times brighter than it is at minimum light. Beating like a cosmic heart, Mira slowly pulsates during a period of 332 days. When brightest, as it is now, Mira also is at its largest — more than 700 times larger than our sun. Red giant stars like Mira are near the ends of their lives and grow unstable. Eventually, Mira will blow away its bloated, outer atmosphere, and expose its burned-out core, destined to become a white dwarf star. Mira reached its maximum brightness last month but still is visible to the naked eye in the eastern sky at about 11 p.m. in mid-September. Catch it now before it disappears until next year.
The second variable star easily visible this fall is Algol, “the Demon Star.” Algol represents the eye of Medusa, whose severed head is held up in the sky by her slayer, the Greek hero Perseus. Medusa’s evil gaze could turn you into stone, if you were foolish enough to glance her way. Perseus avoided this fate by looking only at her reflection in his mirror-like shield.
About every three days, Algol fades to one-third of its normal brightness for two hours and then returns to its original luster. It’s as if Medusa is winking her evil eye at us.
Horrifying to early sky watchers, Algol’s periodic winking no longer is a mystery. Algol actually is a pair of stars, almost in contact as they whirl around each other. When the fainter star eclipses the brighter star, Algol dims for a few hours. Algol was the first eclipsing binary star discovered and continues to be the most famous.
Look for the Demon Star below the familiar W-shaped pattern of Cassiopeia, high in the northeastern sky on September evenings. Algol will be in mid-eclipse at 5:57 a.m. Tuesday, at 2:46 a.m. Friday, at 11:34 p.m. Sunday and at 8:23 p.m. Sept. 18. Start watching Algol several hours before mid-eclipse and then watch it slowly fade compared to the stars around it. Spooky!
Professor Jimmy Westlake teaches astronomy and physics at Colorado Mountain College's Alpine Campus. He is an avid astronomer whose photographs and articles have been published across the world. His "Celestial News" column appears weekly in the Steamboat Today newspaper and his "Cosmic Moment" radio spots can be heard on local radio station KFMU. Also, check out Westlake’s astrophotography website at www.jwestlake.com.