Jimmy Westlake's Celestial News column appears monthly in the Steamboat Today.
Find more columns by Westlake here.
Steamboat Springs Most stars shine with a constant brightness in our sky throughout timescales of centuries, millennia and even eons. But it’s the stars that don’t shine steady that are among the most interesting. These are the variable stars whose brightness 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 unaided eyes this month.
First up is Mira, also called Omicron Ceti. Sky-watchers suspected Mira’s variability throughout the centuries, but it was amateur astronomer David Fabricius who confirmed in 1596 that Mira is a variable star. In 1662, the great astronomer Johann Hevelius coined the name Mira for this star, a name that translates to “the Wonderful Star.”
Modern astronomers recognize Mira as the prime example of a long-period red giant variable star. At its brightest, Mira can shine as a second-magnitude star, similar to the stars that form the Big Dipper. Then, it slowly fades and disappears from view as it drops to magnitude 10. Mira shines nearly 1,600 times brighter in our sky when at maximum light compared to minimum light. Throbbing 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 grow unstable as they near the ends of their lives. Eventually, Mira will shed its bloated, outer layers and expose its burned-out core, destined to become a white dwarf star.
Mira is near maximum brightness and is visible to the unaided eye in our eastern sky around 10 p.m. Look for it in the constellation Cetus the Whale, about one hand-span to the lower right of the dazzling planet Jupiter. Better catch it now before it fades into obscurity for another year.
The second variable star easily visible this fall is Algol, “the Demon Star.” Algol represents the eye of Medusa, a mythological demon whose severed head is held up in the sky by her slayer, the Greek hero Perseus. Medusa’s evil stare could turn you into stone if you were unfortunate 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 second-magnitude luster. It’s as if Medusa is winking her evil eye at us.
Horrifying to early sky-watchers, Algol’s periodic winking is no longer a mystery. Algol is actually 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 in the constellation Perseus, high in the northeastern sky on autumn evenings and about two hand-spans to the left of Jupiter. Algol will be in mid-eclipse at 5:04 a.m. Thursday; 1:53 a.m. Sunday; 10:42 p.m. Oct. 25; and 7:30 p.m. Oct. 28. Start watching Algol a few hours before mid-eclipse and then watch it slowly fade compared to the stars around it. Spooky!
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 all acorss the world. Check out Westlake’s astrophotography website at www.jwestlake.com.