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 throughout the course of centuries, millennia and even eons. But a few do not. These are the variable stars whose light output can change in a matter of minutes or months. Our fall sky holds two of the most spectacular variable stars known to astronomers.
The 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 B.C. 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, a name that translates to “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 three stars that form Orion’s Belt. When brightest, as it is now, Mira also is at its largest — more than 700 times larger than the sun.
Red giants such as 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 is visible to the unaided eye in our eastern sky at about 10 p.m. as it brightens toward its maximum light, expected around mid-October.
The second variable star easily visible this fall is Algol, “the Demon Star.” Algol represents the eye of Medusa. Medusa’s evil gaze 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 typical 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 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 below the familiar W-shaped pattern of Cassiopeia, high in the northeastern sky on fall evenings. Algol will be in mid-eclipse as darkness falls Wednesday, so you can watch it brighten back to maximum light during that evening. The next favorable evening eclipse occurs Oct. 23, with mid-eclipse at 10:10 p.m. Even the full Hunter’s Moon that evening will be no match for the Demon Star. Start watching Algol several hours before mid-eclipse and then watch it slowly fade compared to the stars around it. Spooky!