Jimmy Westlake: From crown to crown across the sky
July 20, 2011
Steamboat Springs — There are two starry crowns that twinkle in our summer sky, one in the north and one in the south. Although each is a tiny constellation, their shapes are so distinctive that locating them is a snap. They lie nearly 90 degrees apart in the sky, on opposite banks of the river of stars we see as the Milky Way arching across our summer sky.
The Northern Crown, called Corona Borealis, represents the golden, star-studded crown of Greek Princess Ariadne. To locate Corona Borealis, look straight overhead at about 10 p.m. for a small half-circle of stars, like a letter C. It's about one-third of the way from the bright star Arcturus toward the comparably bright star Vega to the east. A person with normal vision should be able to see seven glittering stars outlining the celestial crown. The brightest of the seven is a star known by two different names: Alphecca, meaning the broken circle, and Gemma, meaning the jewel of the crown.
Corona Borealis has within its borders one of the most unusual stars known, a "reverse nova" named R Coronae Borealis. Normally a sixth-magnitude star just at the limit of naked-eye visibility, it will occasionally fade to only 1/1,600th its normal brightness before slowly recovering. Astronomers think this peculiar behavior is because of the occasional buildup of carbon soot in the star's atmosphere. R Coronae Borealis bears constant watching because its light variations are completely unpredictable. Its most recent episode of fading happened in 2003. You can find it with binoculars near the center of the C pattern of Ariadne's crown.
The Southern Crown, called Corona Australis, is a little more challenging to locate than its northern counterpart. This is because of its far southern location and its fainter stars. But again, its distinctive semicircular shape will help you find it. This crown belonged to Sagittarius, the centaur-archer also pictured in our summer sky. In fact, Corona Australis can be found just beneath the familiar "Teapot" asterism of Sagittarius. Although it has completely risen by 10 p.m. in mid-July, it is easier to spot at about midnight when it has risen higher in the sky. You'll need an unobstructed view of the southern horizon to check this constellation off your list. At its highest, it is only about a fist-width at arm's length above the southern horizon. In sticking with the whole "teapot" story, you can think of the semicircular pattern of Corona Australis as the slice of lemon awaiting the cup of hot tea.
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 around the world. Check out Jimmy's website at http://www.jwestlake.com.