Corona Borealis, or the Northern Crown, is a small but lovely semicircular constellation just to the east of Böotes, the Herdsman. In the months of May and June, shortly after sunset, the constellation lies nearly overhead for northern-hemisphere observers, and well over the northern horizon for southern stargazers. It’s one of the oldest constellations, and one of the few that vividly resembles its name.
The constellation takes its name from the crown, in Greek legend, given to the maiden Ariadne, the daughter of King Minos of Crete. Ariadne had been abandoned on the island of Naxos by Theseus, the legendary hero who slew the Minotaur. The god Dionysus rescued the maiden, fell in love with her, and gave her a jeweled crown forged by Hephaestus, god of the forge. Ariadne and Dionysus had a happy life together. But she was mortal and eventually died. Dionysus placed her crown in the heavens to remember her.
Since they form a remarkably distinctive pattern, many other cultures have associated legends with these stars. The Australian aboriginals called the constellation the Boomerang. The Pawnee of the American Plains beheld these stars as a council of celestial elders. And the Mi’kmaq of eastern Canada saw the constellation as a den for a celestial bear represented by the stars of what we now call Ursa Major.
It takes little imagination to see a crown here: the little semi-circle of stars, a little less than a fist-width in diameter, is visible in all but the most light-polluted skies. Corona Borealis was listed in Ptolemy’s original atlas of 48 constellations in the first century A.D. The constellation’s brightest star, 2nd-magnitude Alphecca, marks the jewel in the crown. Some maps list Alphecca’s name as Gemma, the Jewel. The other stars in the crown range from 3rd to 5th approximately. Alphecca is an eclipsing binary star that varies in brightness by just 0.1 magnitude every 17.4 days. Both components are workaday main sequence stars like our Sun. The pair is likely part of the Ursa Major Moving Group, a nearby association of stars that likely formed together and which include most of the stars of the Big Dipper. Alphecca is about 75 light years away.
Corona Borealis does have a few pleasing double stars that make for excellent viewing in binoculars and smaller telescopes, and the whole constellation is very beautiful in a pair of binoculars which reveal many chance pairings of colorful stars. Alphecca is attended by a gaggle of 8th and 9th-magnitude stars, an assembly that is quite lovely in binoculars. Off the tip of the crown, halfway to Hercules, look for the pair of orange stars that make up the star ν (nu) Coronae Borealis. The pair is split by 360″, wide enough for sharp-eyed stargazers to resolve without optics. For telescopists, perhaps the prettiest double star in these parts is zeta (ζ) CrB, a blue-white and greenish-white double. The spacing is a little tight, about 6.3”, but still resolvable at moderate magnification in any telescope on all but the most turbulent nights. The 5th and 6th magnitude pair lies on the northern edge of the constellation.
Corona Borealis holds little in the way of bright deep-sky objects. But if you have a 20″ or larger telescope tucked away in your closest, you might spy the brighter members of a distant cluster of galaxies, the Corona Borealis Cluster, itself the anchor of the immense “Cor Bor Supercluster”. These extremely distant galaxies are more than a billion light years away and, from our skies, they shine at 16th magnitude and fainter.