Corona Borealis: The Northern Crown
Corona Borealis is one of the most elegant small constellations in the sky. Its distinctive semicircular arc of stars genuinely resembles a crown or tiara, making it one of the few constellations whose shape matches its name convincingly. Though modest in size, Corona Borealis contains fascinating variable stars, an intriguing galaxy cluster, and one of the most romantic stories in Greek mythology. Locate this celestial crown with StarGlobe by looking between Arcturus and Vega on spring and summer evenings.
Finding Corona Borealis
Corona Borealis is easy to locate once you know its neighbors. Find Arcturus in Bootes (use the "arc to Arcturus" technique from the Big Dipper's handle), then look about 20 degrees to the east-northeast. You will see a neat semicircle of seven stars curving gracefully between Bootes and Hercules. The constellation covers only 179 square degrees, making it the 73rd-largest, but its compact shape makes it instantly recognizable.
The crown is best seen from April through September, reaching its highest point in the evening sky around June. From mid-northern latitudes, it sits at a comfortable altitude and is visible for most of the night during summer months. Our summer sky guide places it in context with surrounding constellations.
Alphecca: The Jewel of the Crown
Alphecca (Alpha Coronae Borealis), also known as Gemma, is the brightest star in the constellation at magnitude 2.2. The name Alphecca derives from Arabic and refers to the bright one of the broken ring, while Gemma is Latin for "jewel." Both names suit a star that serves as the centerpiece of a crown.
Alphecca is an A0 main-sequence star located about 75 light-years from Earth. It is actually an eclipsing binary system, though the brightness variations are extremely small (about 0.1 magnitude over a 17.4-day period) and undetectable without precise instruments. The system consists of a bright primary about 2.6 times the Sun's mass and a fainter companion. Alphecca also has an infrared excess similar to Vega, suggesting a circumstellar debris disk.
Remarkable Variable Stars
Corona Borealis hosts two of the most unusual variable stars known. R Coronae Borealis is the prototype of an entire class of variables called R CrB stars. Normally it shines at about magnitude 5.8, faintly visible to the naked eye. At unpredictable intervals, sometimes years apart, it can suddenly fade dramatically, dropping to magnitude 14 or fainter before slowly recovering. These fading episodes are caused by clouds of carbon dust that condense in the star's atmosphere and temporarily block its light. Monitoring R CrB is a popular project among variable star observers.
T Coronae Borealis, known as the Blaze Star, is equally fascinating but for the opposite reason. It is a recurrent nova, normally sitting at a faint magnitude 10 but capable of suddenly brightening to magnitude 2, making it easily visible to the naked eye. The last recorded eruption occurred in 1946, and astronomers have been watching it closely, as historical records suggest eruptions roughly every 80 years. The nova outbursts occur because T CrB is a binary system in which a white dwarf accumulates hydrogen from a red giant companion until the pressure triggers a thermonuclear explosion on the white dwarf's surface.
Deep Sky Objects
Corona Borealis contains Abell 2065, also known as the Corona Borealis Galaxy Cluster. This is a rich collection of over 400 galaxies located approximately one billion light-years from Earth. Individual galaxies in the cluster are faint, requiring large amateur telescopes to detect, but the cluster represents one of the densest galaxy concentrations in the observable universe at its distance.
For most observers, the constellation's appeal lies in its stars rather than deep sky objects. The crown shape itself, with its clean arc of stars against a relatively empty background, is a beautiful sight in binoculars.
Mythology: Ariadne's Crown
The most widely told myth behind Corona Borealis involves Ariadne, the daughter of King Minos of Crete. Ariadne helped the hero Theseus navigate the Labyrinth to slay the Minotaur by giving him a ball of thread to unwind as he went, allowing him to find his way back out. Theseus promised to marry Ariadne, but he later abandoned her on the island of Naxos.
The god Dionysus found Ariadne on Naxos and fell in love with her. He gave her a beautiful golden crown studded with jewels as a wedding gift. When Ariadne died, Dionysus threw the crown into the sky, where it became the constellation Corona Borealis. The stars are said to represent the gems that adorned the crown, with Alphecca as the brightest jewel. This poignant story of loss, redemption, and immortalization is one of the most emotionally resonant tales attached to any constellation. Find more stories like this in our constellation mythology article.
Neighboring Constellations
Corona Borealis nestles perfectly between Bootes to the west and Hercules to the east. This makes it a natural stepping stone when star-hopping across the spring and summer sky. After identifying the Spring Arc to Arcturus, continue eastward past the kite of Bootes to find the crown, then push onward to the Keystone of Hercules.
To the north lies the long, winding body of Draco, and to the south is Serpens Caput, the head of the serpent held by Ophiuchus. Corona Borealis acts as a compact, memorable landmark in a region of sky filled with larger but less distinctive star patterns.
Observing Tips
The crown is best appreciated from suburban or rural skies where all seven main stars are visible. Under heavy light pollution, only Alphecca and one or two other stars may be apparent. Binoculars are helpful for confirming the arc shape and for searching for R Coronae Borealis during one of its fading events.
Corona Borealis is a wonderful constellation for beginners because its shape is intuitive and satisfying to identify. Use StarGlobe to point out the crown in the sky, and keep an eye on the Blaze Star, T CrB, in case it decides to put on a spectacular show visible to the naked eye.
