Corona Borealis, the Northern Crown, is a small but distinctive constellation whose stars form a graceful semicircle resembling a jeweled crown. Unlike many constellations, its shape is easy to recognize, making it a useful landmark in the spring and summer sky.
Covering approximately 179 square degrees, Corona Borealis is the 73rd-largest of the 88 recognized constellations. Its brightest star, Alphecca, marks the central jewel of the crown. The constellation is also home to two remarkable variable stars, T Coronae Borealis and R Coronae Borealis, along with several distant galaxy clusters.
Where and When to Find Corona Borealis
Corona Borealis is best observed from the Northern Hemisphere between April and July, when it appears high in the evening sky. The constellation lies between Boötes to the west and Hercules to the east, with Serpens to the south.
To find it, first locate Arcturus, the bright orange star in Boötes. Look eastward toward Hercules for a small arc of stars forming an unmistakable crown. The brightest star, Alphecca, shines at magnitude +2.2 and marks the center of the semicircle. The remaining stars are mostly third- and fourth-magnitude objects, making the pattern easier to recognize under reasonably dark skies.
The Northern Crown is an excellent naked-eye target and requires no telescope.
Mythology — The Crown of Ariadne
In Greek mythology, Corona Borealis represents the crown of Ariadne, daughter of King Minos of Crete. Ariadne helped the hero Theseus defeat the Minotaur by giving him a thread to find his way out of the labyrinth. Although Theseus initially promised to marry her, he later abandoned Ariadne on the island of Naxos. The god Dionysus subsequently discovered Ariadne, fell in love with her, and made her his wife.
According to one version of the story, Dionysus placed her jeweled crown among the stars, where its brilliant gems became the constellation Corona Borealis. The crown’s distinctive semicircular shape has made this one of the more recognizable mythological patterns in the night sky.
Notable Stars
- Alphecca (Alpha Coronae Borealis) is the brightest star in the constellation, shining at magnitude +2.2 from approximately 75 light-years away. Also known as Gemma, meaning “jewel,” it is a binary system whose primary component is a white main-sequence star. Its position at the center of the crown makes it an easy star to identify.
- Nusakan (Beta Coronae Borealis) is the constellation’s second-brightest star, with an apparent magnitude of approximately +3.7. Located around 110 light-years away, it is a binary system whose primary star has unusual chemical abundances and a strong magnetic field.
- Gamma Coronae Borealis is a binary system approximately 170 light-years away, shining at around magnitude +3.8. It forms part of the Northern Crown’s curved outline.
T Coronae Borealis (T CrB) is one of the most remarkable variable stars in the sky. Normally too faint to see without a telescope, it is a recurrent nova system consisting of a white dwarf and a red giant star orbiting one another. The white dwarf draws gas from its companion. As this material accumulates on the white dwarf’s surface, it can eventually undergo a thermonuclear explosion, causing the system to brighten dramatically.
T CrB experienced recorded eruptions in 1866 and 1946, reaching approximately second magnitude and becoming easily visible without optical aid. Astronomers are monitoring the system for its next eruption, but the exact timing cannot be predicted reliably. When it erupts, the Blaze Star may briefly appear as a bright new point of light near the familiar Northern Crown.
R Coronae Borealis (R CrB) behaves almost opposite to the Blaze Star. Instead of suddenly brightening, this unusual variable star can fade dramatically and unpredictably. R CrB is a rare hydrogen-deficient, carbon-rich star that periodically produces clouds of carbon dust. When these clouds obscure the star from our viewpoint, its apparent brightness can drop by several magnitudes.
At its brightest, R CrB is close to naked-eye visibility under dark skies, but during deep fading episodes it may require a substantial telescope to detect. The star is the prototype of the R Coronae Borealis class of variable stars.
Notable Deep-Sky Objects
Corona Borealis contains no Messier objects and relatively few bright deep-sky targets. However, it lies in the direction of some remarkable distant galaxy clusters.
Corona Borealis Galaxy Cluster (Abell 2065)
Abell 2065, sometimes called the Corona Borealis Galaxy Cluster, is a distant concentration of hundreds of galaxies approximately one billion light-years away. It is part of the much larger Corona Borealis Supercluster, a collection of galaxy clusters occupying an enormous region of space.
Abell 2065 contains many faint galaxies, with even its brightest members requiring substantial telescopes and dark skies to observe. Unlike the nearby stars forming the Northern Crown, these galaxies lie at distances so vast that their light has traveled for roughly a billion years to reach Earth.
The cluster is primarily an advanced observing and astrophotography target rather than an object suitable for small telescopes.
Abell 2142
Another notable object is Abell 2142, a massive galaxy cluster located approximately 1.2 billion light-years away. X-ray observations reveal extremely hot gas within the cluster, providing astronomers with evidence of its complex history and interactions between galaxy groups. Its galaxies are far too faint for ordinary binocular observing.
Observing Corona Borealis
Corona Borealis is best appreciated first with the naked eye. Find Arcturus in Boötes, then look toward Hercules for the distinctive arc of the Northern Crown. Alphecca marks the brightest jewel in the pattern. Observers interested in variable stars can follow T Coronae Borealis and R Coronae Borealis using comparison-star charts and brightness measurements.
Although the constellation has few bright deep-sky objects, its recognizable shape, fascinating mythology, and unusual variable stars make it one of the most interesting small constellations of the northern sky.