Corona Borealis
“The Northern Crown” · Northern · best around July evenings
Real star positions and magnitudes (HYG database via D3-Celestial, BSD-3). Lines are the conventional stick figure; north is up.
The story
Ariadne's wedding crown, flung into the sky by Dionysus — a perfect semicircle of seven stars. It hides two famous freaks: T CrB, the 'Blaze Star', a recurrent nova that erupts to naked-eye brightness every ~80 years (astronomers are watching for the next outburst now), and R CrB, a star that unpredictably soots itself over.
How to find it
The little arc between Arcturus and Hercules.
✦ What lives inside it
- T Coronae Borealis, the Blaze Star
- R Coronae Borealis
The deep dive
Researched for the Atlas from Wikipedia — Corona Borealis (20,154 characters read) · updated Sep 20, 2026
01 Alphecca: a crown jewel with a hidden twin
Alpha Coronae Borealis carries two common names — Alphecca, its IAU-approved title, and the older Gemma, Latin for jewel. What looks to the naked eye like a steady blue-white star of magnitude 2.2 is actually an Algol-type eclipsing binary: a hidden companion periodically crosses in front of the primary, dimming the combined light by 0.1 magnitude every 17.4 days. The primary is a white main-sequence star of spectral type A0V with 2.91 times the Sun's mass and 57 times its luminosity, encircled by a debris disk stretching to roughly 60 astronomical units — about the distance from the Sun to just beyond the outer edge of our own solar system. Its companion is a more modest yellow main-sequence star of spectral type G5V, slightly smaller than the Sun. Alphecca lies 75 light-years away and is thought to share a common motion through space with other members of the Ursa Major Moving Group, suggesting the stars formed together long ago from the same cloud of gas before drifting to widely separated corners of the galaxy.
02 The Blaze Star: a nova poised to erupt
T Coronae Borealis earns its nickname the Blaze Star through dramatic and sudden violence. For most of its existence it sits quietly at around magnitude 10, barely within reach of binoculars, but in a matter of hours it can surge to magnitude 2 — matching Alphecca, the constellation's brightest steady star — when accumulated hydrogen on the surface of its white dwarf companion ignites in a runaway nuclear reaction and explodes. The system is a binary pairing of a red giant primary and a white dwarf secondary orbiting each other roughly every eight months. Its first recorded outburst came in 1866, and its second in February 1946. When T Coronae Borealis began dimming in March 2023 — a pattern known to precede its nova events by about a year — astronomers anticipated a new eruption between mid-2024 and 2025. Among recurrent novae, it shares its restless category with T Pyxidis and U Scorpii. The outbursts do not destroy the star, making it one of the rare stellar objects known to repeat this extreme performance on a human-observable timescale.
03 R Coronae Borealis and the dust-dimmed giants Deeper
R Coronae Borealis stands apart from nearly every other variable star because its brightness does not follow a clock. Normally hovering near magnitude 6 — just at the naked-eye limit — it can plunge without warning as far as magnitude 15, a drop of nine magnitudes representing a factor of roughly 4,000 in perceived brightness, before slowly climbing back over months. The culprit is not pulsation but self-generated obscuration: the star periodically ejects clouds of dust that drift across our line of sight and block its light. Hubble Space Telescope imaging has revealed these dust clouds extending to a radius of around 2,000 astronomical units. Two distinct populations of dust grains are present: a fine component carried by the stellar wind, with grain diameters of just 5 nanometres, and a coarser ejected component with grain diameters near 0.14 micrometres. R Coronae Borealis is more than 7,000 light-years away and serves as the prototype for its entire class of variable stars — all of which are thought to be extremely hydrogen-deficient, likely born from the merger of two white dwarfs. It is a yellow supergiant, and its peculiar chemistry sets it at the far edge of stellar evolution.
04 Multiple-star complexity: six suns in one system Deeper
The constellation's best showcase of stellar multiplicity is ADS 9731, a system composed of six stars, two of which are themselves spectroscopic binaries — meaning their pairing is detected only through the periodic splitting of spectral lines rather than direct imaging. A close rival in complexity is Sigma Coronae Borealis, resolvable with small amateur telescopes, which turns out to be a layered gravitational family. At its heart, two stars each roughly as massive as the Sun orbit one another every 1.14 days. A third Sun-like star circles that inner pair over a much longer 726-year period. Separated by a staggering 14,000 astronomical units — more than 330 times the Sun-to-Neptune distance — a fourth and fifth star form a binary red dwarf system gravitationally tethered to the whole arrangement. Even the constellation's simpler doubles carry interest: Zeta Coronae Borealis shows two blue-white components 6.3 arcseconds apart, cleanly split at 100× magnification, while Nu Coronae Borealis is an optical double whose members merely share the same line of sight — one a red giant 640 light-years away, the other an orange giant an estimated 590 light-years distant, moving through space at different speeds.
05 Nusakan and Theta: rapid spinners and pulsators Deeper
Beta Coronae Borealis, named Nusakan, presents a spectroscopic binary whose two components are separated by about 10 astronomical units and complete one mutual orbit every 10.5 years. The brighter member is a rapidly oscillating Ap star — a chemically peculiar star with unusual magnetic field structure — that pulsates with a period of just 16.2 minutes, an extraordinarily short heartbeat for a star. With a surface temperature near 7,980 K, it has about 2.1 solar masses, 2.6 solar radii, and 25.3 times the Sun's luminosity. Its companion is cooler at about 6,750 K, carrying about 1.4 solar masses, 1.56 solar radii, and between 4 and 5 solar luminosities. Nearby Theta Coronae Borealis adds its own spectacle: with a combined magnitude of 4.13 and located 380 light-years away, its dominant component spins at approximately 393 kilometres per second — a rotation rate so extreme that the star's equator is measurably flattened. Classed as a Be star, it is surrounded by a debris disk. Such fast rotators likely shed mass into their disks continuously, making Theta an active and dynamically evolving object despite its seemingly unremarkable place in the crown's arc.
06 Delta Coronae Borealis crossing the Hertzsprung gap Deeper
At magnitude 4.06 and 170 light-years distant, Delta Coronae Borealis might seem an unremarkable yellow giant of spectral type G3.5III, but its current state reveals a star caught at a pivotal and brief moment of stellar evolution. For the vast majority of its life it burned as a blue-white main-sequence star of spectral type B. Once hydrogen fuel in its core was exhausted, the star's structure could no longer resist gravitational compression in the same way; it ballooned outward, swelling to 7.4 solar radii and shifting to a cooler yellow color. It now sits at a surface temperature of 5,180 K. Its luminosity and spectral characteristics indicate it has just crossed the Hertzsprung gap — the narrow and relatively short-lived transition region on the Hertzsprung-Russell diagram between the main sequence and the red giant branch — having recently finished core hydrogen burning and begun fusing hydrogen in a shell surrounding the spent core. Catching a star in this transition is relatively rare, as the crossing is astronomically rapid, making Delta Coronae Borealis a useful case study for stellar evolution models.
07 Abell 2065 and the Corona Borealis Supercluster
The constellation's most imposing deep-sky structure is invisible to all but the largest amateur instruments, yet its scale dwarfs anything in the nearby universe. Abell 2065 is a tightly packed galaxy cluster more than one billion light-years from Earth, containing over 400 member galaxies, the brightest of which glow at a faint 16th magnitude. It belongs to an even grander assembly: the Corona Borealis Supercluster, formed alongside Abell 2061, Abell 2067, Abell 2079, Abell 2089, and Abell 2092. Within the same constellation boundary sits Abell 2142, a cluster with a visual magnitude of 16.0 and a redshift of 0.0909, meaning it is racing away from us at 27,250 kilometres per second. It spans approximately six million light-years in diameter and is the product of an ongoing collision between two galaxy clusters, currently being merged by gravity and betrayed by its intense X-ray emission. At about 1.2 billion light-years distant, it and Abell 2065 sit in a region of the sky that, despite Corona Borealis covering a modest 179 square degrees, hosts some of the most massive gravitationally bound structures astronomers have identified.
08 Crown stories from cultures around the world
The semicircle of stars inspired strikingly different stories depending on where observers stood. In Welsh tradition the pattern was Caer Arianrhod, the Castle of the Silver Circle, celestial home of the Lady Arianrhod. To the ancient Balts it was Darželis, a flower garden. Arab astronomers called the whole constellation Alphecca — meaning separated or broken up — picturing a loose string of jewels or a broken dish; Bedouin peoples knew it as qaṣʿat al-masākīn, the bowl of the poor. In Mesopotamia the constellation was linked to the goddess Nanaya. Aboriginal Australians of the Wailwun people of northwestern New South Wales read it as mullion wollai, an eagle's nest. The Wardaman people of northern Australia regarded it as a gathering point where Men's Law, Women's Law, and Law of both sexes convened to consider matters of existence. The Skidi people of Native America saw a council of stars with Polaris as chief, and also interpreted the shape as the smokehole over a fireplace through which messages rose to the gods. The Shawnee saw the Heavenly Sisters descending nightly to dance, with Alphecca representing the youngest sister. The Mi'kmaq of eastern Canada viewed the stars as the den of a celestial bear. Polynesian navigators across the Pacific recognized the pattern under several local names, including Na Kaua-ki-tokerau in the Tuamotus.
09 Ariadne's crown: the Greek myth in depth
Classical Greece knew more than one version of how the northern crown reached the sky. The dominant account ties it to Ariadne, daughter of King Minos of Crete, who helped the Athenian hero Theseus navigate the labyrinth and slay the Minotaur. After Theseus abandoned Ariadne on Naxos, the god Dionysus took her as his bride, and at their wedding placed her crown among the stars as an eternal commemoration. A variant gives the crown a more active role in the myth: Dionysus, who had given the crown to Ariadne, saw Theseus use its light to find his way out of the labyrinth after killing the Minotaur. A third account, drawn from the De astronomia attributed to Hyginus, shifts the crown to Bacchus himself, who wore it as a disguise when approaching Mount Olympus to reveal his divine identity to the gods — concealing that he was yet another product of Jupiter's relationship with a mortal, in his case Semele. Hyginus also noted Corona Borealis's position next to Hercules and Lyra; one tradition ascribed Hercules to Theseus, and Lyra to his lyre, suggesting all three constellations may have been conceived as a connected group of myth-figures placed in the heavens together.
10 TY and UW CrB: white dwarfs and neutron stars Deeper
Corona Borealis contains two compact-object binary systems that test the limits of stellar physics. TY Coronae Borealis is a pulsating white dwarf of the ZZ Ceti class — a type whose surface layers ring like a bell — and its physical proportions are extreme: it has roughly 70 percent of the Sun's mass yet squeezes into only 1.1 percent of the Sun's diameter, making it only slightly larger than Earth. UW Coronae Borealis, discovered in 1990, goes further still. It is a low-mass X-ray binary composed of a star less massive than the Sun paired with a neutron star, the dense collapsed remnant of a massive star's supernova. An accretion disk surrounds the neutron star, funnelling material stripped from the companion. The system's brightness varies in an unusually tangled pattern: the two stars orbit each other every 111 minutes, yet a second cycle of 112.6 minutes traces the orbit of the accretion disk around the neutron star. The interaction between these two near-equal periods produces a beat period of 5.5 days — the time required for the asymmetric, precessing accretion disk to complete one full rotation around the neutron star, adding another rhythm to an already complex variable-brightness signature.
11 Exoplanets: five systems, one hot Jupiter
Five stellar systems inside Corona Borealis are confirmed hosts of planets, most detected through the radial velocity method, which tracks the subtle wobble a planet induces in its star's light. Epsilon Coronae Borealis, an orange giant of 1.7 solar masses swollen to 21 solar radii, hosts a planet about 6.7 times Jupiter's mass taking 418 days to complete one orbit at a distance of roughly 1.3 astronomical units — a slow, wide journey around an already enormous star. Kappa Coronae Borealis, an orange subgiant nearly twice the Sun's mass, carries both a debris disk and a confirmed planet of about 2.5 Jupiter masses on a 3.4-year orbit; the disk's dimensions hint at a possible second unseen companion. Omicron Coronae Borealis holds one of the two least massive planets yet found orbiting a clump giant, at just 0.83 Jupiter masses and a 187-day period. The most dramatically different is XO-1b, found in 2006 by the transit method using the XO Telescope. It is a hot Jupiter — roughly Jupiter-sized — circling a Sun-like star of spectral type G1V, about 560 light-years away, in just three days, far too close for any hope of a temperate environment.
12 The constellation's boundaries and modern identity
Corona Borealis covers 179 square degrees of sky, placing it 73rd out of 88 IAU constellations by area — compact enough that its entire arc fits easily in a binocular field. Belgian astronomer Eugène Delporte drew its formal eight-sided boundary in 1930, defining the limits by right ascension between 15 hours 16.0 minutes and 16 hours 25.1 minutes and by declination between 25.54° and 39.71° north. The IAU adopted its three-letter abbreviation CrB in 1922. Every observer north of latitude 50° south can see the whole constellation, making it accessible from Europe, most of North America, all of Asia, and much of the Southern Hemisphere. Within its borders astronomers count 37 stars brighter than or equal to apparent magnitude 6.5 — the practical limit of the unaided eye under good skies. Johann Bayer assigned Greek letters from Alpha through Upsilon to twenty of those stars in his 1603 Uranometria, a scheme that remains in use. It is one of the 48 constellations catalogued by Ptolemy in the 2nd century Almagest and has retained continuous recognition ever since, making it one of the oldest formally documented star patterns in the Western tradition.