Ophiuchus ( o-fee-YOO-kəs) is a large constellation straddling the celestial equator. Its name comes from the Ancient Greek ὀφιοῦχος (ophioûkhos), meaning "serpent-bearer", and it is commonly represented as a man grasping a snake. The serpent is represented by the constellation Serpens. Ophiuchus was one of the 48 constellations listed by the 2nd-century astronomer Ptolemy, and it remains one of the 88 modern constellations. An old alternative name for the constellation was Serpentarius.
Characteristics
Ophiuchus lies between Aquila, Serpens, Scorpius, Sagittarius, and Hercules, northwest of the center of the Milky Way. The southern part lies between Scorpius to the west and Sagittarius to the east. In the Northern Hemisphere, it is best visible in summer. It is opposite of Orion. Ophiuchus is depicted as a man grasping a serpent; the interposition of his body divides the snake constellation Serpens into two parts, Serpens Caput and Serpens Cauda. Ophiuchus straddles the equator with the majority of its area lying in the Southern Hemisphere. Rasalhague, its brightest star, lies near the northern edge of Ophiuchus at about +12° 30′ declination. The constellation extends southward to −30° declination. Segments of the ecliptic within Ophiuchus are south of −20° declination (see chart at right). In contrast to Orion, from November to January (summer in the Southern Hemisphere, winter in the Northern Hemisphere), Ophiuchus is in the daytime sky and thus not visible at most latitudes. However, for much of the polar region north of the Arctic Circle in the Northern Hemisphere's winter months, the Sun is below the horizon even at midday. Stars (and thus parts of Ophiuchus, especially Rasalhague) are then visible at twilight for a few hours around local noon, low in the south. In the Northern Hemisphere's spring and summer months, when Ophiuchus is normally visible in the night sky, the constellation is actually not visible, because the midnight sun obscures the stars at those times and places in the Arctic. In countries close to the equator, Ophiuchus appears overhead in June around midnight and in the October evening sky.
Features
Stars
The brightest stars in Ophiuchus include α Ophiuchi, called Rasalhague ("head of the serpent charmer"), at magnitude 2.07, and η Ophiuchi, known as Sabik ("the preceding one"), at magnitude 2.43. Alpha Ophiuchi is composed of an A-type (bluish-white) giant star and a K-type main sequence star. The primary is a rapid rotator with an inclined axis of rotation. Eta Ophiuchi is a binary system. Other bright stars in the constellation include β Ophiuchi, Cebalrai ("dog of the shepherd") and λ Ophiuchi, or Marfik ("the elbow"). Beta Ophiuchi is an evolved red giant star that is slightly more massive than the Sun. Lambda Ophiuchi is a binary star system with the primary being more massive and luminous than the Sun. RS Ophiuchi is part of a class called recurrent novae, whose brightness increase at irregular intervals by hundreds of times in a period of just a few days. It is thought to be at the brink of becoming a Type Ia supernova. It erupts around every 15 years and usually has a magnitude of around 5.0 during eruptions, most recently in 2021. Barnard's Star, one of the nearest stars to the Solar System (the only stars closer are the Alpha Centauri binary star system and Proxima Centauri), lies in Ophiuchus. It is located to the left of β and just north of the V-shaped group of stars in an area that was once occupied by the now-obsolete constellation of Taurus Poniatovii (Poniatowski's Bull). The star has historically been subject to multiple claims of planets that were later disproven, until a confirmation of four planets in 2025. In 1998, an intense flare was observed. The star has also been a target of plans for interstellar travel such as Project Daedalus. In 2005, astronomers using data from the Green Bank Telescope discovered a superbubble so large that it extends beyond the plane of the galaxy. It is called the Ophiuchus Superbubble. In April 2007, astronomers announced that the Swedish-built Odin satellite had made the first detection of clouds of molecular oxygen in space, following observations in the constellation Ophiuchus. The supernova of 1604 was first observed on 9 October 1604, near θ Ophiuchi. Johannes Kepler saw it first on 16 October and studied it so extensively that the supernova was subsequently called Kepler's Supernova. He published his findings in a book titled De stella nova in pede Serpentarii (On the New Star in Ophiuchus's Foot). Galileo used its brief appearance to counter the Aristotelian dogma that the heavens are changeless. It was a Type Ia supernova and the most recent Milky Way supernova visible to the unaided eye. In 2009 it was announced that GJ 1214, a star in Ophiuchus, undergoes repeated, cyclical dimming with a period of about 1.5 days consistent with the transit of a small orbiting planet. The planet's low density (about 40% that of Earth) suggests that the planet might have a substantial component of low-density gas—possibly hydrogen or steam. The proximity of this star to Earth (42 light years) makes it a feasible target for further observations. The host star emits X-rays which could have removed mass from the exoplanet. In April 2010, the naked-eye star ζ Ophiuchi was occulted by the asteroid 824 Anastasia.
Deep-sky objects
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