Ophiuchus
Ophiuchus is a large constellation straddling the celestial equator. Its name is from the Greek Ὀφιοῦχος, 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. It was formerly referred to as Serpentarius.
Location
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 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 declination. The constellation extends southward to −30° declination. Segments of the ecliptic within Ophiuchus are south of −20° declination.In contrast to Orion, from November to January, Ophiuchus is in the daytime sky and thus not visible at most latitudes. However, for much of the Arctic Circle in the Northern Hemisphere's winter months, the Sun is below the horizon even at midday. Stars 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, at magnitude 2.07, and η Ophiuchi, known as Sabik, at magnitude 2.43. Other bright stars in the constellation include β Ophiuchi, Cebalrai and λ Ophiuchi, or Marfik.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-1a supernova. Barnard's Star, one of the nearest stars to the Solar System, 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. 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. Galileo used its brief appearance to counter the Aristotelian dogma that the heavens are changeless. 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 suggests that the planet may have a substantial component of low-density gas—possibly hydrogen or steam. The proximity of this star to Earth makes it a tempting target for further observations. In April 2010, the naked-eye star ζ Ophiuchi was occulted by the asteroid 824 Anastasia.
Deep-sky objects
Ophiuchus contains several star clusters, such as IC 4665, NGC 6633, M9, M10, M12, M14, M19, M62, and M107, as well as the nebula IC 4603-4604.M10 is a fairly close globular cluster, only 20,000 light-years from Earth. It has a magnitude of 6.6 and is a Shapley class VII cluster. This means that it has "intermediate" concentration; it is only somewhat concentrated towards its center.
The unusual galaxy merger remnant and starburst galaxy NGC 6240 is also in Ophiuchus. At a distance of 400 million light-years, this "butterfly-shaped" galaxy has two supermassive black holes 3,000 light-years apart. Confirmation of the fact that both nuclei contain black holes was obtained by spectra from the Chandra X-ray Observatory. Astronomers estimate that the black holes will merge in another billion years. NGC 6240 also has an unusually high rate of star formation, classifying it as a starburst galaxy. This is likely due to the heat generated by the orbiting black holes and the aftermath of the collision.
In 2006, a new nearby star cluster was discovered associated with the 4th magnitude star Mu Ophiuchi. The Mamajek 2 cluster appears to be a poor cluster remnant analogous to the Ursa Major Moving Group, but 7 times more distant. Mamajek 2 appears to have formed in the same star-forming complex as the NGC 2516 cluster roughly 135 million years ago.
Barnard 68 is a large dark nebula, located 410 light-years from Earth. Despite its diameter of 0.4 light-years, Barnard 68 only has twice the mass of the Sun, making it both very diffuse and very cold, with a temperature of about 16 kelvins. Though it is currently stable, Barnard 68 will eventually collapse, inciting the process of star formation. One unusual feature of Barnard 68 is its vibrations, which have a period of 250,000 years. Astronomers speculate that this phenomenon is caused by the shock wave from a supernova.
The space probe Voyager 1, the furthest man-made object from earth, is traveling in the direction of Ophiuchus. It is located between α Herculis, α and κ Ophiuchi at right ascension 17h 13m and declination +12° 25’.
History and mythology
There is no evidence of the constellation preceding the classical era, and in Babylonian astronomy, a "Sitting Gods" constellation seems to have been located in the general area of Ophiuchus. However, Gavin White proposes that Ophiuchus may in fact be remotely descended from this Babylonian constellation, representing Nirah, a serpent-god who was sometimes depicted with his upper half human but with serpents for legs.The earliest mention of the constellation is in Aratus, informed by the lost catalogue of Eudoxus of Cnidus :
To the ancient Greeks, the constellation represented the god Apollo struggling with a huge snake that guarded the Oracle of Delphi.
Later myths identified Ophiuchus with Laocoön, the Trojan priest of Poseidon, who warned his fellow Trojans about the Trojan Horse and was later slain by a pair of sea serpents sent by the gods to punish him. According to Roman era mythography, the figure represents the healer Asclepius, who learned the secrets of keeping death at bay after observing one serpent bringing another healing herbs. To prevent the entire human race from becoming immortal under Asclepius' care, Jupiter killed him with a bolt of lightning, but later placed his image in the heavens to honor his good works. In medieval Islamic astronomy, the constellation was known as Al-Ḥawwa', "the snake-charmer".
Aratus describes Ophiuchus as trampling on Scorpius with his feet. This is depicted in Renaissance to Early Modern star charts, beginning with Albrecht Dürer in 1515; in some depictions, Scorpius also seems to threaten to sting Serpentarius in the foot. This is consistent with Azophi, who already included ψ Oph and ω Oph as the snake-charmer's "left foot", and θ Oph and ο Oph as his "right foot", making Ophiuchus a zodiacal constellation at least as regards his feet. This arrangement has been taken as symbolic in later literature and placed in relation to the words spoken by God to the serpent in the Garden of Eden.
Zodiac
Ophiuchus is one of thirteen constellations that cross the ecliptic. It has therefore been called the "13th sign of the zodiac". However, this confuses sign with constellation. The signs of the zodiac are a twelve-fold division of the ecliptic, so that each sign spans 30° of celestial longitude, approximately the distance the Sun travels in a month, and are aligned with the seasons so that the March equinox always falls on the boundary between Pisces and Aries.Constellations, on the other hand, are unequal in size and are based on the positions of the stars. The constellations of the zodiac have only a loose association with the signs of the zodiac, and do not in general coincide with them. In Western astrology the constellation of Aquarius, for example, largely corresponds to the sign of Pisces. Similarly, the constellation of Ophiuchus occupies most of the sign of Sagittarius. The differences are due to the fact that the time of year that the sun passes through a particular zodiac constellation's position has slowly changed over the centuries from when the Babylonians originally developed the Zodiac.