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Ophiuchus (known as the serpent holder) is one of the 88 constellations, and was also one of the 48 listed by Ptolemy. Of the 13 zodiac constellations (constellations that can contain the Sun during the course of a year), Ophiuchus is the only one which is not counted as an astrological sign - see below for more information.
Ophiuchus is depicted as a man supporting a snake, Serpens; the interposition of his body divides the snake into two parts, Serpens Caput and Serpens Cauda, which are nonetheless counted as one constellation.
Notable features
The brightest stars in Ophiuchus include α Ophiuchi, called Rasalhague, at the figure's head; and λ Ophiuchi, a triple star, at his elbow.
RS Ophiuchi, a star too faint to interest amateur skywatchers, is part of a strange class called recurrent novas, whose brightness increases at irregular intervals by hundreds of times in a period of just a few days.
Barnard's Star, the fifth-nearest star to the Solar System, lies in Ophiuchus.
If we were to observe Earth's Sun from Tau Ceti, it would appear as a 2.54 magnitude star in Ophiuchus.
Notable 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. The unusual double galaxy NGC 6240 is also in Ophiuchus.
Mythology
The figure is supposed to represent the legendary physician Asclepius, who learned the secrets of life and death from one serpent bringing another some herbs which healed it (Asclepius had previously tried to kill it). In order to avoid the human race becoming immortal under Asclepius's care, Zeus eventually killed him with a bolt of lightning, but placed him in the heavens to honour his good works. The involvement in the myth of Chiron may be connected to the nearby presence of the constellation Sagittarius, which was in later times occasionally considered to represent Chiron (who was more usually identified as the constellation Centaurus).
It is not, however, easy to find the figure of a man in these stars without some diligence, since Ophiucius is intersected by Serpens, parts of which some people identify as the base of Ophiucius' torso (making identification of other parts quite difficult), rather than a snake below it.
History
 Johannes Kepler's drawing depicting the location of the stella nova in the foot of Ophiuchus. This constellation, known from antiquity, is one of the 48 constellations described by Ptolemy. It has also been known as Serpentarius, a Latin form of its name.
The most important historical event in Ophiuchus was the Supernova 1604, also named Kepler's Supernova, whose explosion was first observerd on October 9, 1604, near θ Ophiuchi. Johannes Kepler saw it first on October 16, but studied it so extensively that the supernova was subsequently named after him. He published his findings in a book entitled 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 occurred only 32 years after another supernova in Cassiopeia that had been observed by Tycho Brahe; the last supernova before then had occurred in 1054 (see Crab Nebula), and after Kepler's no further supernovae were observed until 1987 (see Supernova 1987a.)
Astrology
Although it has been found on the plane of the ecliptic since 1930, i.e. the Sun is "in" Ophiuchus each year (in early December) and hence it can be sometimes be seen against the stars of Ophiuchus during an eclipse, Ophiuchus - unlike the other twelve constellations of the zodiac - does not give its name to one of the zodiac signs of the Tropical Zodiac of Western astrology [the division of the ecliptic into twelve equal segments].
Many people use the fact that there are 13 constellations, of unequal size, along the astronomical ecliptic, and not twelve equally-sized constellations, to try and discredit or otherwise disprove astrology. In astrology, the size of the actual astronomical constellations is a non-issue because most Western astrologers use the tropical zodiac, where the ecliptic is divided into 12 equal portions exactly 30 degrees each to get the 12 astrological signs. (12 signs x 30 degrees each sign = the 360 degrees of the ecliptic). The beginning of the astrological year is the beginning of Spring in the Northern Hemisphere (the vernal equinox - usually around March 21), and not when the Sun crosses an arbitrary dividing line drawn up in modern times to denote where the constellation of Aries begins. The time of the vernal equinox, therefore, is the definition of the start of the astrological sign of Aries, even though the sun is well within the astronomical constellation of Pisces at that time of year. See also the First Point of Aries. Jyotish and western Siderial astrologers avoid this criticism by using the true astronomical positions of the stars.
Of the 13 modern signs of the zodiac (constellations of the ecliptic), Ophiuchus is the only one which is not counted as an astrological sign because Ophiuchus was only added to the astronomical zodiac by the International Astronomical Union in 1930 when it based its zodiac on the 1875 equinox. The constellations were redrawn in 1930 to include a very small portion of Ophiuchus on the ecliptic, possibly in an attempt to try and discredit astrology and astrologers, though scientists at the time claimed that the redrawing took place for the sake of simplicity; this redrawing is where we get our modern 88 constellations. According to the official boundaries of constellations first set out in 1930, the ecliptic now also passes through a thirteenth constellation, Ophiuchus (Serpent-Bearer), but before 1930 the ecliptic never passed through this constellation. Although the constellation Ophiuchus has existed for a very long time (since Ptolemy at least), one must remember that it wasn't until 1930 that the constellation was "officially redrawn" to cross over the ecliptic. The constellations as we know them today were 'standardized' in 1930.
The 12 (equal) sign astrological Zodiac has been in place for well over 2000 years and is still used by most Western astrologers, while Ophiuchus has only been located along the astronomical ecliptic for the past 75 years or so.
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