Constellation of Ophiuchus

Ophiuchus

Ophiuchus is one of Ptolemy’s original 48 constellations and represents the healer Asclepius from Roman legend, 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, Zeus killed him with a bolt of lightning, but later placed his image in the heavens to honour his good works. The name Ophiuchus means “Serepent Bearer” in Greek and on either side are the two parts of Serpens, the Serpent, the sky's only divided constellation. 

Ophiuchus is a large constellation straddling both the celestial equator and the ecliptic. The southern half of the constellation lies on the summer Milky Way. Its brightest star at magnitude 2.08 is Rasalhague (Alpha) which lies not far from similarly named Rasalgethi in Hercules. Rasalhague is 47 light years distant and shines about 25 times brighter than our Sun. Its spectral class is A5 with a surface temperature is about 8,500 degrees Kelvin. Second brightest is Sabik (Eta) at magnitude2.43, which lies at the southern end of the line of stars forming the Serpent Bearer’s lower “skirt”. Sabik is in fact a close double that is very difficult for amateurs to resolve. The orbit is highly eccentric with the separation of the two stars varying by a factor of 32 during the 88 year orbital period. The system lies at a distance of 84 light years and both stars belong to spectral class A. Zeta is third brightest at magnitude 2.56, lying in the middle of the line of stars at the bottom of the constellation. Zeta does not have a proper name, which is rather ironic as it is the real powerhouse amongst the brighter stars of Ophiuchus. It is 458 light years distant and were it not partly obscured by interstellar dust and gas would shine at almost first magnitude. It has a surface temperature of about 32,500 Kelvin and a luminosity of about 68,000 times that of the Sun. Fourth brightest is Yed Prior (Delta) which together with Yed Posterior (Epsilon) is meant to represent the left hand of the Serpent Bearer, which holds the Serpent. Yed Prior is 170 light years distant and shines at magnitude 2.73. It is a class M giant with a radius 58 times that of the Sun and a surface temperature of 3,800 Kelvin. Its total luminosity is 630 times that of the Sun. Despite its “Beta” status Cebalrai is only fifth brightest at magnitude 2.76. Cebalrai is an orange class K2 giant (similar to Arcturus), 82 light years distant with a surface temperature of 4,600 Kelvin and a luminosity of 64 times that of the Sun. 

70 Ophiuchi is a wonderful double star with a separation ranging from two to seven seconds of arc over a period of 88 years, making the components easily separable with a small telescope. The system is only 17 light years from us and consists of two low mass cool yellow-orange class K dwarfs, giving a beautiful colour contrast. Another fine double is Rho which may be found on the border with Scorpius, close to Antares. Both components are hot (22,400 Kelvin) class B stars and they are separated by three seconds of arc. The system is 395 light years from us. Rho is far more famed however for its setting within dusty clouds of the Milky Way. Surrounding Rho is the bright reflection nebula IC 4603-4604 caused by starlight that is scattered from a cloud of interstellar dust grains. Close to Theta Ophiuchi, is another very rich area of the Milky Way in which may be seen several of Barnard’s “dark nebulae” including B78, the "Pipe Nebula" and B72 the “Snake Nebula”. 

Another discovery of Barnard’s in Ophiuchus was “Barnard’s Star”. At just six light years, this tenth magnitude M class dwarf is the closest star to Earth after the the stars of the Alpha Centauri system. It also has a higher proper motion than any other star, moving across the sky at 10.4 arcseconds per year. This is due both to its proximity and its unusually high speed of 139 kilometres per second relative to the Sun. Despite this, Barnard’s Star still takes about 175 years to move the angular diameter of the Full Moon across the sky.

RS Ophiuchi is a member of a rare class of cataclysmic variable star known as recurrent novae. These stars are novae where more than one outburst has been observed and appear to be intermediate in class between the classical novae (single major outburst) and dwarf novae (frequent minor outbursts). Outbursts in RS Oph have been seen in 1933, 1958, 1967, 1985 & 2006 and have peaked around 5th magnitude. When at minimum the star oscillates irregularly between magnitudes 10.0 & 12.5. RS is an interacting binary star system with a long orbital period of 460 days. The components consist of a white dwarf primary and a red giant secondary. About every 20 years, enough material from the red giant builds up on the surface of the white dwarf to produce a thermonuclear explosion. The white dwarf orbits close to the red giant, with an accretion disc concentrating the overflowing atmosphere of the red giant onto the white dwarf. It is conjectured that RS may ultimately explode as a Type Ia supernova. If it did so in the near future this would be quite a coincidence because Ophiuchus played host to the last bright supernova observed in our galaxy – Kepler’s Supernova in 1604.

Ophiuchus boasts seven of the globular clusters from Messier’s catalogue (M9, M10, M12, M14, M19, M62 & M107). This total is only equalled by neighbouring Sagittarius and the prepondeance of these objects in these two constellations arises from their location in the direction of the galactic centre. All of the Ophiuchus globulars are visible in binoculars but require a telescope to do them justice. None match the showpiece globulars of the northern heavens, such as M13 in Hercules or M5 in Serpens.

Finally, Ophiuchus also has two decent open or galactic clusters. These are the large and loose IC 4665 which is well seen in binoculars and the under-rated 5th magnitude NGC 6633.