Ophiuchus is a large constellation straddling the celestial equator. Its name is from the Greek Ὀφιοῦχος Ophioukhos; “serpent-bearer,” and it is commonly represented as a man grasping the snake that is represented by the constellation Serpens; the interposition of his body divides the snake constellation Serpens into two parts, Serpens Caput and Serpens Cauda, which are in modern times taken with Ophiuchus as one constellation. Ophiuchus straddles the equator but lies predominantly to its south. 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 and Anguitenens. Ophiuchus figures between Aquila, Serpens, Serpens and Hercules, northwest of the centre 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. The constellation extends southward to −30° declination. In the period November–January (summer in the Southern Hemisphere, winter in the Northern Hemisphere) when Ophiuchus is in the daytime sky and thus not visible at most latitudes. In the Northern Hemisphere’s spring and summer months, when Ophiuchus is normally visible in the night sky, the constellation is not visible, at those times and places in the Arctic when midnight sun obscures the stars. In countries close to the equator Ophiuchus appears overhead in June around midnight and in the October evening sky.
The brightest stars in Ophiuchus include α Ophiuchi, called Rasalhague (“head of the serpent charmer”) at magnitude 2.07. It is a binary star with an orbital period of 8.62 years. The primary component in the system is a white giant star with the stellar classification of A5 III. It has a mass 2.4 times that of the Sun. The companion is an orange main sequence dwarf with 85 per cent of the Sun’s mass. It belongs to the stellar class K5-7 V. The brighter component is about 25 times more luminous than the Sun. It is a very fast spinner, with a projected rotational velocity of 240 km/s. As a result, it has an equatorial bulge that is about 20 per cent larger than the polar radius, which gives Alpha Ophiuchi the shape of an oblate spheroid. The second brightest star η Ophiuchi, known as Sabik (“the preceding one”) at magnitude 2.43 and is approximately 88 light years distant from the Sun. The system is composed of two white main sequence dwarfs belonging to the spectral classes A1 V and A3 V. They have an orbital period of 87.58 years. The stars have apparent magnitudes of 3.05 and 3.27. Zeta Ophiuchi is the third brightest star in Ophiuchus. It is an extremely large blue main sequence star with the stellar classification of O9.5 V. It has an apparent magnitude of 2.569 and is about 366 light years distant from the solar system. The star is classified as a Beta Cephei variable, a star that exhibits variations in brightness as a result of pulsation of its surface. Within the next few million years, the star will expand into a red supergiant and likely explode as a supernova, leaving behind a pulsar or neutron star. Zeta Ophiuchi has eight times the Sun’s radius and more than 19 solar masses. It is a fast rotating star, spinning close to the velocity at which it could begin to break up. Its estimated rotational velocity may be 400 km/s. The star’s estimated age is only 3 million years. Delta Ophiuchi is a red giant with the stellar classification of M0.5 III. It is the fourth brightest star in the constellation. It has an apparent magnitude of 2.75 and is approximately 171 light years distant from the Sun. It forms an optical double with the star Epsilon Ophiuchi or Yed Posterior. The word “yed” comes from Arabic and means “the hand.” The two stars mark the left hand of the Serpent Bearer, which holds the head of the serpent. Delta Ophiuchi has a mass 1.5 times that of the Sun and a radius about 59 times solar. It is a suspected variable star with possible variations in magnitude by 0.03. Beta Ophiuchi is an orange giant star belonging to the spectral class K2 III. It is the fifth brightest star in the constellation. It has a visual magnitude that ranges from 2.75 to 2.77 and is 81.8 light years distant from Earth. The star’s traditional name, Celbalrai (and variants Cheleb and Kelb Alrai) comes from the Arabic kalb al-rā‘ī, which means “the shepherd dog.” Celbalrai has 113 per cent of the Sun’s mass and a radius 12.42 times solar. The star is 63.4 times more luminous than the Sun. It has an unconfirmed planetary companion in its orbit. Kappa Ophiuchi is another suspected variable star in Ophiuchus. It is an orange giant with the stellar classification of K2 III. It has a mean apparent magnitude of 3.20 and is 91.5 light years distant from the Sun. Kappa Ophiuchi has 119 per cent of the Sun’s mass and 11 times the solar radius. It is 46 times more luminous than the Sun.
Ophiuchus contains several star clusters, such as NGC 6633, IC 4665, 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 centre. 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. NGC 6572 is a planetary nebula in Ophiuchus, and It has an apparent magnitude of 8.1. IC 4603-IC 4604 is a reflection nebula in Ophiuchus. IC 4603 lies near the bright star Antares in Scorpius constellation, and IC 4604 contains several bright stars, the brightest of which is Rho Ophiuchi. The stars that illuminate the two nebulae are 400-440 light years distant from Earth. The planetary nebula M2-9, also known as the Twin Jet nebula, Minkowski’s Butterfly, or the Butterfly Nebula, is another planetary nebula in Ophiuchus. It has an apparent magnitude of 14.7 and is approximately 2,100 light years distant from the solar system. It was discovered by the German-American astronomer Rudolph Minkowski is 1947. It is a bipolar nebula formed in the shape of twin lobes of material ejected from the central star. The shape of the lobes is believed to be caused by polar jets, which is why the nebula is sometimes called the Twin Jet Nebula. The star at the centre of the nebula is a binary system. The primary component is the hot core of a star that ejected its outer layers and contracted into a white dwarf. The secondary component is a smaller star in a close orbit with the primary. The interaction of the two stars is what has created the nebula.
The Rho Ophiuchi cloud complex is another dark nebula in Ophiuchus, located a degree to the south of the star Rho Ophiuchi. The nebula is approximately 460 light years distant from Earth. It is one of the nearest star-forming regions to the Sun. The nebula covers an area of 4.5°x6.5° and consists of two major regions of dust and gas where new stars are being formed. About 425 infrared sources, presumably young stellar objects, have been detected in one of them.
The Little Ghost Nebula is another planetary nebula in Ophiuchus. It has an apparent magnitude of 12.9 and is about 2,000 light years distant from Earth. The nebula was discovered by William Herschel. Its main ring structure spans about a light year across and is illuminated by the central white dwarf. The Dark Horse Nebula, sometimes known as the Great Dark Horse, is a large dark nebula in Ophiuchus. It lies near the border with the zodiac constellations Sagittarius and Scorpius and obscures a part of the upper central bulge of the Milky Way Galaxy. The nebula got the name Dark Horse because its shape resembles that of a side silhouette of a horse. The Pipe Nebula is a dark nebula which is part of the larger Dark Horse Nebula in Ophiuchus. It forms the hindquarters of the Dark Horse. The nebula appears as a pipe-shaped dust lane that obscures the Milky Way in the background. The Pipe Nebula is approximately 600-700 light years distant from the solar system. It consists of two parts: the Pipe Stem, which is composed of Barnard 59, 65, 66 and 67, and the Bowl of the Pipe, which consists of Barnard 78. The Snake Nebula is a dark nebula approximately 650 light years from Earth. It is small but has a distinctive S-shape of a snake, which is how it got its name. The nebula can be found in the north-western part of the bowl of the Pipe Nebula and is part of the larger Dark Horse Nebula. 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 Snake Nebula is a dark nebula approximately 650 light years from Earth. It is small but has a distinctive S-shape of a snake, which is how it got its name. The nebula can be found in the north-western part of the bowl of the Pipe Nebula and is part of the larger Dark Horse Nebula. Credits: Constellation Guide, Wikipedia.