Ophiuchus (/ˌɒfiˈjuːkəs/) is a fascinating constellation that stretches across the celestial equator. Its name originates from the Ancient Greek word "ophioûkhos," which means "serpent-bearer." In ancient times, it was often depicted as a man grasping a snake, with the serpent represented by the constellation Serpens. Ophiuchus is one of the 48 constellations listed by the 2nd-century astronomer Ptolemy and remains one of the 88 modern constellations.
Location
Ophiuchus is located between Aquila, Serpens, Scorpius, Sagittarius, and Hercules, northwest of the center of the Milky Way. In the northern hemisphere, it is best visible during the summer months. It lies opposite of Orion and divides the snake constellation Serpens into two parts: Serpens Caput and Serpens Cauda. With its brightest star, Rasalhague, near its northern edge, Ophiuchus straddles the equator, with the majority of its area lying in the southern hemisphere. The constellation extends southward to -30° declination. Please note that some segments of the ecliptic within Ophiuchus are south of -20° declination.
Features
Stars
The constellation boasts several bright stars, including α Ophiuchi (Rasalhague) and η Ophiuchi (Sabik), with magnitudes of 2.07 and 2.43, respectively. Rasalhague is composed of an A-type (bluish-white) giant star and a K-type main sequence star, while Sabik is a binary system. Other notable stars in Ophiuchus include β Ophiuchi (Cebalrai) and λ Ophiuchi (Marfik). Beta Ophiuchi is an evolved red giant star slightly more massive than the Sun, while Lambda Ophiuchi is a binary star system with the primary star being more massive and luminous than the Sun.
One particularly interesting star in Ophiuchus is RS Ophiuchi, part of a class of stars known as recurrent novae. RS Ophiuchi experiences significant increases in brightness at irregular intervals of just a few days and is believed to be on the verge of becoming a type-1a supernova. It erupts approximately every 15 years, with eruptions reaching a magnitude of around 5.0.
Did you know that Barnard's Star, one of the closest stars to the Solar System, is located in Ophiuchus? It is situated to the left of β Ophiuchi and just north of the V-shaped group of stars that once represented the now-obsolete constellation, Taurus Poniatovii. While there have been claims of an exoplanet orbiting Barnard's Star, subsequent studies have refuted this claim. The star has also been a subject of interest for interstellar travel plans, such as Project Daedalus.
Deep-sky objects
Ophiuchus is also home to several star clusters and nebulae. Notable star clusters in the constellation include IC 4665, NGC 6633, M9, M10, M12, M14, M19, M62, and M107. The nebula IC 4603-4604 is also found within Ophiuchus.
M9 is a globular cluster that may have originated from outside our galaxy. M10, an approximately 20,000 light-year distant globular cluster, has a magnitude of 6.6 and falls under the Shapley class VII cluster, meaning that it has an intermediate concentration towards its center. M12, another globular cluster, is located around 5 kiloparsecs from the Solar System. M14 is a more distant globular cluster, while M62 and M107 are known for their abundance of variable stars.
One of the most intriguing objects in Ophiuchus is the galaxy merger remnant and starburst galaxy NGC 6240. This butterfly-shaped galaxy, located 400 million light-years away, contains two supermassive black holes positioned 3,000 light-years apart. It is estimated that these black holes will merge in around a billion years. NGC 6240 also exhibits a high rate of star formation, classifying it as a starburst galaxy.
Barnard 68, a large dark nebula located 410 light-years from Earth, is another remarkable feature in Ophiuchus. Despite its relatively small size, Barnard 68 has only twice the mass of the Sun. It is both very diffuse and very cold, with a temperature of about 16 kelvins. The nebula has the potential for star formation in the future and exhibits fascinating vibrations with a period of 250,000 years, likely caused by a supernova shock wave.
History and Mythology
Ophiuchus has an intriguing history and mythology. While there is no evidence of the constellation existing before the classical era, it is thought to have been remotely derived from a Babylonian constellation known as the "Sitting Gods." Gavin White proposes that Ophiuchus may represent Nirah, a serpent-god depicted with a human upper half and serpents for legs.
In ancient Greek mythology, Ophiuchus was associated with Apollo's struggle against a large snake guarding the Oracle of Delphi. Later myths identified Ophiuchus with Laocoön, the Trojan priest who warned his fellow Trojans about the Trojan Horse and was subsequently slain by sea serpents sent by the gods as punishment. Roman mythography connects the constellation to Asclepius, the healer who learned the secrets of keeping death at bay by observing one serpent bringing healing herbs to another. Jupiter, wanting to prevent the entire human race from becoming immortal under Asclepius' care, killed him with a bolt of lightning but later honored him by placing his image in the heavens.
It is interesting to note that in medieval Islamic astronomy, Ophiuchus was known as "Al-Ḥawwa'," meaning "the snake-charmer."
Zodiac
Ophiuchus is among the thirteen constellations that intersect the ecliptic. Some have mistakenly suggested it as the "13th sign of the zodiac," but it's important to differentiate between zodiac signs and constellations. Zodiac signs divide the ecliptic into twelve equal parts, each spanning 30° of celestial longitude. They align with the seasons, with the March equinox marking the boundary between Pisces and Aries in Western astrology. Constellations, however, vary in size and are based on the positions of the stars. While the constellations of the zodiac loosely correspond to the zodiac signs, they do not align perfectly.
Ophiuchus, for example, occupies most of the sign of Sagittarius from November 29 to December 18. The differences in alignment are due to the gradual shift of the Sun's passage through the zodiac constellations caused by the precession of the Earth's rotational axis over centuries.
In conclusion, Ophiuchus is a captivating constellation rich in history, mythology, and celestial wonders. Its unique features and intriguing connections make it an exciting area of interest for astronomers and stargazers alike.
References:
- Levy, David H. (2005). Deep Sky Objects. Prometheus Books
- Ridpath, Ian; Tirion, Wil. (2007) Stars and Planets Guide, Collins, Princeton University Press