Serpens is a constellation of the northern hemisphere. One of the 48 constellations listed by the 2nd-century astronomer Ptolemy, it remains one of the 88 modern constellations defined by the International Astronomical Union. It is unique among the modern constellations in being split into two non-contiguous parts, Serpens Caput (Serpent Head) to the west and Serpens Cauda (Serpent Tail) to the east. Between these two halves lies the constellation of Ophiuchus, the “Serpent-Bearer.” In figurative representations, the body of the serpent is represented as passing behind Ophiuchus between Mu Serpentis in Serpens Caput and Nu Serpentis in Serpens Cauda. Serpens Caput is bordered by Libra to the south, Virgo and Bootes to the east, Corona Borealis to the north, and Ophiuchus and Hercules to the west; Serpens Cauda is bordered by Sagittarius to the south, Scutum and Aquila to the east, and Ophiuchus to the north and west. Covering 636.9 square degrees total, it ranks 23rd of the 88 constellations in size. It appears prominently in both the northern and southern skies during the Northern Hemisphere’s summer. Serpens Caput’s boundaries, as set by Eugène Delporte in 1930, are defined by a 15-sided polygon, while Serpens Cauda’s are defined by a 25-sided polygon. In the equatorial coordinate system, the right ascension coordinates of Serpens Caput’s borders lie between 15h 10.4m and 16h 22.5m, while the declination coordinates are between 25.66° and −03.72°. Serpens Cauda’s boundaries lie between right ascensions of 17h 16.9m and 18h 58.3m and declinations of 06.42° and −16.14°.
Marking the heart of the serpent is the constellation’s brightest star, Alpha Serpentis. Traditionally called Unukalhai, is a red giant of spectral type K2III located approximately 23 parsecs distant with a visual magnitude of 2.630 ± 0.009. A faint companion is in orbit around the red giant star. The brightest star in the tail, Eta Serpentis, is similar to Alpha Serpentis’ primary in that it is a red giant of spectral class K. This star, however, is known to exhibit solar-like oscillations over a period of approximately 2.16 hours.
A few structures of the Milky Way Galaxy are present in Serpens Caput, such as Messier 5, a globular cluster positioned approximately 8° southwest of α Serpentis, next to the star 5 Serpentis and is located approximately 25,000 ly distant. Messier 5 contains a large number of known RR Lyrae variable stars and is receding from us at over 50 km/s. The cluster contains two-millisecond pulsars, one of which is in a binary, allowing the proper motion of the cluster to be measured. Another globular cluster is Palomar 5, found just south of Messier 5. Many stars are leaving this globular cluster due to the Milky Way’s gravity, forming a tidal tail over 30000 light-years long. The L134/L183 is a dark nebula complex that, along with a third cloud, is likely formed by fragments of a single original cloud located 36 degrees away from the galactic plane, a large distance for dark nebulae. The entire complex is thought to be around 140 parsecs distant. L183, also referred to as L134N, is home to several infrared sources, indicating pre-stellar sources thought to present the first known observation of the contraction phase between cloud cores and pre-stellar cores. Outside of the Milky Way, there are no bright deep-sky objects in Serpens Caput, with nothing else above 10th magnitude. The brightest is NGC 5962, a spiral galaxy positioned around 28 megaparsecs distant with an apparent magnitude of 11.34. Slightly fainter is NGC 5921, a barred spiral galaxy with a LINER-type active galactic nucleus situated somewhat closer at a distance of 21 megaparsecs. A type II supernova was observed in this galaxy in 2001 and was designated SN 2001X. Fainter still are the spirals NGC 5964 and NGC 6118, with the latter being host to the supernova SN 2004dk. Hoag’s Object, located 600 million light-years from Earth, is a member of the very rare class of galaxies known as ring galaxies. The outer ring is largely composed of young blue stars while the core is made up of older yellow stars. Arp 220 is another unusual galaxy in Serpens. The prototypical ultra-luminous infrared galaxy, Arp 220 is 250 million light-years from Earth. It consists of two large spiral galaxies in the process of colliding with their nuclei orbiting at a distance of 1,200 light-years, causing extensive star formation
throughout both components. It possesses a large cluster of more than a billion stars, partially covered by thick dust clouds near one of the galaxies’ core. Another interacting galaxy pair, albeit in an earlier stage, consists of the galaxies NGC 5953 and NGC 5954. In this case, both are active galaxies, with the former a Seyfert 2 galaxy and the latter a LINER-type galaxy. Both are undergoing a burst of star formation triggered by the interaction. Seyfert’s Sextet is a group of six galaxies, four of which are interacting gravitationally and two of which simply appear to be a part of the group despite their greater distance. The gravitationally bound cluster lies at a distance of 190 million light-years from Earth and is approximately 100,000 light-years across, making Seyfert’s Sextet one of the densest galaxy group known.
Part of the galactic plane passes through the tail, and thus Serpens Cauda is rich in deep-sky objects within our own galaxy. The Eagle Nebula and its associated star cluster, Messier 16 lie 7,000 light-years from Earth in the direction of the galactic centre. The nebula measures 70 light-years by 50 light-years and contains the Pillars of Creation, three dust clouds that became famous for the image taken by the Hubble Space Telescope. The stars being born in the Eagle Nebula added to those with an approximate age of 5 million years have an average temperature of 45,000 kelvins and produce prodigious amounts of radiation that will eventually destroy the dust pillars. Despite its fame, the Eagle Nebula is fairly dim, with an integrated magnitude of approximately 6.0. The star-forming regions in the nebula are often evaporating gaseous globules; unlike Bok globules, they only hold one protostar. Within the Serpens OB2 HII region is the open cluster NGC 6604, which is the same age as the surrounding OB association, and the cluster is now thought to simply be the densest part of it. IC 4756, containing at least one naked-eye star, HD 17236. Positioned approximately 440 parsecs distant, the cluster is estimated to be around 800 million years old, quite old for an open cluster. MWC 922 is a star surrounded by a planetary nebula. Dubbed the Red Square Nebula due to its similarities to the Red Rectangle Nebula, the planetary nebula appears to be a nearly perfect square with a dark band around the equatorial regions. MWC 922 itself is an FS Canis Majoris variable; meaning that it is a Be star containing exceptionally bright hydrogen emission lines as well as select forbidden lines, likely due to the presence of a close binary. East of Xi Serpentis is another planetary nebula, Abell 41, containing the binary star MT Serpentis at its centre. The nebula appears to have a bipolar structure, and the axis of symmetry of the nebula has been found to be within 5° of the line perpendicular to the orbital plane of the stars, strengthening the link between binary stars and bipolar planetary nebulae.n The Serpens cloud is a massive star-forming molecular cloud situated in the southern part of Serpens Cauda. Only two million years old and 420 parsecs distant, the cloud is known to contain many protostars such as Serpens FIRS 1 and Serpens SVS 20. The Serpens South protocluster was uncovered by NASA’s Spitzer Space Telescope in the southern portion of the cloud, and it appears that star formation is still continuing in the region. Another site of star formation is the Westerhout 40 complex, consisting of a prominent HII region adjacent to a molecular cloud. Located around 500 parsecs distant, it is one of the nearest massive regions of star formation, but as the molecular cloud obscures the HII region, rendering it and its embedded cluster tough to see visibly, it is not as well-studied as others.[ The embedded cluster likely contains over 600 stars above 0.1 solar masses, with several massive stars, including at least one O-type star, is responsible for lighting the HII region and the production of a bubble.
There are two daytime meteor showers that radiate from Serpens, the Omega Serpentids and the Sigma Serpentids. Both showers peak between December 18 and December 25. Credit: Wikipedia.