|Right ascension||Serpens Caput: 15h 10.4m to 16h 22.5m
Serpens Cauda: 17h 16.9m to 18h 58.3m
|Declination||Serpens Caput: 25.66° to −03.72°
Serpens Cauda: 06.42° to −16.14°
|Area||Serpens Caput: 428 sq. deg.
Serpens Cauda: 208 sq. deg.
Total: 637 sq. deg. (23rd)
|Stars with planets||15|
|Stars brighter than 3.00m||1|
|Stars within 10.00 pc (32.62 ly)||2|
|Brightest star||α Ser (Unukalhai) (2.63m)|
|Nearest star||GJ 1224
(24.60 ly, 7.54 pc)
|Visible at latitudes between +80° and −80°.
Best visible at 21:00 (9 p.m.) during the month of July.
Serpens ("the Serpent", Greek Ὄφις) 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's Head) to the west and Serpens Cauda (Serpent's 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 μ Ser in Serpens Caput and ν Ser in Serpens Cauda.
The brightest star in Serpens is Unukalhai or Cor Serpentis "Serpent's Heart", with an apparent magnitude of 2.63. Part of the Milky Way passes through Serpens Cauda, which is therefore rich in deep-sky objects, such as the Eagle Nebula (IC 4703) and its associated star cluster Messier 16. 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. Located in Serpens Caput are Seyfert's Sextet, one of the densest galaxy clusters known, and Arp 220, the prototypical ultraluminous infrared galaxy. In addition, it also contains the Hercules–Corona Borealis Great Wall, the largest object in the universe.
In Greek Mythology, Serpens represents a snake held by Asclepius, a healer. Asclepius, represented in the sky by the constellation Ophiuchus, which splits Serpens into two distinct halves, was known for killing a snake that was resurrected because a different snake had placed a certain herb on it before its "death". Serpens is depicted as either winding around Ophiuchus in the night sky or simply passing through him, although the precise reason for either of these is unknown.
In some ancient atlases, the constellations Serpens and Ophiuchus were depicted as two separate constellations, although in most they were shown as a single constellation. Back in this time, there were no official constellation boundaries, so when depicted separately, their bodies were not intertwined with each other.
In Chinese astronomy, most of the stars of Serpens represented part of a wall surrounding a marketplace, known as Tianshi, which was in Ophiuchus and part of Hercules. Serpens also contains a few Chinese constellations. Two stars in the tail represented part of Shilou, the tower with the market office. Another star in the tail represented Liesi, jewel shops. One star in the head (Mu Serpentis) marked Tianru, the crown prince's wet nurse, or sometimes rain.
There were two "serpent" constellations in Babylonian astronomy, known as Mušḫuššu and Bašmu. It appears that Mušḫuššu was depicted as a hybrid of a dragon, a lion and a bird, and loosely corresponded to Hydra. Bašmu was a horned serpent (c.f. Ningishzida) and loosely corresponds to the Ὄφις constellation of Eudoxus of Cnidus on which the Ὄφις (Serpens) of Ptolemy is based.
Serpens is unique among the 88 modern constellations in that it is split into two disconnected regions in the sky: Serpens Caput (the head) and Serpens Cauda (the tail). The constellation is also one of the only ones that is dependent on another constellation for context; specifically, it is being held by the Serpent Bearer Ophiuchus.
Serpens Caput is bordered by Libra to the south, Virgo and Boötes 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 the both the northern and southern skies during the Northern Hemisphere's summer. Its main asterism consists of 11 stars, and 108 stars in total are brighter than magnitude 6.5.
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°. The International Astronomical Union (IAU) adopted the three-letter abbreviation "Ser" for the constellation in 1922.
The brightest star in Serpens, Alpha Serpentis, or Unukalhai, is a red giant of spectral type K2III located approximately 22.68 parsecs (74.0 ly) away which marks the snake's heart. With a visual magnitude of 2.63, it can easily be seen with the naked eye even in areas with substantial light pollution. A faint companion is in orbit around the red giant star, although it is not visible to the naked eye. Located near Alpha is Lambda Serpentis, a magnitude 4.42 star rather similar to the Sun located only 12.12 parsecs (39.5 ly) away. Another solar analog in Serpens is the primary of Psi Serpentis, a binary star located slightly further away at approximately 14.64 parsecs (47.7 ly).
Beta, Gamma, and Iota Serpentis form a distinctive triangular shape marking the head of the snake, with Kappa Serpentis being roughly midway between Gamma and Iota. The brighest of the four with an apparent magnitude of 3.67, Beta Serpentis is a white main-sequence star roughly 155.0 parsecs (506 ly) distant. It is likely that a nearby star of magnitude 9.95 is physically associated with Beta, although it is not certain. The Mira variable R Serpentis, located between Beta and Gamma, is visible to the naked eye at its maximum brightness of 5.16, but typical of Mira variables, it can fade to below magnitude 14. Gamma Serpentis itself is an F-type subgiant located only 11.25 parsecs (36.7 ly) distant and thus is quite bright, being of magnitude 3.84. The star is known to show solar-like oscillations
Delta Serpentis, forming part of the body of the snake between the heart and the head, is a multiple star system located 69.93 parsecs (228.1 ly) from Earth. Consisting of four stars, the system has a total apparent magnitude of 3.79 as viewed from Earth, although two of the stars, with a combined apparent magntitude of 3.80, provide nearly all the light that reaches Earth. The primary, a white subgiant, is a Delta Scuti variable with an average apparent magnitude of 4.23. Located very near Delta, both in the night sky and likely in actual space at an estimated distance of 70.87 parsecs (231.1 ly), is the barium star 16 Serpentis. Another notable variable star visible to the naked eye is Chi Serpentis, an Alpha2 Canum Venaticorum variable located midway between Delta and Beta which varies from its median brightness of 5.33 by 0.03 magnitudes over a period of approximately 1.5 days.
The two stars in Serpens Caput that form part of the Snake's body below the heart are Epsilon and Mu Serpentis, both third-magnitude A-type main-sequence stars. Both have a peculiarity: Epsilon is an Am star, while Mu is a binary. Located slightly northwest of Mu is 36 Serpentis, another A-type main-sequence star. This star also has a peculiarity; it is a binary with the primary component being a Lambda Boötis star, meaning that it has solar-like amounts of carbon, nitrogen, and oxygen, while containing very low amounts of iron peak elements. 25 Serpentis, located a few degrees northeast of Mu Serpentis, is a spectroscopic binary consisting of a hot B-type giant and an A-type main-sequence star. The primary is a slowly pulsating B star, which causes the system to vary by 0.03 magnitudes.
Serpens Caput contains many RR Lyrae variables, although most are too faint to be seen without professional equipment. The brightest is VY Serpentis, only of 10th magnitude. Interestingly, this star's period has been increasing by approximately 1.2 seconds per century. A variable star of a different kind is Tau4 Serpentis, a cool red giant that pulsates between magnitudes 5.89 and 7.07 in 87 days. This star has been found to display an inverse P Cygni profile.
Several stars in Serpens have been found to have planets. The brightest, Omega Serpentis, located between Epsilon and Mu, is an orange giant with a planet of approximately 1.7 Jupiter-masses. NN Serpentis, an eclipsing post-common-envelope binary consisting of a white dwarf and a red dwarf, is very likely to have two planets causing variations in the period of the eclipses. Although it does not have a planet, the solar analog HD 137510 has been found to have a brown dwarf companion within the brown dwarf desert.
One of the most interesting stellar remnants in the night sky is PSR B1534+11, a system consisting of two neutron stars in orbit around each other. One of the neutron stars is a pulsar with a period of 37.9 milliseconds. Located approximately 1,051 parsecs (3,430 ly) distant, the system demonstrates one of the best examples of general relativity. The X-ray emission from the system has been found to be present when the non-pulsar star intersects the equatorial pulsar wind of the pulsar, and the system's orbit has been found to vary slightly.
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. Eta Serpentis was previously classified as a carbon star, which would have made it the brightest carbon star in the sky, although this classification was found to be erroneous.
The other two stars in Serpens Cauda forming its asterism are Theta and Xi Serpentis. Xi, where the asterism crosses over to Mu Serpentis in the head, is a triple star system located approximately 105.2 parsecs (343 ly) away. Two of the stars, with a combined apparent magnitude of 3.54, form a spectroscopic binary, and thus cannot be resolved with modern equipment. The primary is a white giant with an excess of strontium. Theta, forming the tip of the tail, is also a multiple system, consisting of two A-type main-sequence stars with a combined apparent magnitude of 4.10 separated by almost half an arcminute.
Lying near the boundary with Ophiuchus are Zeta (23.55 parsecs (76.8 ly) distant), Nu (209.3 parsecs (683 ly) distant), and Omicron Serpentis (173.1 parsecs (565 ly) distant). All three are 4th-magnitude main-sequence stars, with Nu and Omicron being of spectral type A and Zeta being of spectral type F. Nu is a binary star with a secondary component of magnitude 9.3, while Omicron is a Delta Scuti variable with amplitude variations of 0.01 magnitudes. In 1909, the symbiotic nova RT Serpentis appeared near Omicron, although it only reached a maximum magnitude of 10.2.
The star system 59 Serpentis, also known as d Serpentis, is a triple star system  consisting of a spectroscopic binary containing an A-type star and an orange giant and an orange giant secondary. The system shows irregular variations in brightness between magnitudes 5.17 and 5.2. In 1970, the nova FH Serpentis appeared just slightly north of 59 Serpentis, reaching a maximum brightness of 4.5. Also near 59 Serpentis in the Serpens Cloud are several Orion variables. MWC 297 is a Herbig Be star that in 1994 exhibited a large X-ray flare and increased in X-ray luminosity by five times before returning to the quiescent state. The star also appears to possess a circumstellar disk. Another Orion variable in the region is VV Serpentis, a Herbig Ae star that has been found to exhibit Delta Scuti pulsations. VV Serpentis has also, like MWC 297, been found to have a dusty disk surrounding it, and is also a UX Orionis star, meaning that it shows irregular variations in its brightness.
The star HR 6958, also known as MV Serpentis, is an Alpha2 Canum Venaticorum variable that is faintly visible to the naked eye. The star's metal abundance is incredibly high for most metals at the iron peak and heavier, and has also been found to contain excess silicon. Barely visible to the naked eye is HD 172365, a likely post-blue straggler in the open cluster IC 4756 that contains a large excess of lithium. HD 172189, also located in IC 4756, is an Algol variable eclipsing binary with a 5.70 day period. The primary star in the system is also a Delta Scuti variable, undergoing multiple pulsation frequencies, which, combined with the eclipses, causes the system to vary by around a tenth of a magnitude.
As the Milky Way passes through it, Serpens Cauda contains many massive OB stars. Several of these are visible to the naked eye, such as NW Serpentis, an early Be star that has been found to be somewhat variable. The variability is interesting; according to one study, it could be one of the first discovered hybrids between Beta Cephei variables and slowly pulsating B stars. Although not visible to the naked eye, HD 167971 (MY Serpentis) is a Beta Lyrae variable triple system consisting of three very hot O-type stars. A member of the cluster NGC 6604, the two eclipsing stars are both blue giants, with one being of the very early spectral type O7.5III. The remaining star is either a blue giant or supergiant of a late O or early B spectral type. Also an eclipsing binary, the HD 166734 system consists of two O-type blue supergiants in orbit around each other. Less extreme in terms of mass and temperature is HD 161701, a spectroscopic binary consisting of a B-type primary and an Ap secondary, although it is the only known spectroscopic binary to consist of a star with excess of mercury and manganese and an Ap star.
South of the Eagle Nebula on the border with Sagittarius is the eclipsing binary W Serpentis, whose primary is a white giant that is interacting with the secondary. The system has been found to contain an accretion disk, and was one of the first discovered Serpentids, which are eclipsing binaries containing exceptionally strong far-ultraviolet spectral lines. It is suspected that such Serpentids are in an earlier evolutionary phase, and will evolve first into double periodic variables and then classical Algol variables. Also near the Eagle Nebula is the eclipsing Wolf–Rayet binary CV Serpentis, consisting of a Wolf–Rayet star and a hot O-type subgiant. The system is surrounded by a ring-shaped nebula, likely formed during the Wolf–Rayet phase of the primary. The eclipses of the system very erratic, and although there are two theories as to why, neither of them is completely consistent with current understanding of stars.
Serpens Cauda contains a few X-ray binaries. One of these, GX 17+2, is a low-mass X-ray binary consisting of a neutron star and, as in all low-mass X-ray binaries, a low-mass star. The system has been classified as a Sco-like Z source, meaning that its accretion is near the Eddington limit. The system has also been found to approximately every 3 days brighten by around 3.5 K-band magnitudes, possibly due to the presence of a synchrotron jet. Another low-mass X-ray binary, Serpens X-1, undergoes occasional X-ray bursts. One in particular lasted nearly four hours, possibly explained by the burning of carbon in "a heavy element ocean".
As the Milky Way does not pass through this part of Serpens, a view to many galaxies beyond it is possible. However, a few structures of the Milky Way Galaxy are present in Serpens Caput, such as Messier 5, a globular cluster located approximately 8° southwest of α Serpentis, next to the star 5 Serpentis. Barely visible to the naked eye under good conditions, and is located approximately 25000 ly distant. Messier 5 contains a large number of known RR Lyrae variable stars, and is receding from us at over 50 km/s. Interestingly, the cluster contains two millisecond pulsars, one of which is in a binary. The cluster has been used to test for magnetic dipole moments in neutrinos, which could shed light on some hypothetical particles such as the axion. Another globular cluster is Palomar 5, located just south of Messier 5. Interestingly, many stars are leaving this globular cluster due to the Milky Way's gravity, forming a tidal tail over 30,000 light-years (9,200 pc) long.
Hoag's Object, located 600 million light-years from Earth, is one of the most famous of a 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. The predominant theory regarding its formation is that the progenitor galaxy was a barred spiral galaxy whose arms had velocities too great to keep the galaxy's coherence and therefore detached.
Arp 220 is another unusual galaxy in Serpens. The prototypical ultraluminous infrared galaxy, Arp 220 is located 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.
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. Astronomers predict that the four interacting galaxies will eventually merge to form a large elliptical galaxy.
Part of the Milky Way 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 center. 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.
North of Messier 16, at a distance of approximately 1,900 parsecs (6,200 ly), is the OB association Serpens OB2, containing over 100 OB stars. Around 5 million years old, the association appears to still contain star-forming regions, and the light from its stars is illuminating the HII region S 54. Within this 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. Interestingly, the cluster appears to be producing a thermal chimney of ionized gas, caused by the interaction of the gas from the galactic disk with the galactic halo.
Another open cluster in Serpens Cauda is IC 4756, containing at least one naked-eye star (HD 172365) (another naked-eye star in the vicinity, HD 171586, is most likely likely unrelated). Located approximately 437 parsecs (1,430 ly) distant, the cluster is estimated to be around 800 million years old, which is quite old for an open cluster. Despite the presence of the Milky Way in Serpens Cauda, one globular cluster can be found: NGC 6535, although invisible to the naked eye, can be made out in small telescopes just north of Zeta Serpentis. Rather small and sparse for a globular cluster, this cluster contains no known RR Lyrae variables, which is unusual for a globular 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. The nebula contains concentric rings, which are similar to those seen in the supernova SN 1987A. 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 center. 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. On the other end of the stellar age spectrum is L483, a dark nebula which contains the protostar IRAS 18418-0440. Although classified as a class 0 protostar, it has some unusual features for such an object, such as a lack of high-velocity stellar winds, and it has been proposed that this object is in transition between class 0 and class I. A variable nebula exists around the protostar, although it is only visible in infrared light.
The Serpens cloud is a massive star-forming molecular cloud located in the southern part of Serpens Cauda. Only two million years old and 415 parsecs (1,350 ly) 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.
Despite the presence of the Milky Way, several active galaxies are visible in Serpens Cauda as well, such as PDS 456, located near Xi Serpentis. The most intrinsically luminous nearby active galaxy, this AGN has been found to be extremely variable in the X-ray spectrum. This has allowed light to be shed on the nature of the supermassive black hole at the center, likely a Kerr black hole. It is possible that the quasar is undergoing a transition from an ultraluminous infrared galaxy to a classical radio-quiet quasar, but there are problems with this theory, and the object appears to be an exceptional object that does not completely lie within current classification systems. Nearby is NRAO 530, a blazar that has been known to flare in the X-rays occasionally. One of these flares was for less than 2000 seconds, making it the shortest flare ever observed in a blazar as of 2004. The blazar also appears to show periodic variability in its radio wave output over two different periods of six and ten years.
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