HD 172189, also located in IC 4756, is an Algol variable eclipsing binary with a 5.70 day period.
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.
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 unrelated).
In 1909, the symbiotic nova RT Serpentis appeared near Omicron, although it only reached a maximum magnitude of 10. 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.
When Eugène Delporte established modern constellation boundaries in the 1920s, he elected to depict the two separately.
The International Astronomical Union (IAU) adopted the three-letter abbreviation "Ser" for the constellation in 1922. ==Features== ===Stars=== ====Head stars==== Marking the heart of the serpent is the constellation's brightest star, Alpha Serpentis.
Its main asterism consists of 11 stars, and 108 stars in total are brighter than magnitude 6.5, the traditional limit for naked-eye visibility. Serpens Caput's boundaries, as set by Belgian astronomer Eugène Delporte in 1930, are defined by a 10-sided polygon, while Serpens Cauda's are defined by a 22-sided polygon.
In 1970, the nova FH Serpentis appeared just slightly north of 59 Serpentis, reaching a maximum brightness of 4.5.
First thought to be a single overluminous radio galaxy with a redshift of z = 1.206, it was found in 1987 to actually be two galaxies, with the radio galaxy at the aforementioned redshift being lensed by another galaxy at redshift z = 0.845.
The nebula contains concentric rings, which are similar to those seen in the supernova SN 1987A.
However, in 1990, it was shown that the source is instead a brighter, smaller galaxy a few arcseconds north.
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-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 2000 parsecs, is the OB association Serpens OB2, containing over 100 OB stars.
A type II supernova was observed in this galaxy in 2001 and was designated SN 2001X.
The galaxy has over 40,000 known globular clusters, the highest known total of any galaxy as of 2002. Consisting of two quasars with a separation of less than 5 arcseconds, the quasar pair 4C 11.50 is one of the visually closest pairs of quasars in the sky.
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.
This object is the most distant found to emit photons with energies in the TeV range as of 2007.
In 2012, the object flared in the gamma-ray spectrum, tripling in luminosity for two nights, allowing the redshift to be accurately measured as z = 0.49. Several gamma-ray bursts (GRBs) have been observed in Serpens Caput, such as GRB 970111, one of the brightest GRBs observed.
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