Serpens is a constellation in the northern celestial hemisphere. One of the 48 constellations listed by the 2nd-century astronomer Ptolemy, it remains one of the 88 modern constellations designated by the International Astronomical Union. It is unique among the modern constellations in being split into two non-contiguous parts, Serpens Caput to the west and Serpens Cauda 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.
Messier 23, also known as NGC 6494, is an open cluster of stars in the northwest of the southern constellation of Sagittarius. It was discovered by Charles Messier in 1764. It can be found in good conditions with binoculars or a modestly sized telescope. It is in front of "an extensive gas and dust network", which there may be no inter-association. It is within 5° the sun's position so can be occulted by the moon.
Messier 35 or M35, also known as NGC 2168, is a relatively close open cluster of stars in the west of Gemini, at about the declination of the sun when the latter is at June solstice. It was discovered by Philippe Loys de Chéseaux around 1745 and independently discovered by John Bevis before 1750. It is scattered over part of the sky almost the size of the full moon and is 2,970 light-years away. The compact open cluster NGC 2158 lies directly southwest of it.
Messier 36 or M36, also known as NGC 1960, is an open cluster of stars in the somewhat northern Auriga constellation. It was discovered by Giovanni Batista Hodierna before 1654, who described it as a nebulous patch. The cluster was independently re-discovered by Guillaume Le Gentil in 1749, then Charles Messier observed it in 1764 and added it to his catalogue. It is about 1,330 pc away from Earth. The cluster is very similar to the Pleiades cluster (M45), and if as far away it would be of similar apparent magnitude.
The Beehive Cluster, is an open cluster in the constellation Cancer. One of the nearest open clusters to Earth, it contains a larger population of stars than other nearby bright open clusters holding around 1,000 stars. Under dark skies, the Beehive Cluster looks like a small nebulous object to the naked eye, and has been known since ancient times. Classical astronomer Ptolemy described it as a "nebulous mass in the breast of Cancer". It was among the first objects that Galileo studied with his telescope.
Merope, designated 23 Tauri, is a star in the constellation of Taurus and a member of the Pleiades star cluster. It is approximately 380 light years away.
NGC 663 is a young open cluster in the constellation of Cassiopeia. It has an estimated 400 stars and spans about a quarter of a degree across the sky. It can reportedly be detected with the unaided eye, although a telescope is recommended for best viewing. The brightest members of the cluster can be viewed with binoculars. Although the listed visual magnitude is 7.1, several observers have reported higher estimates.
NGC 5822 is an open cluster of stars in the southern constellation of Lupus. It was discovered by English Astronomer John Herschel on July 3, 1836, and lies close to another cluster, NGC 5823, which suggests there may be a physical association.
NGC 6193 is open cluster containing 27 stars in the constellation Ara, visible to the unaided eye. NGC 6193 lies at the center of the Ara OB1 association, which extends over a square degree. The cluster is associated with neighboring regions of the nebulosity NGC 6188.
NGC 6811 is an open cluster in the constellation of Cygnus, near the constellation of Lyra. It has an angular size half that of the full Moon and includes about 1000 stars of roughly similar magnitude. It has also been called "The Hole in the Cluster" because of its dark center.
NGC 654 is an open cluster in the constellation Cassiopeia. It was discovered by William Herschel in 1787. With apparent magnitude 6.5, it can be observed by binoculars. It is located 2,5° northeast of the star Delta Cassiopeiae. In the same low power field can also be seen the open clusters NGC 663 and NGC 659. It surrounds a 7th magnitude yellowish star, an F5Ia supergiant, which is a possible member of the group.
NGC 6939 is an open cluster in the constellation Cepheus. It was discovered by William Herschel in 1798. The cluster lies 2/3° northwest from the spiral galaxy NGC 6946. The cluster lies approximately 4.000 light years away and it is over a billion years old.
NGC 7209 is an open cluster in the constellation Lacerta. It was discovered by William Herschel on 19 October 1787. The cluster lies 3,810 light years away from Earth. It has been suggested that there is another cluster at a distance of 2,100 light years projected in front of a cluster lying at 3,800 light years away, based on the reddening of the cluster, however, further photometric studies of the cluster did not support that claim.
NGC 2539 is an open cluster in the constellation Puppis, located at the north edge of the constellation. It was discovered by William Herschel on January 31, 1785. It is a moderately rich cluster and with little central concentration, with Trumpler class II1m.
NGC 5460 is an open cluster in the constellation Centaurus. It is a bright but loose cluster of intermediate age located approximately 2,300 light years away from Earth. It is located nearly 2 degrees east-southeast of Zeta Centauri.
NGC 5617 is an open cluster in the constellation Centaurus. NGC 5617 forms a binary open cluster with Trumpler 22. It lies one degree west-northwest of Alpha Centauri.
NGC 5316 is an open cluster in the constellation Centaurus. It was discovered by James Dunlop in 1826. Located approximately 4,000 light years away from Earth, it is located in the Carina-Sagittarius arm.
NGC 4103 is an open cluster in the constellation Crux. It was discovered by James Dunlop in 1826. It is located approximately 5,000 light years away from Earth, in the Carina-Sagittarius arm.
NGC 6910 is an open cluster in the constellation Cygnus. It was discovered by William Herschel on October 17, 1786. The cluster was also observed by John Herschel on September 18, 1828. It is a poor cluster and with prominent central concentration, with Trumpler class I2p. NGC 6910 is the core cluster of the stellar association Cygnus OB9.
