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.
The Helix Nebula is a planetary nebula (PN) located in the constellation Aquarius. Discovered by Karl Ludwig Harding, most likely before 1824, this object is one of the closest of all the bright planetary nebulae to Earth. The distance, measured by the Gaia mission, is 655±13 light-years. It is similar in appearance to the Cat's Eye Nebula and the Ring Nebula, whose size, age, and physical characteristics are similar to the Dumbbell Nebula, varying only in its relative proximity and the appearance from the equatorial viewing angle. The Helix Nebula has sometimes been referred to as the "Eye of God" in pop culture, as well as the "Eye of Sauron".
The Cat's Eye Nebula is a planetary nebula in the northern constellation of Draco, discovered by William Herschel on February 15, 1786. It was the first planetary nebula whose spectrum was investigated by the English amateur astronomer William Huggins, demonstrating that planetary nebulae were gaseous and not stellar in nature. Structurally, the object has had high-resolution images by the Hubble Space Telescope revealing knots, jets, bubbles and complex arcs, being illuminated by the central hot planetary nebula nucleus (PNN). It is a well-studied object that has been observed from radio to X-ray wavelengths.
NGC 2346 is a planetary nebula near the celestial equator in the constellation of Monoceros, less than a degree to the ESE of Delta Monocerotis. It is informally known as the Butterfly Nebula. The nebula is bright and conspicuous with a visual magnitude of 9.6, and has been extensively studied. Among its most remarkable characteristics is its unusually cool central star, which is a spectroscopic binary, and its unusual shape.
NGC 3132 is a bright and extensively studied planetary nebula in the constellation Vela. Its distance from Earth is estimated at 613 pc or 2,000 light-years.
NGC 1514, also known as the Crystal Ball Nebula, is a planetary nebula in the zodiac constellation of Taurus, positioned to the north of the star Psi Tauri along the constellation border with Perseus. Distance to the nebula is 466 pc, according to GAIA DR2 data.
NGC 6302 is a bipolar planetary nebula in the constellation Scorpius. The structure in the nebula is among the most complex ever seen in planetary nebulae. The spectrum of Butterfly Nebula shows that its central star is one of the hottest stars known, with a surface temperature in excess of 250,000 degrees Celsius, implying that the star from which it formed must have been very large.
NGC 7354 is a planetary nebula located in the northern circumpolar constellation of Cepheus, at a distance of approximately 5.5 kly from the Sun. It was discovered by German-born astronomer William Herschel on November 3, 1787. John L. E. Dreyer described it as, "a planetary nebula, bright, small, round, pretty gradually a very little brighter middle".
NGC 6790 is a young, compact planetary nebula with a high surface brightness located in the equatorial constellation of Aquila. Imaging by the Hubble Space Telescope shows elongated shells surrounding the central star. The distance to this nebula is poorly known, but is estimated at 19 kilolight-years, and it is roughly 6,000 years old. The expansion velocity of the neutral hydrogen component is in the range 15−19 km s−1. The central star is a white dwarf with a temperature of around 73,500 K and a photographic magnitude of 11.1. It has a mass of 0.6 M☉, having evolved from a star with a mass about the same as the Sun.
NGC 3195 is a planetary nebula located in the southern constellation of Chamaeleon. Discovered by Sir John Herschel in 1835, this 11.6 apparent magnitude planetary nebula is slightly oval in shape, with dimensions of 40×35 arc seconds, and can be seen visually in telescopic apertures of 10.5 centimetres (4.1 in) at low magnifications.
NGC 6072 is a planetary nebulae in the southern constellation of Scorpius. It has a dynamical age of 104 years. Its circumstellar envelope is likely to be rich in carbon as it has very strong CN (cyanide) spectral lines. CN spectral lines are generally not detected in oxygen rich AGB (asymptotic giant branch) circumstellar envelopes. NGC 6072 also shows H2 (hydrogen) emission and intense CO (carbon monoxide) emission which has been mapped displaying bipolarity and some gas at high velocity. The evolution of this planetary nebulae is likely to be dominated by photodissociation and ion/radical molecular reactions. Shock chemistry is also likely to be important.
Little Ghost Nebula, also known as NGC 6369, is a planetary nebula in the constellation Ophiuchus. It was discovered by William Herschel.
NGC 7026 is a planetary nebula located 6000 light-years away, in the constellation of Cygnus. The central star of the planetary nebula has a spectral type of [WO3], indicating a spectrum similar to that of an oxygen-rich Wolf–Rayet star. An analysis of Gaia data suggests that it is a binary system.
NGC 6153 is a planetary nebula in the constellation Scorpius. It was discovered in 1883 by Ralph Copeland.
NGC 1501 is a complex planetary nebula located in the constellation of Camelopardalis, it was discovered on 27 August 1787 by William Herschel.
Cometary knots, also referred as globules, are structures observed in several nearby planetary nebulae (PNe), including the Helix Nebula, the Ring Nebula, the Dumbbell Nebula, the Eskimo Nebula, and the Retina Nebula. They are believed to be a common feature of the evolution of planetary nebulae, but can only be resolved in the nearest examples. They are generally larger than the size of the Solar System, with masses of around 0.00001 times the mass of the Sun, which is comparable to the mass of the Earth. There are about 40,000 cometary knots in the Helix Nebula.
NGC 6441, sometimes also known as the Silver Nugget Cluster, is a globular cluster in the southern constellation of Scorpius. It was discovered by the Scottish astronomer James Dunlop on May 13, 1826, who described it as "a small, well-defined rather bright nebula, about 20″ in diameter". The cluster is located 5 arc minutes east-northeast of the star G Scorpii, and is some 43,000 light-years from the Sun.
NGC 6905, also known as the Blue Flash Nebula, is a planetary nebula in the constellation Delphinus. It was discovered by William Herschel in 1784. The central star is 14.0 mag. The distance of the nebula, as with most planetary nebulae, is not well determined and estimates range between 1.7 and 2.6 kpc.
NGC 6778 is a planetary nebula (PN) located about 10,300 light years away from the Sun in the equatorial constellation of Aquila. It is positioned 5° to the SSW of the prominent star Delta Aquilae. This nebula was discovered by German astronomer Albert Marth during the period 1863–1865. English astronomer John Herschel may have mistakenly catalogued it as NGC 6785, as nothing can be found now at the coordinates he gave for it. In the New General Catalogue it was described as a "small, elongated, ill-defined disc".
LoTr 5 is a large, faint planetary nebula in the constellation of Coma Berenices. In 2018, its parallax was measured by Gaia, giving a distance of about 1,650 light-years.
