A planetary nebula is a type of emission nebula consisting of an expanding, glowing shell of ionized gas ejected from red giant stars late in their lives.
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".
An H II region or HII region is a region of interstellar atomic hydrogen that is ionized. It is typically in a molecular cloud of partially ionized gas in which star formation has recently taken place, with a size ranging from one to hundreds of light years, and density from a few to about a million particles per cubic centimetre. The Orion Nebula, now known to be an H II region, was observed in 1610 by Nicolas-Claude Fabri de Peiresc by telescope, the first such object discovered.
Wolf–Rayet stars, often abbreviated as WR stars, are a rare heterogeneous set of stars with unusual spectra showing prominent broad emission lines of ionised helium and highly ionised nitrogen or carbon. The spectra indicate very high surface enhancement of heavy elements, depletion of hydrogen, and strong stellar winds. The surface temperatures of known Wolf–Rayet stars range from 20,000 K to around 210,000 K, hotter than almost all other kinds of stars. They were previously called W-type stars referring to their spectral classification.
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.
IC 4406, sometimes known as the Retina Nebula, is a planetary nebula near the western border of the constellation Lupus, the Wolf. It has dust clouds and has the shape of a torus. Despite this, it looks somewhat rectangular because it is seen from its side as viewed from Earth, almost in the plane of its equator.
NGC 5882 is a small planetary nebula in the southern constellation of Lupus, positioned about 1.5° to the southwest of the star Epsilon Lupi. It was discovered by English astronomer John Herschel on July 2, 1834 from the Cape of Good Hope observatory. John L. E. Dreyer described it as "very small, round, quite sharp". It is located at a distance of approximately 7.7 kilolight-years from the Sun.
NGC 5307 is a planetary nebula in the southern constellation of Centaurus, positioned less than 3° to the northeast of the star Epsilon Centauri. It was discovered by English astronomer John Herschel on April 15, 1836. The nebula is located at a distance of approximately 10.6 kilolight-years from the Sun. The central star, designated PNG 312.3+10.5, is a weak emission-line star, superficially similar to the WC subtype of Wolf–Rayet stars. It has a spectral class of O(H)3.5 V.
NGC 6884 is a planetary nebula located in the constellation Cygnus, less than a degree to the southwest of the star Ο1 Cygni. It lies at a distance of approximately 12.5 kly from the Sun. The nebula was discovered on May 8, 1883, by American astronomer Edward C. Pickering.
IC 2149 is a planetary nebula in the constellation of Auriga. It is a small, bright planetary nebula with something to offer in telescopes of most sizes.
IRAS 23304+6147 is a protoplanetary nebula in the constellation Cassiopeia, 16,000 light years away. The central star is a G-type supergiant.
HD 179821 or V1427 Aquilae is either a post-red supergiant yellow hypergiant or a post-AGB yellow supergiant star in the constellation of Aquila, surrounded by a detached dust shell. It is a semi-regular variable nearing the end of its life.
IRAS 19475+3119 is a protoplanetary nebula in the constellation of Cygnus, 15,000 light-years away. The central star, V2513 Cygni, is an F-type post-AGB star.
WR 102 is a Wolf–Rayet star in the constellation Sagittarius, an extremely rare star on the WO oxygen sequence. It is a luminous and very hot star, highly evolved and close to exploding as a supernova.
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 6445, also known as the Little Gem Nebula or Box Nebula, is a planetary nebula in the constellation Sagittarius. It was discovered by William Herschel on May 28, 1786. The distance of NGC 6445 is estimated to be slightly more than 1,000 parsecs based on the parallax measured by Gaia, which was measured at 0.9740±0.3151 mas.
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.
M33-013406.63, also known as B416 or UIT301, is a O-type blue evolved supergiant star in the constellation of Triangulum. It is located within the Triangulum Galaxy, which is approximately 2,380,000–3,070,000 light years away from Earth.
HM Sagittae is a dusty-type symbiotic nova in the northern constellation of Sagitta. It was discovered by O. D. Dokuchaeva and colleagues in 1975 when it increased in brightness by six magnitudes. The object displays an emission line spectrum similar to a planetary nebula and was detected in the radio band in 1977. Unlike a classical nova, the optical brightness of this system did not rapidly decrease with time, although it showed some variation. It displays activity in every band of the electromagnetic spectrum from X-ray to radio.
AF Andromedae is a luminous blue variable (LBV), a type of variable star. The star is one of the most luminous variables in M31, the Andromeda Galaxy.
