An emission nebula is a nebula formed of ionized gases that emit light of various wavelengths. The most common source of ionization is high-energy ultraviolet photons emitted from a nearby hot star. Among the several different types of emission nebulae are H II regions, in which star formation is taking place and young, massive stars are the source of the ionizing photons; and planetary nebulae, in which a dying star has thrown off its outer layers, with the exposed hot core then ionizing them.
NGC 2264 is the designation number of the New General Catalogue that identifies two astronomical objects as a single object: the Cone Nebula, and the Christmas Tree Cluster. Two other objects are within this designation but not officially included, the Snowflake Cluster, and the Fox Fur Nebula. All of the objects are located in the Monoceros constellation and are located about 720 parsecs or 2,300 light-years from Earth. Due to its relative proximity and large size, it is extremely well studied. NGC 2264 is sometimes referred to as the Christmas Tree Cluster and the Cone Nebula. However, the designation of NGC 2264 in the New General Catalogue refers to both objects and not the cluster alone. In December 2023, NASA released Christmas holiday-related images by the James Webb Space Telescope, including the Christmas Tree Galaxy Cluster and others.
A starburst is an astrophysical process that involves star formation occurring at a rate that is large compared to the rate that is typically observed. This starburst activity will consume the available interstellar gas supply over a timespan that is much shorter than the lifetime of the galaxy. For example, the nebula NGC 6334 has a star formation rate estimated to be 3600 solar masses per million years compared to the star formation rate of the entire Milky Way of about seven million solar masses per million years. Due to the high amount of star formation a starburst is usually accompanied by much higher gas pressure and a larger ratio of hydrogen cyanide to carbon monoxide emission-lines than are usually observed.
The Crescent Nebula is an emission nebula in the constellation Cygnus, about 5000 light-years away from Earth. It was discovered by William Herschel in 1792. It is formed by the fast stellar wind from the Wolf-Rayet star WR 136 colliding with and energizing the slower moving wind ejected by the star when it became a red giant around 250,000 to 400,000 years ago. The result of the collision is a shell and two shock waves, one moving outward and one moving inward. The inward moving shock wave heats the stellar wind to X-ray-emitting temperatures.
NGC 3132 is a bright and extensively studied planetary nebula in the constellation Vela. Its distance from Earth is estimated at about 613 pc. or 2,000 light-years.
NGC 4656/57 is a highly warped edge-on barred spiral galaxy located in the local universe 30 million light years away from earth in the constellation Canes Venatici. This galaxy is sometimes called the Hockey Stick Galaxy or the Crowbar Galaxy. Its unusual shape is thought to be due to an interaction between NGC 4656, NGC 4631, and NGC 4627. The galaxy is a member of the NGC 4631 Group.
NGC 2023 is an emission and reflection nebula in the equatorial constellation of Orion. It was discovered by the German-born astronomer William Herschel on 6 January 1785. This reflection nebula is one of the largest in the sky, with a size of 10 × 10 arcminutes. It is located at a distance of 1,300 ly (400 pc) from the Sun, and is positioned ~15′ to the northeast of the Horsehead Nebula.
NGC 6334, colloquially known as the Cat's Paw Nebula, or Gum 64, is an emission nebula and star-forming region located in the constellation Scorpius. NGC 6334 was discovered by astronomer John Herschel in 1837, who observed it from the Cape of Good Hope in South Africa. The nebula is located in the Carina–Sagittarius Arm of the Milky Way, at a distance of approximately 5.5 kilolight-years from the Sun.
NGC 3576 is a bright emission nebula in the Sagittarius arm of the galaxy a few thousand light-years away from the Eta Carinae nebula. It is also approximately 100 light years across and 9000 light-years away from Earth. It was discovered by John Frederick William Herschel on 16 March 1834. This nebula has received six different classification numbers. Currently, astronomers call the entire nebula NGC 3576. A popular nickname is "The Statue of Liberty Nebula" because of the distinctive shape in the middle of the nebula. The name was first suggested in 2009 by Dr. Steve Mazlin, a member of Star Shadows Remote Observatory (SSRO). Within the nebula, episodes of star formation are thought to contribute to the complex and suggestive shapes. Powerful winds from the nebula's embedded, young, massive stars shape the looping filaments.
The Sadr Region is the diffuse emission nebula surrounding Sadr at the center of Cygnus's cross. The Sadr Region is one of the surrounding nebulous regions; others include the Butterfly Nebula and the Crescent Nebula. It contains many dark nebulae in addition to the emission diffuse nebulae.
NGC 2359 is an emission nebula in the constellation Canis Major. The nebula is approximately 3,670 parsecs away and 30 light-years in size. The central star is the Wolf-Rayet star WR7, an extremely hot star thought to be in a brief pre-supernova stage of evolution. It is similar in nature to the Bubble Nebula, but interactions with a nearby large molecular cloud are thought to have contributed to the more complex shape and curved bow-shock structure of Thor's Helmet.
NGC 1931 is an emission and reflection nebula in the constellation Auriga. The nebula has been referred to as a "miniature version of the Orion Nebula", as it shares some of the same characteristics. It is a mixed emission-reflection nebula, and contains a smaller version of the Trapezium in its hot young star cluster centered in the emission nebula. The entire cluster/nebula complex is only about 3 arcmin in size. The distance from earth is estimated at about 7000 light years.
Sh2-54 is an extended bright nebula in the constellation of Serpens.
NGC 2032 is an emission nebula in the Dorado constellation and near the supershell LMC-4 and it consists of NGC 2029, NGC 2035 and NGC 2040. It was first discovered by James Dunlop on 27 September 1826, and John Herschel rerecorded it on 2 November 1834. NGC 2032 is located in the Large Magellanic Cloud.
NGC 1624, also known as Sh2-212 in the Sharpless catalog, is a very young open cluster in the constellation Perseus inside an emission nebula. It was discovered by German-British astronomer William Herschel in 1790. NGC 1624 is about 20,000 ly from Earth, and latest estimates give it an age of less than 4 million years. Its apparent magnitude is 11.8, and apparent diameter is about 3.0 arc minutes. Its celestial location is right ascension (α) 04h 40m 36.0s and declination (δ) +50° 27′ 42″. It is potentially an area of massive star formation.
N11 is the brightest emission nebula in the north-west part of the Large Magellanic Cloud in the Dorado constellation. The N11 complex is the second largest H II region of that galaxy, the largest being the Tarantula Nebula. It covers an area approximately 6 arc minutes across. It has an elliptical shape and consists of a large bubble, generally clear interstellar area, surrounded by nine large nebulae. It was named by Karl Henize in 1956.
NGC 3199 is an emission nebula in the constellation Carina. It is commonly known as the Banana Nebula. The object was discovered in 1826 by the Scottish astronomer James Dunlop. It was thought to be the bow shock around the central star, WR 18, an especially hot and luminous Wolf–Rayet star; however, it was determined that the nebula formed due to the composition of local space, not because of the star's movement.
NGC 592 is an H II region type emission nebula located in the Triangulum galaxy (M33) and thus in the constellation of Triangulum. The nebula contains an open cluster of stars and is approximately 2.86 million light-years away from Earth.
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.
