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.
7-456: The Crescent Nebula (also known as NGC 6888 , Caldwell 27 , Sharpless 105 ) 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 (HD 192163) colliding with and energizing the slower moving wind ejected by the star when it became
14-403: 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. The Crescent Nebula is a rather small object located about 2 degrees Southwest of Sadr . While considered bright by astronomical imaging standards, visually it
21-418: Is relatively faint. For most telescopes it requires a UHC or OIII filter to see. Under favorable circumstances a telescope as small as 8 cm (with filter) can see its nebulosity. Larger telescopes (20 cm or more) reveal the crescent or a Euro sign shape which makes some to call it the "Euro sign nebula". Emission nebula Usually, a young star will ionize part of the same cloud from which it
28-470: The Balmer series . If more energy is available, other elements will be ionized, and green and blue nebulae become possible. By examining the spectra of nebulae, astronomers infer their chemical content. Most emission nebulae are about 90% hydrogen, with the remaining helium , oxygen , nitrogen , and other elements. Some of the most prominent emission nebulae visible from the northern celestial hemisphere are
35-744: The North America Nebula (NGC 7000) and Veil Nebula NGC 6960/6992 in Cygnus , while in the south celestial hemisphere, the Lagoon Nebula M8 / NGC 6523 in Sagittarius and the Orion Nebula M42. Further in the southern hemisphere is the bright Carina Nebula NGC 3372. Emission nebulae often have dark areas in them which result from clouds of dust which block the light. Many nebulae are made up of both reflection and emission components such as
42-472: The reflection nebulae. The radiation emitted by cooler stars is generally not energetic enough to ionize hydrogen, which results in the reflection nebulae around these stars giving off less light than the emission nebulae. The nebula's color depends on its chemical composition and degree of ionization. Due to the prevalence of hydrogen in interstellar gas, and its relatively low energy of ionization, many emission nebulae appear red due to strong emissions of
49-399: Was born, although only massive, hot stars can release sufficient energy to ionize a significant part of a cloud. In many emission nebulae, an entire cluster of young stars is contributing energy. Stars that are hotter than 25,000 K generally emit enough ionizing ultraviolet radiation (wavelength shorter than 91.2 nm) to cause the emission nebulae around them to be brighter than
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