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.
5-603: The Sadr Region (also known as IC 1318 or the Gamma Cygni Nebula ) is the diffuse emission nebula surrounding Sadr ( γ Cygni ) 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 . Sadr itself has approximately
10-447: A magnitude of 2.2. The nebulous regions around the region are also fairly bright. This nebula-related article is a stub . You can help Misplaced Pages by expanding it . Emission nebula Usually, a young star will ionize part of the same cloud from which it 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
15-429: 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 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
20-530: 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 the North America Nebula (NGC 7000) and Veil Nebula NGC 6960/6992 in Cygnus , while in the south celestial hemisphere,
25-458: 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 the Balmer series . If more energy is available, other elements will be ionized, and green and blue nebulae become possible. By examining
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