nebula

a medicinal liquid preparation intended for use in an atomizer cloudiness of the urine an immense cloud of gas (mainly hydrogen) and dust in interstellar space (pathology) a faint cloudy spot on the cornea

lies in a spiral arm of Galaxy M33, 2.7 million light-years from Earth. This nebula is a region in which stars are forming.]] ary nebula located about 8,000 light years away. This image was taken with the Wide Field and Planetary Camera2 aboard NASA's Hubble Space Telescope.]]A nebula (Latin for "mist"; pl. nebulae) is an interstellar cloud of dust, gas and plasma. Originally ''nebula'' was a general name for any extended astronomyastronomical astronomical objectobject, including galaxygalaxies beyond the Milky Way (some examples of the older usage survive; for example, the Andromeda Galaxy is sometimes referred to as the ''Andromeda Nebula'').Nebulae can be classified by how they are illuminated:

Diffuse nebulae are illuminated nebulae

* Emission nebulae are internally illuminated clouds of ionized gas. Two of the most common types of emission nebula are H II regions and Planetary nebulae

* Reflection nebulae are illuminated by reflections from nearby stars. An example is the nebulosity inside the Pleiades (astronomy)Pleiades star cluster.

Dark nebulae are unilluminated. They can be detected when they obscure stars or other nebulae. Famous examples include the Horsehead nebula in Orion (constellation)Orion, and the Coalsack Nebula in the CruxSouthern Cross.HII regions are the Stellar evolutionbirthplace of stars. They are formed when very diffuse molecular clouds begin to collapse under their own gravity, often due to the influence of a nearby supernova explosion. The cloud collapses and fragments, forming sometimes hundreds of new stars. The newly-formed stars ionizationionize the surrounding gas to produce an emission nebula.Other nebulae are formed by the death of stars. A star that undergoes the transition to a white dwarf blows off its outer layer to form a planetary nebula. Novae and supernovae can also create nebulae known as nova remnants and supernova remnants respectively.See also:

Solar nebula

Timeline of the interstellar medium and intergalactic medium

Crab nebula

Messier object

:Category:Nebular imagesImages of nebulaeThe Nebula Awards, annually given out by the Science Fiction and Fantasy Writers of America, are named after !nebulae.Category:Nebulae *Category:Space plasmasCategory:Plasma physicsca:Nebulosa !planetàriada:Stjernetågede:N ebel? !(Astronomie)eo:Nebulozofr:Néb uleuseio:Nebulosoit:Nebulosaja :星雲? nl:Nevels en !gaswolkenpl:Mgławicaro:Nebulo asăsk:Hmlovinasl:meglicafi:Ka asusumuhe:ערפ ;יליתz h:星云

Students for the Exploration and the Development of Space
Comprehensive overview with newsletters, forums, projects facts of a worldwide student space organization.
http://www.seds.org

Nebula Books
Books, comics and magazines devoted to "the fantastic, the imaginative and the weird". It's located in Montreal, but has an online catalog and shopping cart.
http://www.nebulabooks.com/

National Optical Astronomy Observatories (NOAO)
A national center for optical sky observations, which is operated by the Association of Universities for Research in Astronomy.
http://www.noao.edu/