nebula

1. Astronomy.
   1. A diffuse mass of interstellar dust or gas or both, visible as luminous patches or areas of darkness depending on the way the mass absorbs or reflects incident radiation.
   2. See galaxy (sense 1).
2. Pathology.
   1. A cloudy spot on the cornea.
   2. Cloudiness in the urine.
3. A liquid medication applied by spraying.

[Middle English nebule, cloud, mist, from Latin nebula.]

Originally, any fixed, extended, and usually fuzzy luminous object seen in a telescope. Nebulae are now distinguished from star clouds that can be resolved into individual stars, but earlier workers were unable to differentiate between white nebulae, which are stellar systems so remote as to show no individual stars, and gaseous or diffuse nebulae in the Milky Way Galaxy. See also Star clouds.

Extragalactic nebulae are stellar systems comparable with the Milky Way Galaxy or the Magellanic Clouds in size and number of stars, and are more properly termed external galaxies. See also Galaxy, external.

This article deals with gaseous nebulae. This class of objects includes diffuse nebulae which contain dust and gas of the interstellar medium, excited and caused to fluoresce by embedded stars. Gaseous nebulae are members of the Milky Way galactic system, and small compared with its overall dimensions. Various types of gaseous nebulae have been identified. See also Interstellar matter.

Diffuse nebulae range in density from a few atoms per cubic centimeter to 10,000 or more atoms per cubic centimeter (as in the Orion Nebula). Some are compact objects less than a parsec in diameter. Both dust and gas are excited by ultraviolet radiation of stars. Some diffuse nebulae such as Orion occur at the edges of large clouds of cool dust and gas, mostly in molecular form. Those of lower density are found from the faint glow in the red hydrogen line produced as hydrogen ions recapture electrons. For this reason they are also called H II regions, indicating regions of ionized hydrogen. They are also found in external galaxies such as the Magellanic Clouds and M33. See also Magellanic Clouds; Orion Nebula.

Reflection nebulae show no bright line spectra. Dust grains simply reflect the light of nearby embedded stars. Hydrogen gas is present but mostly neutral. The Pleiades nebulosity is an example of this type. See also Pleiades.

Nebulae associated with star formation include the so-called fan-shaped nebulae associated with T Tauri stars, certain bipolar nebulae, and Herbig-Haro Objects. Some, such as Hubble's variable nebula, associated with the variable star R Monocerotis, show brightness fluctuations. In many instances, a newly formed star excites and ionizes the gas in its immediate neighborhood, although the star itself is quite concealed by its dusty surroundings. See also Stellar evolution.

Planetary nebulae are so denoted because they often show small greenish disks in the telescope, not unlike the images of the planets Uranus and Neptune. The energy emitted by planetary nebulae is derived mostly from the ultraviolet emission of the central star, although in some objects an important component may be due to shock waves. See also Planetary nebula.

The detonation of a star in a supernova event causes the ejection of the outer layers into the surrounding interstellar medium. In early stages as in the Crab Nebula, the radiating material consists of ejecta from the star. In the later stages this rapidly moving material is slowed down as it mixes with the surrounding dust and gas of the interstellar medium. Heating by shock waves causes the material to radiate optically. Sometimes, the temperature behind the shock front can rise to more than 10⁶ K, but the gas is so rarefied that the intensity of the emitted radiation is extremely low. Supernovae remnants characteristically emit nonthermal radio-frequency emission, whereby they are often detected in nearby galaxies as well as in the Milky Way system. See also Crab Nebula; Supernova.

Cocoon nebulae are associated with very massive stars. At a late stage in its evolution a massive star may eject a dense shell of material that effectively hides it from view temporarily. Although the extended Carina Nebula appears to be a normal H II region, η Carinae itself is a dense, compact object which hides the central star and emits a remarkable spectrum dominated by forbidden lines of ionized iron.

For more information on nebula, visit Britannica.com.

plur. nebulae

In astronomy, a hazy patch of light visible in the sky. Some nebulae are clouds of gas within the Milky Way; others are distant galaxies. (See photo, next page.)

1. a slight corneal opacity.
2. an oily preparation for use in an atomizer.

For other uses, see Nebula (disambiguation).

The Triangulum Emission Garren Nebula NGC 604

The "Pillars of Creation" from the Eagle Nebula

A nebula (from Latin: "cloud" [1]; pl. nebulae or nebulæ, with ligature or nebulas) is an interstellar cloud of dust, hydrogen gas, helium gas and plasma. Originally nebula was a general name for any extended astronomical object, including galaxies beyond the Milky Way (some examples of the older usage survive; for example, the Andromeda Galaxy was referred to as the Andromeda Nebula before galaxies were discovered by Edwin Hubble). Nebulae often form star-forming regions, such as in the Eagle Nebula. This nebula is depicted in one of NASA's most famous images, the "Pillars of Creation". In these regions the formations of gas, dust and other materials "clump" together to form larger masses, which attract further matter, and eventually will become big enough to form stars. The remaining materials are then believed to form planets, and other planetary system objects.

Contents 1 Formation 2 Diffuse nebulae 3 Specific types of nebulae 3.1 Planetary nebulae 3.1.1 Protoplanetary nebula 3.2 Supernova remnants 4 Notable named nebulae 5 See also 6 References 7 External links

Formation

NGC 2024, The Flame Nebula

Many nebulae form from the gravitational collapse of gas in the interstellar medium or ISM. As the material collapses under its own weight, massive stars may form in the center, and their ultraviolet radiation ionises the surrounding gas, which creates plasma, making it visible at optical wavelengths. An example of this type of nebula is the Rosette Nebula or the Pelican Nebula. The size of these nebulae, known as HII regions, varies depending on the size of the original cloud of gas, and the number of stars formed can vary too. As the sites of star formation, the formed stars are sometimes known as a young, loose cluster. Some nebulae are formed as the result of supernova explosions, the death throes of massive, short-lived stars. The material thrown off from the supernova explosion is ionized by the supernova remnant. One of the best examples of this is the Crab Nebula, in Taurus. It is the result of a recorded supernova, SN 1054, in the year 1054 and at the centre of the nebula is a neutron star, created during the explosion.

Other nebulae may form as planetary nebulae. This is the final stage of a low-mass star's life, like Earth's Sun. Stars with a mass up to 8-10 solar masses evolve into red giants and slowly lose their outer layers during pulsations in their atmospheres. When a star has lost a sufficient amount of material, its temperature increases and the ultraviolet radiation it emits is capable of ionizing the surrounding nebula that it has thrown off. It is 97% Hydrogen and 3% Helium. The main goal in this stage is to achieve equilibrium.

Diffuse nebulae

The Omega Nebula, an example of an emission nebula.

The Pleiades. The diffuse nebulae near the stars are examples of reflection nebula.

Most nebulae can be described as diffuse nebulae, which means that they are extended and contain no well-defined boundaries.^[1] In visible light these nebulae may be divided into emission nebulae and reflection nebulae, a categorization that depends on how the light we see is created. Emission nebulae contain ionized gas (mostly ionized hydrogen) that produces spectral line emission.^[2] These emission nebulae are often called HII regions; the term "HII" is used in professional astronomy to refer to ionized hydrogen. In contrast to emission nebulae, reflection nebulae do not produce significant amounts of visible light by themselves but instead reflect light from nearby stars.^[2]

The Horsehead Nebula, an example of a dark nebula.

Dark nebulae are similar to diffuse nebulae, but they are not seen by their emitted or reflected light. Instead, they are seen as dark clouds in front of more distant stars or in front of emission nebulae.^[2]

Although these nebulae appear different at optical wavelengths, they all appear to be bright sources of emission at infrared wavelengths. This emission comes primarily from the dust within the nebulae.^[2]

Specific types of nebulae

While diffuse nebulae have poorly-defined boundaries, a few nebulae may actually be described as discrete objects with identifiable boundaries.

Planetary nebulae

The Cat's Eye Nebula, an example of a planetary nebula.

Planetary nebulae are nebulae that form from the gaseous shells that are ejected from low-mass asymptotic giant branch stars when they transform into white dwarfs.^[2] These nebulae are emission nebulae with spectral emission that is similar to the emission nebulae found in star formation regions.^[2] Technically, they are a type of HII region because the majority of hydrogen will be ionized. However, planetary nebulae are denser and more compact than the emission nebulae in star formation regions.^[2] Planetary nebulae are so called because the first astronomers who observed these objects thought that the nebulae resembled the disks of planets, although they are not at all related to planets.^[3]

Protoplanetary nebula

The Red Rectangle Nebula, an example of a protoplanetary nebula.

A protoplanetary nebula (PPN) is an astronomical object which is at the short-lived episode during a star's rapid stellar evolution between the late asymptotic giant branch (LAGB) phase and the subsequent planetary nebula (PN) phase.^[4] A PPN emits strong infrared radiation, and is a kind of reflection nebula. The exact point when a PPN becomes a planetary nebula (PN) is defined by the temperature of the central star.

Supernova remnants

The Crab Nebula, an example of a supernova remnant.

A supernova occurs when a high-mass star reaches the end of its life. When nuclear fusion ceases in the core of the star, the star collapses inward on itself. The gas falling inward either rebounds or gets so strongly heated that it expands outwards from the core, thus causing the star to explode.^[2] The expanding shell of gas form a supernova remnant, a special type of diffuse nebula.^[2] Although much of the optical and X-ray emission from supernova remnants originates from ionized gas, a substantial amount of the radio emission is a form of non-thermal emission called synchrotron emission.^[2] This emission originates from high-velocity electrons oscillating within magnetic fields.

Notable named nebulae

• Ant Nebula
• Barnard's Loop
• Boomerang Nebula
• Cat's Eye Nebula
• Crab Nebula
• Eagle Nebula
• Eskimo Nebula
• Eta Carinae Nebula
• Hourglass Nebula
• Horsehead Nebula
• Orion Nebula
• Pelican Nebula
• Red Square Nebula
• Ring Nebula
• Rosette Nebula
• Tarantula Nebula

See also

• H I region
• H II region
• List of diffuse nebulae
• Magellanic Clouds
• Messier object
• Orion Molecular Cloud Complex
• Timeline of the interstellar medium and intergalactic medium

References

1. ^ "The Messier Catalog: Diffuse Nebulae". University of Illinois SEDS. http://seds.lpl.arizona.edu/messier/diffuse.html. Retrieved on 2007-06-12. 
2. ^ ^{a b c d e f g h i j} F. H. Shu (1982). The Physical Universe. Mill Valley, California: University Science Books. ISBN 0-935702-05-9. 
3. ^ E. Chaisson, S. McMillan (1995). Astronomy: a beginner's guide to the universe (2nd ed.). Upper Saddle River, New Jersey: Prentice-Hall. ISBN 0-13-733916-X. 
4. ^ R. Sahai, C. Sánchez Contreras, M. Morris (2005). "A Starfish Preplanetary Nebula: IRAS 19024+0044". Astrophysical Journal 620: 948–960. doi:10.1086/426469. http://adsabs.harvard.edu/abs/2005ApJ...620..948S. 

External links

Wikimedia Commons has media related to: Nebula

• fusedweb.pppl.gov
• Information on star formation
• Clickable table of Messier objects

This entry is from Wikipedia, the leading user-contributed encyclopedia. It may not have been reviewed by professional editors (see full disclaimer)

Dansk (Danish)
n. - tågeplet, stjernetåge, plet på hornhinden

Nederlands (Dutch)
nevel, nebula

Français (French)
n. - nébuleuse

Deutsch (German)
n. - Nebel, Hornhauttrübung

Ελληνική (Greek)
n. - (αστρον.) νεφέλωμα

Italiano (Italian)
nebulosa

Português (Portuguese)
n. - nebulosa (f) (Astr.), nébula (f) (Med.)

Русский (Russian)
туманность

Español (Spanish)
n. - nebulosa

Svenska (Swedish)
n. - nebulosa, hornhinnegrumling

中文（简体）(Chinese (Simplified))
星云, 云翳

中文（繁體）(Chinese (Traditional))
n. - 星雲, 雲翳

한국어 (Korean)
n. - 성운, (병)각막예

日本語 (Japanese)
n. - 星雲

العربيه (Arabic)
‏(الاسم) غمامه ( على قرينه العين), سديم, غيمه سديميه‏

עברית (Hebrew)
n. - ‮ערפילית (ענן גז ואבק בחלל), איזור מואר בחלל, נקודה מעורפלת על קרנית העין הגורמת לטשטוש בראייה‬

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