AskDefine | Define nebula

Dictionary Definition



1 a medicinal liquid preparation intended for use in an atomizer
2 cloudiness of the urine
3 an immense cloud of gas (mainly hydrogen) and dust in interstellar space
4 (pathology) a faint cloudy spot on the cornea [also: nebulae (pl)]

User Contributed Dictionary



  • , /ˈnebjələ/, /"nebj@l@/


nubis, "cloud", nebula, "little cloud", "mist". Akin to Greek νεφέλη, "cloud", German Nebel, "mist", "nebula", Old Norse nifl.


  1. A cloud in outer space consisting of gas or dust (e.g. a cloud formed after a star explodes).


a space cloud

Related terms



ia-noun s


Related terms



Cognate with Ancient Greek νέφος, Sanskrit sc=Deva.



Extensive Definition

A nebula (from Latin: "mist"; pl. nebulae or nebulæ, with ligature or nebulas) is an interstellar cloud of dust, hydrogen gas and plasma. It is the first stage of a star's cycle. 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.


Many nebulae form from the gravitational collapse of diffuse 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, 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 ionised 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 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.

Diffuse nebulae

Most nebulae can be described as diffuse nebulae, which means that they are extended and contain no well-defined boundaries. 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. 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.

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. A PPN emits strong in 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

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. The expanding shell of gas form a supernova remnant, a special type of diffuse nebula. 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. This emission originates from high-velocity and electrons oscillating within magnetic fields.If the star cannot support itself, it can form a black hole, or also a gamma ray burst


Further reading

Lightner, G. Samuel. "Nebulae: Fuzzy Patches in Space." 18 December 2000.
---. "Nebulae." 17 November 2005.
---. "Reflection Nebulae." 18 December 2000.
---. "Emission Nebulae." 18 December 2000.
---. "Planetary Nebulae." 18 December 2000.
---. "Supernova Remnants." December 2000.

External links

nebula in Arabic: سديم
nebula in Bosnian: Maglica
nebula in Bulgarian: Мъглявина
nebula in Catalan: Nebulosa
nebula in Czech: Mlhovina
nebula in Danish: Stjernetåge
nebula in German: Nebel (Astronomie)
nebula in Modern Greek (1453-): Νεφέλωμα
nebula in Spanish: Nebulosa
nebula in Esperanto: Nebulozo
nebula in Basque: Nebulosa
nebula in Persian: سحابی
nebula in French: Nébuleuse
nebula in Galician: Nebulosa
nebula in Gujarati: નિહારિકા
nebula in Croatian: Nebula
nebula in Ido: Nebuloso
nebula in Indonesian: Nebula
nebula in Italian: Nebulosa
nebula in Hebrew: ערפילית
nebula in Georgian: ნისლეულები
nebula in Swahili (macrolanguage): Nebula
nebula in Latin: Nebula (astronomia)
nebula in Luxembourgish: Niwwel (Astronomie)
nebula in Lithuanian: Ūkas
nebula in Hungarian: Csillagköd
nebula in Dutch: Nevels en gaswolken
nebula in Japanese: 星雲
nebula in Norwegian: Stjernetåke
nebula in Novial: Nebula
nebula in Polish: Mgławica
nebula in Portuguese: Nebulosa
nebula in Romanian: Nebuloasă
nebula in Russian: Туманность
nebula in Slovak: Hmlovina
nebula in Slovenian: Meglica
nebula in Finnish: Kaasusumu
nebula in Swedish: Nebulosa
nebula in Thai: เนบิวลา
nebula in Turkish: Nebula (astronomi)
nebula in Ukrainian: Галактична туманність
nebula in Urdu: سحابیہ
nebula in Contenese: 星雲
nebula in Chinese: 星云

Synonyms, Antonyms and Related Words

Black Magellanic Cloud, Crab Nebula, Gegenschein, bright diffuse nebula, coalsack, counterglow, dark cloud, dark nebula, diffuse nebula, dust cloud, gaseous nebula, nebula of Lyra, nebular hypothesis, nebulosity, nebulous stars, planetary nebula, ring nebula, the Coalsack, zodiacal light
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