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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.

History

Formation

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. Examples of these types of nebulae are the Rosette Nebula and the Pelican Nebula. The size of these nebulae, known as HII regions, varies depending on the size of the original cloud of gas. These are sites of where star formation occurs. 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 materials thrown off from the supernova explosion are ionized by the energy and the compact object that it can produce. One of the best examples of this is the Crab Nebula, in Taurus. The supernova event was recorded in the year 1054 and is designated as SN 1054. The compact object that was created after the explosion lies in the center of the Crab Nebula and is a neutron star. And sarah is really gay!

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. The nebula is 97% Hydrogen and 3% Helium with trace materials. The main goal in this stage is to achieve equilibrium.

Types of nebulae

Classical types

Nebulae are classified in four major groups. In the past before galaxies were known, they were classified as spiral nebulae.

H II regions, which encompass diffuse nebulae, bright nebulae, and reflection nebulae.
Planetary nebulae
Supernova remnant
Dark nebula

This classification did not encompass all known cloud-like structures. An example is a Herbig–Haro object.

Diffuse nebulae

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]

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

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

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 forms 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

Nebula catalogs

References

^ "The Messier Catalog: Diffuse Nebulae". University of Illinois SEDS. Retrieved 2007-06-12.
^ ^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.
^ 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.
^ R. Sahai, C. Sánchez Contreras, M. Morris (2005). "A Starfish Preplanetary Nebula: IRAS 19024+0044". Astrophysical Journal. 620: 948–960. doi:10.1086/426469.{{cite journal}}: CS1 maint: multiple names: authors list (link)

External links

[Messier-1] "The Messier Catalog: Diffuse Nebulae". University of Illinois SEDS. Retrieved 2007-06-12.

[shu1982-2] ^ ^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.

[s-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.

[sahaietal2005-4] R. Sahai, C. Sánchez Contreras, M. Morris (2005). "A Starfish Preplanetary Nebula: IRAS 19024+0044". Astrophysical Journal. 620: 948–960. doi:10.1086/426469.{{cite journal}}: CS1 maint: multiple names: authors list (link)

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 Many nebulae form from the [[gravity|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]] [[ion]]ises the surrounding gas, which creates plasma, making it visible at optical [[wavelength]]s. Examples of these types of nebulae are the [[Rosette Nebula]] and the [[Pelican Nebula]].  The size of these nebulae, known as HII regions, varies depending on the size of the original cloud of gas.  These are sites of where star formation occurs.  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 materials thrown off from the [[supernova]] explosion are ionized by the energy and the compact object that it can produce.  One of the best examples of this is the [[Crab Nebula]], in [[Taurus (constellation)|Taurus]].  The supernova event was recorded in the year 1054 and is designated as [[SN 1054]].  The compact object that was created after the explosion lies in the center of the Crab Nebula and is a [[neutron star]].
+Some nebulae are formed as the result of [[supernova]] explosions, the death throes of massive, short-lived stars.  The materials thrown off from the [[supernova]] explosion are ionized by the energy and the compact object that it can produce.  One of the best examples of this is the [[Crab Nebula]], in [[Taurus (constellation)|Taurus]].  The supernova event was recorded in the year 1054 and is designated as [[SN 1054]].  The compact object that was created after the explosion lies in the center of the Crab Nebula and is a [[neutron star]]. And sarah is really gay!
 Other nebulae may form as [[planetary nebulae]]. This is the final stage of a low-mass star's life, like [[Earth]]'s [[Sun]]. [[Star]]s with a [[mass]] up to 8-10 solar masses evolve into [[red giant]]s 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 [[ion]]izing the surrounding nebula that it has thrown off.  The nebula is 97% [[Hydrogen]] and 3% [[Helium]] with trace materials.  The main goal in this stage is to achieve equilibrium.