Anebula (Latin for 'cloud, fog';^[1]pl. nebulae ornebulas)[2]^[3][4][5] is a distinct luminescent part ofinterstellar medium, which can consist of ionized, neutral, or molecularhydrogen and alsocosmic dust. Nebulae are often star-forming regions, such as in thePillars of Creation in theEagle Nebula. In these regions, the formations of gas, dust, and other materials "clump" together to form denser regions, which attract further matter and eventually become dense enough to formstars. The remaining material is then thought to formplanets and otherplanetary system objects.

Most nebulae are of vast size; some are hundreds oflight-years in diameter. A nebula that is visible to thehuman eye fromEarth would appear larger, but no brighter, from close by.^[6] TheOrion Nebula, the brightest nebula in the sky and occupying an area twice the angular diameter of the fullMoon, can be viewed with the naked eye but was missed by early astronomers.^[7] Although denser than the space surrounding them, most nebulae are far less dense than anyvacuum created on Earth (10⁵ to 10⁷ molecules per cubic centimeter) – a nebular cloud the size of theEarth would have a total mass of only a fewkilograms. Earth's air has a density of approximately 10¹⁹ molecules per cubic centimeter; by contrast, the densest nebulae can have densities of 10⁴ molecules per cubic centimeter. Many nebulae are visible due to fluorescence caused by embedded hot stars, while others are so diffused that they can be detected only with long exposures and special filters. Some nebulae are variably illuminated byT Tauri variable stars.

Originally, the term "nebula" was used to describe any diffusedastronomical object, includinggalaxies beyond theMilky Way. TheAndromeda Galaxy, for instance, was once referred to as theAndromeda Nebula (andspiral galaxies in general as "spiral nebulae") before the true nature of galaxies was confirmed in the early 20th century byVesto Slipher,Edwin Hubble, and others. Edwin Hubble discovered that most nebulae are associated with stars and illuminated by starlight. He also helped categorize nebulae based on the type of light spectra they produced.^[8]

Around 150 AD,Ptolemy recorded, in books VII–VIII of hisAlmagest, five stars that appeared nebulous. He also noted a region of nebulosity between theconstellationsUrsa Major andLeo that was not associated with anystar.^[9] The first true nebula, as distinct from astar cluster, was mentioned by theMuslim Persian astronomerAbd al-Rahman al-Sufi in hisBook of Fixed Stars (964).^[10] He noted "a little cloud" where theAndromeda Galaxy is located.^[11] He also cataloged theOmicron Velorum star cluster as a "nebulous star" and other nebulous objects, such asBrocchi's Cluster.^[10] Thesupernovas that created theCrab Nebula,SN 1054, was observed by Arabic andChinese astronomers in 1054.^[12][13]

In 1610,Nicolas-Claude Fabri de Peiresc discovered theOrion Nebula using a telescope. This nebula was also observed byJohann Baptist Cysat in 1618. However, the first detailed study of the Orion Nebula was not performed until 1659 byChristiaan Huygens, who also believed he was the first person to discover this nebulosity.^[11]

In 1715,Edmond Halley published a list of six nebulae.^[14] This number steadily increased during the century, withJean-Philippe de Cheseaux compiling a list of 20 (including eight not previously known) in 1746. From 1751 to 1753,Nicolas-Louis de Lacaille cataloged 42 nebulae from theCape of Good Hope, most of which were previously unknown.Charles Messier then compiled a catalog of 103 "nebulae" (now calledMessier objects, which included what are now known to be galaxies) by 1781; his interest was detectingcomets, and these were objects that might be mistaken for them.^[15]

The number of nebulae was then greatly increased by the efforts ofWilliam Herschel and his sister,Caroline Herschel. TheirCatalogue of One Thousand New Nebulae and Clusters of Stars^[16] was published in 1786. A second catalog of a thousand was published in 1789, and the third and final catalog of 510 appeared in 1802. During much of their work, William Herschel believed that these nebulae were merely unresolved clusters of stars. In 1790, however, he discovered a star surrounded by nebulosity and concluded that this was a true nebulosity rather than a more distant cluster.^[15]

Beginning in 1864,William Huggins examined the spectra of about 70 nebulae. He found that roughly a third of them had theemission spectrum of agas. The rest showed a continuous spectrum and were thus thought to consist of a mass of stars.^[17][18] A third category was added in 1912 whenVesto Slipher showed that the spectrum of the nebula that surrounded the starMerope matched the spectra of thePleiadesopen cluster. Thus, the nebula radiates by reflected star light.^[19]

In 1923, following theGreat Debate, it became clear that many "nebulae" were in fact galaxies far from theMilky Way.

Slipher andEdwin Hubble continued to collect the spectra from many different nebulae, finding 29 that showed emission spectra and 33 that had the continuous spectra of star light.^[18] In 1922, Hubble announced that nearly all nebulae are associated with stars and that their illumination comes from star light. He also discovered that the emission spectrum nebulae are nearly always associated with stars having spectral classifications of B or hotter (including allO-type main sequence stars), while nebulae with continuous spectra appear with cooler stars.^[20] Both Hubble andHenry Norris Russell concluded that the nebulae surrounding the hotter stars are transformed in some manner.^[18]

There are a variety of formation mechanisms for the different types of nebulae. Some nebulae form from gas that is already in theinterstellar medium while others are produced by stars. Examples of the former case aregiant molecular clouds, the coldest, densest phase of interstellar gas, which can form by the cooling and condensation of more diffuse gas. Examples of the latter case are planetary nebulae formed from material shed by a star in late stages of itsstellar evolution.

Star-forming regions are a class of emission nebula associated with giant molecular clouds. These form as a molecular cloud collapses under its own weight, producing stars. Massive stars may form in the center, and theirultraviolet radiationionizes the surrounding gas, making it visible at opticalwavelengths. The region of ionized hydrogen surrounding the massive stars is known as anH II region while the shells of neutral hydrogen surrounding the H II region are known asphotodissociation region. Examples of star-forming regions are theOrion Nebula, theRosette Nebula and theOmega Nebula. Feedback from star-formation, in the form of supernova explosions of massive stars, stellar winds or ultraviolet radiation from massive stars, or outflows from low-mass stars may disrupt the cloud, destroying the nebula after several million years.

Other nebulae form as the result ofsupernova explosions; the death throes of massive, short-lived stars. The materials thrown off from the supernova explosion are then ionized by the energy and the compact object that its core produces. One of the best examples of this is theCrab Nebula, inTaurus. The supernova event was recorded in the year 1054 and is labeledSN 1054. The compact object that was created after the explosion lies in the center of the Crab Nebula and its core is now aneutron star.

Still other nebulae form asplanetary 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 intored giants and slowly lose their outer layers during pulsations in their atmospheres. When a star has lost enough material, its temperature increases and theultraviolet radiation it emits canionize the surrounding nebula that it has thrown off. The Sun will produce a planetary nebula and its core will remain behind in the form of awhite dwarf.

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  Herbig–HaroHH 161 andHH 164.^[21]
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  TheOmega Nebula, an example of anemission nebula
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  TheHorsehead Nebula, an example of adark nebula.
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  TheCat's Eye Nebula, an example of aplanetary nebula.
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  TheRed Rectangle Nebula, an example of aprotoplanetary nebula.
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  The delicate shell ofSNR B0509-67.5
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  Tycho Supernova remnant in X-ray light
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  Southern Ring Nebula, Planetary Nebula
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  Ring Nebula in the northern constellation ofLyra

Objects named nebulae belong to four major groups. Before their nature was understood,galaxies ("spiral nebulae") andstar clusters too distant to be resolved as stars were also classified as nebulae, but no longer are.

• H II regions, large diffuse nebulae containing ionized hydrogen
• Planetary nebulae
• Supernova remnants (e.g., Crab Nebula)
• Dark nebulae

Not all cloud-like structures are nebulae;Herbig–Haro objects are an example.

Most nebulae can be described as diffuse nebulae, which means that they are extended and contain no well-defined boundaries.^[24] Diffuse nebulae can be divided intoemission nebulae,reflection nebulae anddark nebulae.

Visible light nebulae may be divided into emission nebulae, which emitspectral line radiation from excited orionized gas (mostly ionizedhydrogen);^[25] they are often calledH II regions, H II referring to ionized hydrogen), and reflection nebulae which are visible primarily due to the light they reflect.

Reflection nebulae themselves do not emit significant amounts of visible light, but are near stars and reflect light from them.^[25] Similar nebulae not illuminated by stars do not exhibit visible radiation, but may be detected as opaque clouds blocking light from luminous objects behind them; they are calleddark nebulae.^[25]

Although these nebulae have different visibility at optical wavelengths, they are all bright sources ofinfrared emission, chiefly fromdust within the nebulae.^[25]

Planetary nebulae are the remnants of the final stages of stellar evolution for mid-mass stars (varying in size between 0.5-~8 solar masses). Evolvedasymptotic giant branch stars expel their outer layers outwards due to strong stellar winds, thus forming gaseous shells while leaving behind the star's core in the form of awhite dwarf.^[25] Radiation from the hot white dwarf excites the expelled gases, producing emission nebulae with spectra similar to those of emission nebulae found instar formation regions.^[25] They areH II regions, because mostly hydrogen is ionized, but planetary are denser and more compact than nebulae found in star formation regions.^[25]

Planetary nebulae were given their name by the first astronomical observers who were initially unable to distinguish them from planets, which were of more interest to them. The Sun is expected to spawn a planetary nebula about 12 billion years after its formation.^[26]

Asupernova occurs when a high-mass star reaches the end of its life. Whennuclear fusion in the core of the star stops, the star collapses. The gas falling inward either rebounds or gets so strongly heated that it expands outwards from the core, thus causing the star to explode.^[25] The expanding shell of gas forms asupernova remnant, a specialdiffuse nebula.^[25] Although much of the optical andX-ray emission from supernova remnants originates from ionized gas, a great amount of theradio emission is a form of non-thermal emission calledsynchrotron emission.^[25] This emission originates from high-velocityelectrons oscillating withinmagnetic fields.

• Gum catalog (emission nebulae)
• RCW Catalogue (emission nebulae)
• Sharpless catalog (emission nebulae)
• Messier Catalogue
• Caldwell Catalogue
• Abell Catalog of Planetary Nebulae
• Barnard Catalogue (dark nebulae)
• Lynds' Catalogue of Bright Nebulae
• Lynds' Catalogue of Dark Nebulae

• H I region
• H II region
• List of largest nebulae
• List of diffuse nebulae
• Lists of nebulae
• Molecular cloud
• Magellanic Clouds
• Messier object
• Nebular hypothesis
• Orion molecular cloud complex
• Timeline of knowledge about the interstellar and intergalactic medium

• Nebulae, SEDS Messier Pages
• Fusedweb.pppl.gov
• Historical pictures of nebulae, digital library of Paris Observatory