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True color image of the Trifid Nebula, showing complex gas and plasma structure Trifid Nebula by Deddy Dayag.jpg
True color image of the Trifid Nebula, showing complex gas and plasma structure

A nebula ('cloud' or 'fog' in Latin; [1] pl. nebulae, nebulæ or nebulas [2] [3] [4] [5] ) is a distinct luminescent part of interstellar medium, which can consist of ionized, neutral or molecular hydrogen and also cosmic dust. Nebulae are often star-forming regions, such as in the "Pillars of Creation" in the Eagle Nebula. In these regions, the formations of gas, dust, and other materials "clump" together to form denser regions, which attract further matter, and eventually will become dense enough to form stars. The remaining material is then thought to form planets and other planetary system objects.


Most nebulae are of vast size; some are hundreds of light-years in diameter. A nebula that is visible to the human eye from Earth would appear larger, but no brighter, from close by. [6] The Orion Nebula, the brightest nebula in the sky and occupying an area twice the angular diameter of the full Moon, 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 any vacuum created on Earth – a nebular cloud the size of the Earth would have a total mass of only a few kilograms. Earth's air has a density of approximately 1019 molecules per cubic centimeter; by contrast the densest nebulae can have densities of 10,000 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 by T Tauri variable stars.

Originally, the term "nebula" was used to describe any diffused astronomical object, including galaxies beyond the Milky Way. The Andromeda Galaxy, for instance, was once referred to as the Andromeda Nebula (and spiral galaxies in general as "spiral nebulae") before the true nature of galaxies was confirmed in the early 20th century by Vesto 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]

Observational history

Portion of the Carina Nebula Hs-2009-25-e-full.jpg
Portion of the Carina Nebula

Around 150 AD, Ptolemy recorded, in books VII–VIII of his Almagest , five stars that appeared nebulous. He also noted a region of nebulosity between the constellations Ursa Major and Leo that was not associated with any star. [9] The first true nebula, as distinct from a star cluster, was mentioned by the Muslim Persian astronomer Abd al-Rahman al-Sufi, in his Book of Fixed Stars (964). [10] He noted "a little cloud" where the Andromeda Galaxy is located. [11] He also cataloged the Omicron Velorum star cluster as a "nebulous star" and other nebulous objects, such as Brocchi's Cluster. [10] The supernova that created the Crab Nebula, the SN 1054, was observed by Arabic and Chinese astronomers in 1054. [12] [13]

In 1610, Nicolas-Claude Fabri de Peiresc discovered the Orion Nebula using a telescope. This nebula was also observed by Johann Baptist Cysat in 1618. However, the first detailed study of the Orion Nebula was not performed until 1659, by Christiaan 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, with Jean-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 the Cape of Good Hope, most of which were previously unknown. Charles Messier then compiled a catalog of 103 "nebulae" (now called Messier objects, which included what are now known to be galaxies) by 1781; his interest was detecting comets, and these were objects that might be mistaken for them. [15]

The number of nebulae was then greatly increased by the efforts of William Herschel and his sister Caroline Herschel. Their Catalogue 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 the emission spectrum of a gas. The rest showed a continuous spectrum and thus were thought to consist of a mass of stars. [17] [18] A third category was added in 1912 when Vesto Slipher showed that the spectrum of the nebula that surrounded the star Merope matched the spectra of the Pleiades open cluster. Thus the nebula radiates by reflected star light. [19]

About 1923, following the Great Debate, it had become clear that many "nebulae" were in fact galaxies far from the Milky Way.

Slipher and Edwin 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 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 all O-type main sequence stars), while nebulae with continuous spectra appear with cooler stars. [20] Both Hubble and Henry Norris Russell concluded that the nebulae surrounding the hotter stars are transformed in some manner. [18]


NGC 604, a nebula in the Triangulum Galaxy Nursery of New Stars - GPN-2000-000972.jpg
NGC 604, a nebula in the Triangulum Galaxy

There are a variety of formation mechanisms for the different types of nebulae. Some nebulae form from gas that is already in the interstellar medium while others are produced by stars. Examples of the former case are giant 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 its stellar 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 their ultraviolet radiation ionizes the surrounding gas, making it visible at optical wavelengths. The region of ionized hydrogen surrounding the massive stars is known as an H II region while the shells of neutral hydrogen surrounding the H II region are known as photodissociation region. Examples of star-forming regions are the Orion Nebula, the Rosette Nebula and the Omega 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 of supernova 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 the Crab Nebula, in Taurus. The supernova event was recorded in the year 1054 and is labeled SN 1054. The compact object that was created after the explosion lies in the center of the Crab Nebula and its core is now a neutron star.

Still other nebulae 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 enough material, its temperature increases and the ultraviolet radiation it emits can ionize 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 a white dwarf.


Classical types

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

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

Flux Nebula

Diffuse nebulae

The Carina Nebula is an example of a diffuse nebula Carina Nebula by ESO.jpg
The Carina Nebula is an example of a diffuse nebula

Most nebulae can be described as diffuse nebulae, which means that they are extended and contain no well-defined boundaries. [22] Diffuse nebulae can be divided into emission nebulae, reflection nebulae and dark nebulae.

Visible light nebulae may be divided into emission nebulae, which emit spectral line radiation from excited or ionized gas (mostly ionized hydrogen); [23] they are often called H 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. [23] 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 called dark nebulae. [23]

Although these nebulae have different visibility at optical wavelengths, they are all bright sources of infrared emission, chiefly from dust within the nebulae. [23]

Planetary nebulae

The Oyster Nebula is a planetary nebula located in the constellation of Camelopardalis A hazy nebula.jpg
The Oyster Nebula is a planetary nebula located in the constellation of Camelopardalis

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). Evolved asymptotic 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 a white dwarf. [23] Radiation from the hot white dwarf excites the expelled gases, producing emission nebulae with spectra similar to those of emission nebulae found in star formation regions. [23] They are H II regions, because mostly hydrogen is ionized, but planetary are denser and more compact than nebulae found in star formation regions. [23]

Planetary nebulae were given their name by the first astronomical observers who were initially unable to distinguish them from planets, and who tended to confuse them with planets, which were of more interest to them. The Sun is expected to spawn a planetary nebula about 12 billion years after its formation. [24]

Protoplanetary nebula

The Westbrook Nebula is an example of a protoplanetary nebula located in the constellation of Auriga Westbrook Nebula.tif
The Westbrook Nebula is an example of a protoplanetary nebula located in the constellation of Auriga

A protoplanetary nebula (PPN) is an astronomical object at the short-lived episode during a star's rapid stellar evolution between the late asymptotic giant branch (LAGB) phase and the following planetary nebula (PN) phase. [25] During the AGB phase, the star undergoes mass loss, emitting a circumstellar shell of hydrogen gas. When this phase comes to an end, the star enters the PPN phase.

The PPN is energized by the central star, causing it to emit strong infrared radiation and become a reflection nebula. Collimated stellar winds from the central star shape and shock the shell into an axially symmetric form, while producing a fast moving molecular wind. [26] The exact point when a PPN becomes a planetary nebula (PN) is defined by the temperature of the central star. The PPN phase continues until the central star reaches a temperature of 30,000 K, after which it is hot enough to ionize the surrounding gas. [27]

Supernova remnants

The Crab Nebula, an example of a supernova remnant Crab Nebula.jpg
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 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. [23] The expanding shell of gas forms a supernova remnant, a special diffuse nebula. [23] Although much of the optical and X-ray emission from supernova remnants originates from ionized gas, a great amount of the radio emission is a form of non-thermal emission called synchrotron emission. [23] This emission originates from high-velocity electrons oscillating within magnetic fields.


Close up on the Orion Arm, with major stellar associations (yellow), nebulae (red) and dark nebulae (grey) around the Local Bubble. LocalSpur.png
Close up on the Orion Arm, with major stellar associations (yellow), nebulae (red) and dark nebulae (grey) around the Local Bubble.


See also

Related Research Articles

<span class="mw-page-title-main">Planetary nebula</span> Type of emission nebula

A planetary nebula is a type of emission nebula consisting of an expanding, glowing shell of ionized gas ejected from red giant stars late in their lives.

<span class="mw-page-title-main">Interstellar medium</span> Matter and radiation in the space between the star systems in a galaxy

In astronomy, the interstellar medium (ISM) is the matter and radiation that exist in the space between the star systems in a galaxy. This matter includes gas in ionic, atomic, and molecular form, as well as dust and cosmic rays. It fills interstellar space and blends smoothly into the surrounding intergalactic space. The energy that occupies the same volume, in the form of electromagnetic radiation, is the interstellar radiation field.

<span class="mw-page-title-main">Ring Nebula</span> Planetary nebula in Lyra

The Ring Nebula is a planetary nebula in the northern constellation of Lyra. Such a nebula is formed when a star, during the last stages of its evolution before becoming a white dwarf, expels a vast luminous envelope of ionized gas into the surrounding interstellar space.

<span class="mw-page-title-main">Messier 100</span> Grand design intermediate spiral galaxy in the constellation Coma Berenices

Messier 100 is a grand design intermediate spiral galaxy in the southern part of the mildly northern Coma Berenices. It is one of the brightest and largest galaxies in the Virgo Cluster and is approximately 55 million light-years from our galaxy, its diameter being 107,000 light years, and being about 60% as large. It was discovered by Pierre Méchain in 1781 and 29 days later seen again and entered by Charles Messier in his catalogue "of nebulae and star clusters".. It was one of the first spiral galaxies to be discovered, and was listed as one of fourteen spiral nebulae by Lord William Parsons of Rosse in 1850. NGC 4323 and NGC 4328 are satellite galaxies of M100; the former is connected with it by a bridge of luminous matter.

<span class="mw-page-title-main">Emission nebula</span> Nebula formed of ionized gases that emit light of various wavelengths

An emission nebula is a nebula formed of ionized gases that emit light of various wavelengths. The most common source of ionization is high-energy ultraviolet photons emitted from a nearby hot star. Among the several different types of emission nebulae are H II regions, in which star formation is taking place and young, massive stars are the source of the ionizing photons; and planetary nebulae, in which a dying star has thrown off its outer layers, with the exposed hot core then ionizing them.

<span class="mw-page-title-main">Orion Nebula</span> Diffuse nebula (M42) in the constellation Orion

The Orion Nebula is a diffuse nebula situated in the Milky Way, being south of Orion's Belt in the constellation of Orion. It is one of the brightest nebulae and is visible to the naked eye in the night sky with apparent magnitude 4.0. It is 1,344 ± 20 light-years (412.1 ± 6.1 pc) away and is the closest region of massive star formation to Earth. The M42 nebula is estimated to be 24 light-years across. It has a mass of about 2,000 times that of the Sun. Older texts frequently refer to the Orion Nebula as the Great Nebula in Orion or the Great Orion Nebula.

<span class="mw-page-title-main">Triangulum Galaxy</span> Spiral galaxy in the constellation Triangulum

The Triangulum Galaxy is a spiral galaxy 2.73 million light-years (ly) from Earth in the constellation Triangulum. It is catalogued as Messier 33 or NGC (New General Catalogue) 598. With the D25 isophotal diameter of 18.74 kiloparsecs (61,100 light-years), the Triangulum Galaxy is the third-largest member of the Local Group of galaxies, behind the Andromeda Galaxy and the Milky Way.

<span class="mw-page-title-main">H II region</span> Large, low-density interstellar cloud of partially ionized gas

An H II region or HII region is a region of interstellar atomic hydrogen that is ionized. It is typically in a molecular cloud of partially ionized gas in which star formation has recently taken place, with a size ranging from one to hundreds of light years, and density from a few to about a million particles per cubic centimetre. The Orion Nebula, now known to be an H II region, was observed in 1610 by Nicolas-Claude Fabri de Peiresc by telescope, the first such object discovered.

<span class="mw-page-title-main">Wolf–Rayet star</span> Heterogeneous stars with unusual spectra

Wolf–Rayet stars, often abbreviated as WR stars, are a rare heterogeneous set of stars with unusual spectra showing prominent broad emission lines of ionised helium and highly ionised nitrogen or carbon. The spectra indicate very high surface enhancement of heavy elements, depletion of hydrogen, and strong stellar winds. The surface temperatures of known Wolf–Rayet stars range from 20,000 K to around 210,000 K, hotter than almost all other kinds of stars. They were previously called W-type stars referring to their spectral classification.

<span class="mw-page-title-main">Trifid Nebula</span> Emission nebula in the constellation Sagittarius

The Trifid Nebula is an H II region in the north-west of Sagittarius in a star-forming region in the Milky Way's Scutum-Centaurus Arm. It was discovered by Charles Messier on June 5, 1764. Its name means 'three-lobe'. The object is an unusual combination of an open cluster of stars, an emission nebula, a reflection nebula, and a dark nebula. Viewed through a small telescope, the Trifid Nebula is a bright and peculiar object, and is thus a perennial favorite of amateur astronomers.

<span class="mw-page-title-main">Astronomical spectroscopy</span> Study of astronomy using spectroscopy to measure the spectrum of electromagnetic radiation

Astronomical spectroscopy is the study of astronomy using the techniques of spectroscopy to measure the spectrum of electromagnetic radiation, including visible light, ultraviolet, X-ray, infrared and radio waves that radiate from stars and other celestial objects. A stellar spectrum can reveal many properties of stars, such as their chemical composition, temperature, density, mass, distance and luminosity. Spectroscopy can show the velocity of motion towards or away from the observer by measuring the Doppler shift. Spectroscopy is also used to study the physical properties of many other types of celestial objects such as planets, nebulae, galaxies, and active galactic nuclei.

<span class="mw-page-title-main">Tarantula Nebula</span> H II region in the constellation Dorado

The Tarantula Nebula is a large H II region in the Large Magellanic Cloud (LMC), forming its south-east corner.

<span class="mw-page-title-main">Pinwheel Galaxy</span> Face-on spiral galaxy in the constellation Ursa Major

The Pinwheel Galaxy is a face-on spiral galaxy 21 million light-years away from Earth in the constellation Ursa Major. It was discovered by Pierre Méchain in 1781 and was communicated that year to Charles Messier, who verified its position for inclusion in the Messier Catalogue as one of its final entries.

<span class="mw-page-title-main">Eagle Nebula</span> Open cluster in the constellation Serpens

The Eagle Nebula is a young open cluster of stars in the constellation Serpens, discovered by Jean-Philippe de Cheseaux in 1745–46. Both the "Eagle" and the "Star Queen" refer to visual impressions of the dark silhouette near the center of the nebula, an area made famous as the "Pillars of Creation" imaged by the Hubble Space Telescope. The nebula contains several active star-forming gas and dust regions, including the aforementioned Pillars of Creation. The Eagle Nebula lies in the Sagittarius Arm of the Milky Way.

<span class="mw-page-title-main">Lagoon Nebula</span> Emission nebula in Sagittarius

The Lagoon Nebula is a giant interstellar cloud in the constellation Sagittarius. It is classified as an emission nebula and as an H II region.

<span class="mw-page-title-main">Herbig–Haro object</span> Small patches of nebulosity associated with newly born stars

Herbig–Haro (HH) objects are bright patches of nebulosity associated with newborn stars. They are formed when narrow jets of partially ionised gas ejected by stars collide with nearby clouds of gas and dust at several hundred kilometres per second. Herbig–Haro objects are commonly found in star-forming regions, and several are often seen around a single star, aligned with its rotational axis. Most of them lie within about one parsec of the source, although some have been observed several parsecs away. HH objects are transient phenomena that last around a few tens of thousands of years. They can change visibly over timescales of a few years as they move rapidly away from their parent star into the gas clouds of interstellar space. Hubble Space Telescope observations have revealed the complex evolution of HH objects over the period of a few years, as parts of the nebula fade while others brighten as they collide with the clumpy material of the interstellar medium.

<span class="mw-page-title-main">Cosmic dust</span> Dust floating in space

Cosmic dust, also called extraterrestrial dust, star dust or space dust, is dust which exists in outer space, or has fallen on Earth. Most cosmic dust particles measure between a few molecules and 0.1 mm, such as micrometeoroids. Larger particles are called meteoroids. Cosmic dust can be further distinguished by its astronomical location: intergalactic dust, interstellar dust, interplanetary dust and circumplanetary dust. There are several methods to obtain space dust measurement.

<span class="mw-page-title-main">Protoplanetary nebula</span> Nebula surrounding a dying star

A protoplanetary nebula or preplanetary nebula is an astronomical object which is at the short-lived episode during a star's rapid evolution between the late asymptotic giant branch (LAGB) phase and the subsequent planetary nebula (PN) phase. A PPN emits strongly in infrared radiation, and is a kind of reflection nebula. It is the second-from-the-last high-luminosity evolution phase in the life cycle of intermediate-mass stars.

<span class="mw-page-title-main">NGC 4889</span> Supergiant elliptical galaxy in the constellation Coma Berenices

NGC 4889 is an E4 supergiant elliptical galaxy. It was discovered in 1785 by the British astronomer Frederick William Herschel I, who catalogued it as a bright, nebulous patch. The brightest galaxy within the northern Coma Cluster, it is located at a median distance of 94 million parsecs from Earth. At the core of the galaxy is a supermassive black hole that heats the intracluster medium through the action of friction from infalling gases and dust. The gamma ray bursts from the galaxy extend out to several million light years of the cluster.


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