A nebula (; : nebulae, nebulæ, or nebulas) is a distinct luminescent part of

interstellar medium In astronomy, the interstellar medium 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 interstella ...

, 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 ''Pillars of Creation'' is a photograph taken by the Hubble Space Telescope of elephant trunks of interstellar gas and dust in the Eagle Nebula, in the Serpens constellation, some from Earth. These elephant trunks had been discovered by Jo ...

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 become dense enough to form

star A star is an astronomical object comprising a luminous spheroid of plasma (physics), plasma held together by its gravity. The List of nearest stars and brown dwarfs, nearest star to Earth is the Sun. Many other stars are visible to the naked ...

s. The remaining material is then thought to form planets and other

planetary system A planetary system is a set of gravitationally In physics, gravity () is a fundamental interaction which causes mutual attraction between all things with mass or energy. Gravity is, by far, the weakest of the four fundamental interacti ...

objects. Most nebulae are of vast size; some are hundreds of

light-year A light-year, alternatively spelled light year, is a large unit of length used to express astronomical distances and is equivalent to about 9.46 trillion kilometers (), or 5.88 trillion miles ().One trillion here is taken to be 1012 ...

s in diameter. A nebula that is visible to the

human eye The human eye is a sensory organ, part of the sensory nervous system, that reacts to visible light and allows humans to use visual information for various purposes including seeing things, keeping balance, and maintaining circadian rhythm. ...

from Earth would appear larger, but no brighter, from close by. The

Orion Nebula The Orion Nebula (also known as Messier 42, M42, or NGC 1976) 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 nig ...

, 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. Although denser than the space surrounding them, most nebulae are far less dense than any vacuum created on Earth (10 to 10 molecules per cubic centimeter) – a nebular cloud the size of the Earth would have a total mass of only a few

kilogram The kilogram (also kilogramme) is the unit of mass in the International System of Units (SI), having the unit symbol kg. It is a widely used measure in science, engineering and commerce worldwide, and is often simply called a kilo colloquially ...

s. Earth's air has a density of approximately 10 molecules per cubic centimeter; by contrast, the densest nebulae can have densities of 10 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 An astronomical object, celestial object, stellar object or heavenly body is a naturally occurring physical entity, association, or structure that exists in the observable universe. In astronomy, the terms ''object'' and ''body'' are often us ...

, including

galaxies A galaxy is a system of stars, stellar remnants, interstellar gas, dust, dark matter, bound together by gravity. The word is derived from the Greek ' (), literally 'milky', a reference to the Milky Way galaxy that contains the Solar System. ...

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 Edwin Powell Hubble (November 20, 1889 – September 28, 1953) was an Americans, American astronomer. He played a crucial role in establishing the fields of extragalactic astronomy and observational cosmology. Hubble proved that many objects ...

, 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.

Observational history

Around 150 AD, Ptolemy recorded, in books VII–VIII of his ''

Almagest The ''Almagest'' is a 2nd-century Greek-language mathematical and astronomical treatise on the apparent motions of the stars and planetary paths, written by Claudius Ptolemy ( ). One of the most influential scientific texts in history, it canoni ...

'', five stars that appeared nebulous. He also noted a region of nebulosity between the

constellation A constellation is an area on the celestial sphere in which a group of visible stars forms Asterism (astronomy), a perceived pattern or outline, typically representing an animal, mythological subject, or inanimate object. The origins of the e ...

s Ursa Major and

Leo Leo or Léo may refer to: Acronyms * Law enforcement officer * Law enforcement organisation * ''Louisville Eccentric Observer'', a free weekly newspaper in Louisville, Kentucky * Michigan Department of Labor and Economic Opportunity Arts an ...

that was not associated with any

. The first true nebula, as distinct from a

star cluster Star clusters are large groups of stars. Two main types of star clusters can be distinguished: globular clusters are tight groups of ten thousand to millions of old stars which are gravitationally bound, while open clusters are more loosely clust ...

, was mentioned by the Muslim Persian astronomer

Abd al-Rahman al-Sufi ʿAbd al-Rahman al-Sufi ( fa, عبدالرحمن صوفی; December 7, 903 – May 25, 986) was an iranianRobert Harry van Gent. Biography of al-Sūfī'. "The Persian astronomer Abū al-Husayn ‘Abd al-Rahmān ibn ‘Umar al-Sūfī was born in ...

in his '' Book of Fixed Stars'' (964). He noted "a little cloud" where the Andromeda Galaxy is located. He also cataloged the Omicron Velorum star cluster as a "nebulous star" and other nebulous objects, such as

Brocchi's Cluster Brocchi's Cluster (also known as Collinder 399, Cr 399 or Al Sufi's Cluster) is a asterism of six stars in an apparent row, across 1.3° of the night sky and four others, in the south of the constellation Vulpecula, thus near Sagitta. Its nickn ...

. The

supernova A supernova is a powerful and luminous explosion of a star. It has the plural form supernovae or supernovas, and is abbreviated SN or SNe. This transient astronomical event occurs during the last evolutionary stages of a massive star or when ...

that created the

Crab Nebula The Crab Nebula (catalogue designations Messier object, M1, New General Catalogue, NGC 1952, Taurus (constellation), Taurus A) is a supernova remnant and pulsar wind nebula in the constellation of Taurus (constellation), Taurus. The common name ...

SN 1054 SN 1054 is a supernova that was first observed on 1054, and remained visible until 1056. The event was recorded in contemporary Chinese astronomy, and references to it are also found in a later (13th-century) Japanese document, and in a doc ...

, was observed by Arabic and

Chinese astronomers Astronomy in China has a long history stretching from the Shang dynasty, being refined over a period of more than 3,000 years. The Ancient China, ancient Chinese people have identified stars from 1300 BCE, as Chinese star names later categoriz ...

in 1054. In 1610,

Nicolas-Claude Fabri de Peiresc Nicolas-Claude Fabri de Peiresc (1 December 1580 – 24 June 1637), often known simply as Peiresc, or by the Latin form of his name, Peirescius, was a French astronomer, antiquary and savant, who maintained a wide correspondence with scientis ...

discovered the

using a telescope. This nebula was also observed by

Johann Baptist Cysat Johann Baptist Cysat (Latinized as Cysatus; in French, Jean-Baptiste Cysat) (c. 1587 – March 17, 1657) was a Swiss Jesuit mathematician and astronomer, after whom the lunar crater Cysatus is named. He was born in Lucerne, as the eighth of 14 ...

in 1618. However, the first detailed study of the Orion Nebula was not performed until 1659 by

Christiaan Huygens Christiaan Huygens, Lord of Zeelhem, ( , , ; also spelled Huyghens; la, Hugenius; 14 April 1629 – 8 July 1695) was a Dutch mathematician, physicist, engineer, astronomer, and inventor, who is regarded as one of the greatest scientists of ...

, who also believed he was the first person to discover this nebulosity. In 1715, Edmond Halley published a list of six nebulae. This number steadily increased during the century, with

Jean-Philippe de Cheseaux Jean-Philippe Loys de Cheseaux (; 4 May 1718 – 30 November 1751) was a Swiss astronomer. Biography Loys de Cheseaux was born on 4 May 1718 in Lausanne, Vaud, to Paul-Etienne Loys de Cheseaux, a banneret, and Estienne-Judith de Crousaz. His ...

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 The Cape of Good Hope ( af, Kaap die Goeie Hoop ) ;''Kaap'' in isolation: pt, Cabo da Boa Esperança is a rocky headland on the Atlantic coast of the Cape Peninsula in South Africa. A common misconception is that the Cape of Good Hope is t ...

, 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. 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'' 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. Beginning in 1864, William Huggins examined the spectra of about 70 nebulae. He found that roughly a third of them had the

emission spectrum The emission spectrum of a chemical element or chemical compound is the spectrum of frequencies of electromagnetic radiation emitted due to an electron making a atomic electron transition, transition from a high energy state to a lower energy st ...

of a gas. The rest showed a continuous spectrum and were thus thought to consist of a mass of stars. 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. In 1923, following the Great Debate, it became clear that many "nebulae" were in fact galaxies far from the Milky Way. Slipher and

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. 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 all O-type main sequence stars), while nebulae with continuous spectra appear with cooler stars. Both Hubble and

Henry Norris Russell Henry Norris Russell ForMemRS HFRSE FRAS (October 25, 1877 – February 18, 1957) was an American astronomer who, along with Ejnar Hertzsprung, developed the Hertzsprung–Russell diagram (1910). In 1923, working with Frederick Saunders, he deve ...

concluded that the nebulae surrounding the hotter stars are transformed in some manner.

Formation

There are a variety of formation mechanisms for the different types of nebulae. Some nebulae form from gas that is already in the

while others are produced by stars. Examples of the former case are

giant molecular clouds A molecular cloud, sometimes called a stellar nursery (if star formation is occurring within), is a type of interstellar cloud, the density and size of which permit absorption nebulae, the formation of molecules (most commonly molecular hydrogen, ...

, 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 Stellar evolution is the process by which a star changes over the course of time. Depending on the mass of the star, its lifetime can range from a few million years for the most massive to trillions of years for the least massive, which is cons ...

Star-forming region Star formation is the process by which dense regions within molecular clouds in The "medium" is present further soon.-->interstellar space

s 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 In astrophysics, photodissociation regions (or photon-dominated regions, PDRs) are predominantly neutral regions of the interstellar medium in which far ultraviolet photons strongly influence the gas chemistry and act as the most important sourc ...

. Examples of star-forming regions are the

, the

Rosette Nebula The Rosette Nebula (also known as Caldwell 49) is an H II region located near one end of a giant molecular cloud in the Monoceros region of the Milky Way Galaxy. The open cluster NGC 2244 (Caldwell 50) is closely associated with the nebulosity, th ...

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

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

, in Taurus. The supernova event was recorded in the year 1054 and is labeled

. 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 giant A red giant is a luminous giant star of low or intermediate mass (roughly 0.3–8 solar masses ()) in a late phase of stellar evolution. The outer atmosphere is inflated and tenuous, making the radius large and the surface temperature around or ...

s 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.

Types

File:Hubble Sees a Stellar "Sneezing Fit" (11467249715).jpg, Herbig–Haro

HH 161 HH may refer to: Organizations * Happy Hippie Foundation, a non-profit organization founded by Miley Cyrus * Hartmann House Preparatory School, an independent preparatory school in Harare, Zimbabwe * Heirs Holdings, a Nigerian conglomerate with ...

and

HH 164 HH may refer to: Organizations * Happy Hippie Foundation, a non-profit organization founded by Miley Cyrus * Hartmann House Preparatory School, an independent preparatory school in Harare, Zimbabwe * Heirs Holdings, a Nigerian conglomerate with ...

. File:Omega Nebula.jpg, The Omega Nebula, an example of an emission nebula File:Horsehead-Hubble.jpg, The Horsehead Nebula, an example of a

dark nebula A dark nebula or absorption nebula is a type of interstellar cloud, particularly molecular clouds, that is so dense that it obscures the visible wavelengths of light from objects behind it, such as background stars and emission or reflection nebu ...

. File:NGC6543.jpg, The Cat's Eye Nebula, an example of a planetary nebula. File:PIA04533.jpg, The

Red Rectangle Nebula The Red Rectangle Nebula, so called because of its red color and unique rectangular shape, is a protoplanetary nebula in the Monoceros constellation. Also known as HD 44179, the nebula was discovered in 1973 during a rocket flight associated w ...

, an example of a protoplanetary nebula. File:SNR 0509.jpg, The delicate shell of SNR B0509-67.5 File:Tycho xrayonly.jpg, Tycho Supernova remnant in X-ray light File:Southern Ring Nebula by Webb Telescope (2022).jpg, Southern Ring Nebula, Planetary Nebula File:Webb captures detailed beauty of Ring Nebula (NIRCam and MIRI images) (weic2320a).jpg,

Ring Nebula The Ring Nebula (also catalogued as Messier 57, M57 and NGC 6720) 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 v ...

in the northern constellation of Lyra

Classical types

Objects named nebulae belong to four major groups. Before their nature was understood,

("spiral nebulae") and

s 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 nebula A dark nebula or absorption nebula is a type of interstellar cloud, particularly molecular clouds, that is so dense that it obscures the visible wavelengths of light from objects behind it, such as background stars and emission or reflection nebu ...

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

Flux Nebulae

Diffuse nebulae

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

reflection nebula Reflection or reflexion may refer to: Science and technology * Reflection (physics), a common wave phenomenon ** Specular reflection, reflection from a smooth surface *** Mirror image, a reflection in a mirror or in water ** Signal reflection, in s ...

e and

e. Visible light nebulae may be divided into emission nebulae, which emit spectral line radiation from excited or ionized gas (mostly ionized hydrogen); 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. 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

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

Planetary nebulae

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 The asymptotic giant branch (AGB) is a region of the Hertzsprung–Russell diagram populated by evolved cool luminous stars. This is a period of stellar evolution undertaken by all low- to intermediate-mass stars (about 0.5 to 8 solar masses) lat ...

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. 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 Star formation is the process by which dense regions within molecular clouds in The "medium" is present further soon.-->interstellar space

regions. They are H II regions, because mostly hydrogen is ionized, but planetary are denser and more compact than nebulae found in star formation regions. 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.

Protoplanetary nebulae

Supernova remnants

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. The expanding shell of gas forms a supernova remnant, a special

diffuse nebula A nebula ('cloud' or 'fog' in Latin; pl. nebulae, nebulæ or nebulas) 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 region ...

. 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. This emission originates from high-velocity electrons oscillating within

magnetic field A magnetic field is a vector field that describes the magnetic influence on moving electric charges, electric currents, and magnetic materials. A moving charge in a magnetic field experiences a force perpendicular to its own velocity and to ...

Examples

Ant Nebula Mz 3 (Menzel 3) is a young bipolar planetary nebula (PN) in the constellation Norma that is composed of a bright core and four distinct high-velocity outflows that have been named lobes, columns, rays, and chakram. These nebulosities are descri ...

* Barnard's Loop * Boomerang Nebula * Cat's Eye Nebula *

* Eagle Nebula *

Eskimo Nebula The Eskimo Nebula (NGC 2392), also known as the Clown-faced Nebula, Lion Nebula, or Caldwell 39, is a bipolar double-shell planetary nebula (PN). It was discovered by astronomer William Herschel in 1787. The formation resembles a person's head s ...

Carina Nebula The Carina Nebula or Eta Carinae Nebula (catalogued as NGC 3372; also known as the Great Carina Nebula) is a large, complex area of bright and dark nebulosity in the constellation Carina, located in the Carina–Sagittarius Arm of the Milky Wa ...

Fox Fur Nebula The Fox Fur Nebula is a nebula (a formation of gas and dust) located in the constellation of Monoceros (the Unicorn) not far off the right arm of Orion and included in the NGC 2264 Region. In the Sharpless catalog it is number 273. The image ...

Helix Nebula The Helix Nebula (also known as NGC 7293 or Caldwell 63) is a planetary nebula (PN) located in the constellation Aquarius. Discovered by Karl Ludwig Harding, probably before 1824, this object is one of the closest of all the bright planetary neb ...

* Horsehead Nebula *

Engraved Hourglass Nebula The Engraved Hourglass Nebula (also known as MyCn 18) is a young planetary nebula in the southern constellation Musca. It was discovered by Annie Jump Cannon and Margaret W. Mayall during their work on an extended Henry Draper Catalogue (the ...

Lagoon Nebula The Lagoon Nebula (catalogued as Messier 8 or M8, NGC 6523, Sharpless 25, RCW 146, and Gum 72) is a giant interstellar cloud in the constellation Sagittarius. It is classified as an emission nebula and as an H II region. The Lagoon Nebula was di ...

Pelican Nebula The Pelican Nebula (also known as IC 5070 and IC 5067) is an H II region associated with the North America Nebula in the constellation Cygnus. The gaseous contortions of this emission nebula bear a resemblance to a pelican, giving rise to its n ...

* Red Square Nebula *

* Tarantula Nebula

Catalogs

* Gum catalog (emission nebulae) *

RCW Catalogue The RCW Catalogue (from Rodgers, Campbell & Whiteoak) is an astronomical catalogue of Hα-emission regions in the southern Milky Way, described in . It contains 182 objects, including many of the earlier Gum catalogue (84 items) objects. The la ...

(emission nebulae) *

Sharpless catalog The Sharpless catalog is a list of 313 H II regions (emission nebulae) intended to be comprehensive north of declination −27°. (It does include some nebulae south of that declination as well.) The first edition was published in 1953 with 14 ...

(emission nebulae) * Messier Catalogue * Caldwell Catalogue *

Abell Catalog of Planetary Nebulae The Abell Catalog of Planetary Nebulae was created in 1966 by George O. Abell and was composed of 86 entries thought to be planetary nebulae that were collected from discoveries, about half by Albert George Wilson and the rest by Abell, Rob ...

Barnard Catalogue The astronomer Edward Emerson Barnard compiled a list of dark nebulae known as the ''Barnard Catalogue of Dark Markings in the Sky'', or the ''Barnard Catalogue'' for short. The nebulae listed by Barnard have become known as Barnard objects. A 1919 ...

(dark nebulae) *

Lynds' Catalogue of Bright Nebulae Lynds' Catalogue of Bright Nebulae is an astronomical catalogue of bright nebulae. Objects listed in the catalogue are numbered with the prefix LBN (not to be confused with LDN, or Lynds' Catalogue of Dark Nebulae), though, many entries also have ...

* Lynds' Catalogue of Dark Nebulae

References

External links

Nebulae
SEDS Messier Pages
Fusedweb.pppl.govHistorical pictures of nebulae
digital library of Paris Observatory {{Authority control Space plasmas Concepts in astronomy Interstellar media