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nebula (nĕbˈyo͝olə) [Lat.,=mist], in astronomy, observed manifestation of a collection of highly rarefied gas and dust in interstellar space. Prior to the 1960s this term was also applied to bodies later discovered to be galaxies, e.g., the so-called Great Nebula in the constellation Andromeda. In 1864, William Huggins confirmed William Herschel's conclusion that nebulae are not swarms of stars by determining that the spectra of nebulae are made of bright lines characteristic of radiating gases. Diffuse nebulae and planetary nebulae are two major classifications of these objects.
Diffuse nebulae appear as light or dark clouds (called bright and dark nebulae), are irregular in shape, and range up to 100 light-years in diameter. Some bright nebulae, composed primarily of hydrogen gas ionized by nearby hot blue-white stars, radiate their own light; they are called emission nebulae and are characterized by narrow spectral emission lines. Other bright nebulae, existing near cooler stars and not receiving the radiation necessary to make them self-luminous, reflect the starlight and are called reflection nebulae. Over 300 bright nebulae have been cataloged; prime examples are the Orion Nebula, visible to the unaided eye, the Eta Carinae Nebula, and the smaller North America Nebula. Dark nebulae are detected as empty patches in a field of stars or as dark clouds obscuring part of a bright nebula in the background, as in the case of the Horsehead Nebula. Smaller bodies of dark nebulous matter having unusually high densities have been observed in some bright nebulous regions. Many astronomers believe that these bodies, called globules, are in the process of condensation and are the initial stages in the birth of stars.
Planetary nebulae appear through the telescope as small disks with well-defined boundaries. They are the last stage of evolution for most stars, including the sun. Each consists of a shell of gaseous material surrounding a central hot star that emits radiation causing this material to glow. These shells measure about 20,000 AU in diameter (1 AU is the mean distance between the earth and the sun) and are slowly expanding, which suggests that they were expelled by the stars in nova eruptions (see variable star).
See L. Allen, Atoms, Stars, and Nebulae, (3d ed. 1991).
nebula(neb -yŭ-lă) A cloud of interstellar gas and dust that can be observed either as a luminous patch of light – a bright nebula – or as a dark hole or band against a brighter background – a dark nebula. Interstellar clouds cannot usually be detected optically but various processes can cause them to become visible.
With emission nebulae, ultraviolet radiation, generally coming from nearby or embedded hot stars, ionizes the interstellar gas atoms and light is emitted by the atoms as they interact with the free electrons in the nebula. Emission nebulae can be in the form of H II (ionized hydrogen) regions, planetary nebulae, or supernova remnants. With reflection nebulae light from a nearby star or stellar group is scattered (irregularly reflected) by the dust grains in the cloud. Reflection and emission nebulae are bright nebulae. In contrast dark nebulae are detected by what they obscure: the light from stars and other objects lying behind the cloud along our line of sight is significantly decreased or totally obscured by interstellar extinction. Dark nebulae contain approximately the same mixture of gas and dust as bright nebulae but there are no nearby stars to illuminate them. If the column density is sufficiently high, the majority of the hydrogen is likely to be present in molecular form (see molecular clouds).
The term ‘nebula’ was originally applied to any object that appeared fuzzy and extended in a telescope: over 100 were listed in the 18th-century Messier Catalog. The majority of these objects were later identified as galaxies and star clusters.
(or galactic nebula), a luminous or dark cloud of interstellar gas and, sometimes, dust (seeINTERSTELLAR MEDIUM). Now restricted to gaseous clouds located in or near a galaxy, the term “nebula” was formerly applied also to galaxies, which were called extragalactic nebulas, in contrast to the galactic nebulas discussed in the present article. The following types of nebulas are distinguished: diffuse nebulas, planetary nebulas, the remnants of Supernovae, and nebulas around Wolf-Rayet stars.
Diffuse nebulas. Diffuse nebulas are bright or dark formations of irregular shape with angular dimensions ranging from a few minutes to several degrees. They are divided into emission nebulas, whose spectra consist primarily of emission lines; reflection nebulas, which have a continuous spectrum with weak absorption lines; and dark nebulas, which are dense, nonluminous clouds of gas and dust that absorb the radiation of the bright background of the sky. All three types of diffuse nebulas are formed in complexes of gas and dust; the particular type observed depends on the presence of exciting stars and on the spectral class of the stars. Sometimes one part of a complex appears as an emission nebula, another part as a reflection nebula, and a third part as a dark nebula. A bright emission nebula is often surrounded by a fainter region of glowing gas.
EMISSION NEBULAS. Emission nebulas are H II (ionized hydrogen) regions. They glow as a result of excitation by stars of spectral class O (seeSPECTRAL CLASSIFICATION OF STARS), which have a surface temperature of 25,000°-50,000°K and a mass of about ten times that of the sun. The ultraviolet radiation of a star ionizes and excites hydrogen at a distance ranging from a few parsecs to several tens of parsecs, depending on the density of the interstellar gas. The recombination radiation of H and He and the excitation of O, S, and N atoms through collision with electrons determine the optical spectra of emission nebulas: such spectra exhibit bright spectral lines of the Balmer series; forbidden lines of, for example, [OII], [O III], [N II], and [S II]; and a weak continuous background spectrum. In the radio-frequency region of the spectrum, nebulas of this type emit a continuous thermal spectrum, recombination lines of H and He, and lines of, for example, OH and H2O that result from transitions between very high energy levels.
The methods of investigating the physical conditions in diffuse nebulas were worked out by H. Zanstra of the Netherlands, L. Spitzer of the USA, B. Strömgren of Sweden, M. Seaton of Great Britain, and V. I. Pronik of the USSR. The structure and masses of the nebulas have been investigated by the Soviet astronomers G. A. Shain and V. F. Gaze.
The temperature of emission nebulas is about 8000°K. As the distance from the excitation center to the periphery increases, a slight decrease in temperature is observed. The gas density is 10–1,000 atoms/cm3, which amounts to 10–23–10–21 g/cm3, a figure that is, on the average, 100 times greater than the dust density. The dust and gas are intermixed, but fluctuations of density are observed. The masses of individual nebulas range from 1 solar mass to several tens of solar masses. Diffuse nebulas exhibit a tendency to form large complexes that include several objects of different types and different brightnesses; the masses of large complexes reach hundreds or thousands of solar masses. The boundary between an emission nebula (H II region) and the surrounding gas (H I, or neutral hydrogen, region) is sharp: the thickness of the transition layer is about 0.05 parsec. The H II region expands under the pressure of the hot gas, and the sharp boundary, or ionization front, travels through the surrounding cold gas. The front bends around and “squeezes” local concentrations of interstellar gas. In this way various bright and dark small-scale structures are formed in emission nebulas—for example, globules, rims, “elephant trunks, ” and comet-tail structures.
REFLECTION NEBULAS. Reflection nebulas are a result of the reflection of light from bright stars of spectral classes B5-B9 by dense clouds of gas and dust. The spectrum of a reflection nebula is similar to that of the star or stars illuminating the nebula. Reflection nebulas are smaller and fainter than emission nebulas; the luminosities of the responsible stars are tens of times greater than those of the nebulas. If a reflection nebula is illuminated by a class Bl star, the emission lines of the radiation of the gas of the nebula are superposed on the reflected spectrum of the star.
DARK NEBULAS. Dark nebulas are dense clouds of gas and dust near which there are no exciting or illuminating stars. Such nebulas are visible as dark formations against the background of the Milky Way or of a bright nebula. The Coalsacks are the densest dark nebulas. The physical conditions and kinematics of dark nebulas have been investigated through observations of the interstellar absorption lines of Ca II, Na I, Ca I, K I, Ti II, and Fe II and of molecules of, for example, CN, CH, and CH II. Since the 1950’s dark nebulas have been investigated through observations of the 22-cm radio line emitted by H I and the radio lines emitted by, for example, OH, NH3, CO, CH3, OH, and HCN. The temperature is about 50°K in H I regions and 5°–10°K in the densest complexes of gas and dust. The average density is about lOMO4molecules/cm3.
According to theoretical studies, the relation of diffuse nebulas to stars is genetic in character: the process of the condensation of stars from a diffuse medium occurs in dense complexes of gas and dust. Large complexes—those having a mass of 103–104 solar masses, a temperature of about 50°K, and dimensions of up to tens of parsecs—undergo contraction as a result of gravitational instability. When it reaches a sufficient density, the complex breaks up into parts that contract independently and form condensations called protostars. Some of the gravitational energy is used to heat the protestar. With the beginning of thermonuclear reactions the protestar becomes an ordinary star and ionizes and illuminates the uncondensed remnants of gas and dust, thereby forming diffuse nebulas. Some observational confirmations of this hypothesis were obtained in the 1970’s. Cold, dense molecular clouds were detected that have a temperature of about 5°K and an average density of molecular hydrogen of 104molecules/cm3; in some cases the density reaches 107molecules/cm3. In addition, compact sources of maser (OH and H2O) radiation were observed. Having a diameter of about 1–10 astronomical units and a density of lOMO7 molecules/cm3, these objects were found to be moving with respect to each other at speeds of a few kilometers per second. The Soviet astronomer I. S. Shklovskii has suggested that protostars whose infrared radiation “pumps” the masers are located at the center of such super-dense formations.
Planetary nebulas. Planetary nebulas are emission nebulas that have the appearance of a disk or ring of small angular dimension, ranging from a few seconds to a few minutes of arc. Two of the best known planetary nebulas are NGC 6720 and NGC 6853 (nebulas are designated by the abbreviation of the reference catalog and the number under which they are listed in the catalog).
At the center of a planetary nebula is a star called the nucleus, which has given rise to the nebula and excites the nebula’s radiation. The spectra of many central stars are like those of Wolf-Rayet stars, which have broad emission lines, or young O stars and indicate a temperature reaching 50,000°-100,000°K. The intense ultraviolet radiation of the hot central star is the energy source for the ionization and excitation of atoms in the nebula. The strongest lines in the spectra of planetary nebulas are nebular lines of [O III]. Also observed are recombination radiation of H and He and collisionally excited lines of [O II], [N II], [Ne III], [Ne IV], [Ne V], [SII], [S III], [A III], and other elements.
The classical astrophysical methods of determining the temperature, density, and chemical composition of nebulas and of determining the temperature of central stars were developed on the basis of observations of planetary nebulas. Important contributions in this regard were made by Seaton and by the Americans I. Bowen, L. Aller, and D. Menzel.
The temperature of planetary nebulas is 10,000°-20,000°K; their density is several thousand atoms/cm3 and reaches tens of thousands of atoms/cm3 in bright dense nebulas. The degree of ionization of elements is higher than in diffuse nebulas and decreases from the center of a planetary nebula to its periphery. Because of the pressure of the hot gas, planetary nebulas expand. The rate of expansion is 10–40 km/sec and increases toward the periphery. As the nebula expands, its surface luminosity decreases. One method of estimating the distances and diameters of planetary nebulas is based on this fact. The diameters of planetary nebulas reach 0.1–1 parsec; the mass of the gas in an average nebula is about 0.1 solar mass.
A definite relation exists between the character of a nucleus and the type of its nebula: small, bright planetary nebulas have central stars of the Wolf-Rayet type; ring-like nebulas have central stars with a continuous spectrum; and large, irregular nebulas have ordinary stars of spectral class O as nuclei. This situation indicates that the nucleus undergoes considerable alteration over the characteristic period of evolution of a planetary nebula, which is tens of thousands of years. The current theory of stellar evolution holds that the formation of planetary nebulas and their nuclei is a natural process in the evolution of red giants. In a late stage of evolution, a red giant sheds its outer layers, which form a slowly expanding shell. The “bared” hot interior part of the star contracts and turns into the small, dense, hot nucleus of a planetary nebula. Over tens or hundreds of thousands of years the nucleus gradually cools and becomes an ordinary white dwarf, and the planetary nebula disperses in the interstellar medium. The statistics and distribution of planetary nebulas, red giants, and white dwarfs in space basically confirm this conception of the evolution of planetary nebulas.
Supernova remnants. The remnant of a supernova explosion is a glowing filamentary nebula of, generally, symmetric shape. In a supernova explosion a star ejects a substantial part of its mass (about 1 solar mass) at a speed of about 10,000 km/sec. The resulting spherically symmetric shock wave travels through the interstellar gas.
The remnant observed after a few hundred years at the site of the explosion may take the form of separate wisps of ejecta, as is the case with Cassiopeia A, or may appear as a filamentary nebula, such as the Crab Nebula. Spectral observations have shown that young supernova remnants expand at a rate of a few thousand kilometers per second. The shock wave bends around and squeezes the density fluctuations of the interstellar gas; in this way stationary condensations are formed in young remnants. The shock wave sweeps up interstellar gas and gradually slows. At some stage an intensely radiating shell is formed (part of the kinetic energy of the explosion goes to heating, ionizing, and exciting the gas).
Tens of thousands of years after a supernova explosion there may be observed an old remnant—such as IC 443 or the Loop Nebula in the constellation Cygnus—or a filamentary spherically symmetric emission nebula of low surface luminosity. Two of the best known nebulas of this kind are the Cygnus Loop and Simeiz 147. Their rates of expansion reach 20–100 km/sec.
The strongest lines in the optical spectra of supernova remnants are of Hα, [N II], [S II], [O II], [O III], and Hβ. In contrast to other types of nebulas, supernova remnants exhibit “coronal” lines of highly ionized elements, such as Fe × and Fe XIV in the Cygnus Loop and the nebula in the constellation Vela. Filamentary supernova remnants are powerful sources of radio-frequency synchrotron radiation; it may be noted that the first use of the synchrotron mechanism of radio emission in astronomical theory was in the explanation of the radiation of the Crab Nebula. With the development of X-ray astronomy the majority of optical nebulas of this type were found to be extended sources of soft × rays with a thermal spectrum. Pulsars are found in some supernova remnants; the pulsar is the stellar remnant of the explosion.
Observations in the optical, radio-frequency, and X-ray regions of the spectrum have led to several conclusions regarding the nature of supernova remnants. The interior of such a nebula is a hot plasma with a low density of about 0.1 particles/cm3 and a temperature of 107°-106°K. Optical nebulas of this type are a thin shell on the shock front; the shell is of high density—about 103cm3—and has cooled to a temperature of about 1040K. A network of fine filaments is formed by the passage of the shock wave around density fluctuations of the interstellar gas. The mass of an optical nebula is determined by the mass of the interstellar gas swept up and ionized by the shock wave and may be as great as a few solar masses. Beyond the outer boundary of the optical nebula is a dense, cold shell of neutral gas, whose mass may reach a few tens of solar masses. The linear dimension of a filamentary nebula reaches 20–40 parsecs, and the nebula’s age may be tens or hundreds of thousands of years. The rate of expansion of such a nebula decreases with age; after the rate of expansion has decreased to about 10 km/sec—the average speed of interstellar gas clouds—the nebula disperses in the interstellar medium.
Nebulas around Wolf-Rayet stars. Ring-like emission nebulas around Wolf-Rayet stars were classified as a separate type of nebula in the mid-1960’s. Nine filamentary shell nebulas are known. They are associated with single Wolf-Rayet stars of types WN5, WN6, and WN8. The brightest such nebula is NGC 6888 around the star HD 192163. Nebulas of this type are formed through the interaction of the stellar wind with interstellar gas. Wolf-Rayet stars eject about 10–4–10–5 solar mass per year at a speed of about 1,000 km/sec. As a result, a shock wave forms that propagates through the surrounding gas. At a certain stage, a considerable part of the kinetic energy of the ejecta goes to produce radiation; at this time a shell nebula is observed. The main lines of the optical spectrum of the nebula are the Balmer series of H and lines of [O II], [O III], [N II], and [S II]. Extended radio sources with a thermal spectrum are associated with nebulas of this type. The ring-like nebulas are usually observed against the background of a diffuse nebula, which is an ordinary H II region around the Wolf-Rayet star. The ring-like nebulas have been found to be expanding at a speed of 50–100 km/sec.
Cycle of matter. The genetic relation between stars and nebulas largely determines the cycle of matter in the universe. Stars are formed through condensation from dense clouds of interstellar gas. In turn the stars, as they evolve, eject into space part of their mass through their stellar wind, through the shedding of shells, and through supernova explosions. This ejected matter has an increased content of heavy elements because of nuclear reactions.
REFERENCESVorontsov-Vel’iaminov, B. A. Gazovye tumannosti i novye zvezdy. Moscow-Leningrad, 1948.
Pikel’ner, S. B. Fizika mezhzvezdnoi sredy. Moscow, 1959.
Kaplan, S. A., and S. B. Pikel’ner. Mezhzvezdnaia sreda. Moscow, 1963.
Shklovskii, I. S. Zvezdy: Ikh rozhdenie, zhizn’ i smert’. Moscow, 1975.
Aller, L., and W. Liller. Planetarnye tumannosti. Moscow, 1971. (Translated from English.)
T. A. LOZINSKAIA
["NEBULA - A Programming Language for Data Processing", T.G. Braunholtz et al, Computer J 4(3):197-201 (1961)].