NEBULA

((Latin: mist, or cloud), ) plural nebulae, or nebulas, any of various tenuous clouds of gas and dust that occurs in interstellar space. The term was formerly applied to any object outside the solar system that had a diffuse appearance and could not be resolved telescopically into a pointlike image, as in the case of a star. Today, astronomers use the word only in reference to clouds of interstellar matter within a single galaxy, such as the Milky Way system. Collectively, nebulae constitute only a small percentage of the mass of a galaxy. These interstellar clouds are commonly divided into two general classes according to appearance: dark nebulae and bright nebulae. Dark nebulae are extremely dense, cold molecular clouds that contain roughly 50 percent of the entire interstellar matter of a galaxy. Their densities typically range from hundreds to millions or more hydrogen molecules per cubic centimetre. They appear as large, obscure, irregularly shaped areas in the sky. Their solid particles of dust either dim or completely blot out the light of the stars behind them as they absorb, scatter, or partially polarize radiation of wavelengths in the visible portion of the electromagnetic spectrum. Until the early 20th century, it was thought that dark nebulae were tunnels or holes in space, for some reason devoid of stars. The photographic work of the astronomers E.E. Barnard and Max Wolf provided evidence of their true nature. Hourglass nebula MyCn18. This picture is a composite of three images taken by Bright nebulae appear as faintly luminous, glowing surfaces. They emit their own light or reflect that of stars in their vicinity. Bright nebulae are frequently divided into five subclasses on the basis of their origin and visual properties: reflection, diffuse, planetary, supernova remnants, and diffuse ionized gas. The first two are essentially the same type of phenomenon observed under different conditions. Reflection nebulae consist of cold molecular clouds of neutral hydrogen atoms. Dust particles in such clouds reflect the light of a nearby star. Nebulae of this kind would be seen as dark nebulae were it not for the bright star. Diffuse nebulae are composed of hydrogen atoms ionized by a massive, hot neighbouring stari.e., one that is of stellar type O or B. This ionized gas radiates light in the form of bright emission lines, the most readily observable of which belong to the Balmer spectral line series (see spectral line series). Diffuse nebulae are commonly observed on the outer edge of a large molecular cloud wherein a star is forming. (The presence of such a nebula is thought to induce star formation.) Planetary nebulae bear a superficial resemblance to planets, when viewed through a small telescope, hence the name. Most known planetary nebulae have at their centre an extremely hot star with a temperature of up to roughly 150,000 K. Gas ejected by the central star forms the shell or ring of such a nebula; the ultraviolet radiation from the stellar body ionizes the atoms of the gas, causing them to glow. Planetary nebulae expand at a rate of 24 to 56 km (14 to 35 miles) per second and are difficult to detect when they are more than 10,000 years old. More than 20,000 nebulae of this variety are known to exist in the Milky Way Galaxy, though only about 1,500 have been catalogued due to the obscuration by interstellar dust particles. Detail of the Cygnus Loop. This nebula is the product of a supernova Supernova remnants are violently expanding clouds of gaseous matter that result from a stellar explosion known as a supernova. During the catastrophic explosion, most of a star's mass is hurled into space at speeds of many hundreds or even thousands of kilometres per second. At the brightest phase of the supernova event, the expanding cloud emits as much energy in one day as the Sun has done in the past 3,000,000 years. The Crab Nebula in the constellation Taurus and the Cygnus Loop in the constellation Cygnus are prime examples of supernova remnants. Diffuse ionized gas pervades nebular clouds. Astronomers have found that such gas constitutes a major component of the Milky Way Galaxy: it occurs not only close to the galactic plane but far from it as well. The diffuse ionized gas has been observed by faint emissions of positive hydrogen, nitrogen, and sulfur ions. Its origin remains a point of controversy. ((Latin: mist, or cloud), ) plural nebulae, or nebulas, any of the various tenous clouds of gas and dust that occur in interstellar space. The term was formerly applied to any object outside the solar system that had a diffuse appearance rather than a pointlike image, as in the case of a star. This definition, adopted at a time when very distant objects could not be resolved into great detail, unfortunately includes two unrelated classes of objects: the extragalactic nebulae, now called galaxies, which are enormous collections of stars and gas; and the galactic nebulae, which are composed of the interstellar medium (the gas between the stars, with its accompanying small solid particles) within a single galaxy. Today the term nebula generally refers exclusively to the interstellar medium. In a spiral galaxy the interstellar medium makes up 3 to 5 percent of the galaxy's mass, but within a spiral arm its mass fraction increases to about 20 percent. About 1 percent of the mass of the interstellar medium is in the form of dustsmall, solid particles that are efficient in absorbing and scattering radiation. Much of the rest of the mass within a galaxy is concentrated in visible stars, but there are strong indications from the rotation of galaxies about their centres that there is also some form of dark matter accounting for a substantial fraction of the mass in the outer regions. This material might consist of dim stars or compact objects such as neutron stars or black holes. The most conspicuous property of interstellar gas is its clumpy distribution on all size scales observed, from the size of the entire Milky Way Galaxy (about 1020 metres, or hundreds of thousands of light-years) down to the distance from the Earth to the Sun (less than 1011 metres, or a few light-minutes). The large-scale variations are seen by direct observation; the smallest are observed by fluctuations in the intensity of radio waves, similar to the twinkling of starlight caused by unsteadiness in the Earth's atmosphere. Various regions exhibit an enormous range of densities and temperatures. Within the Galaxy's spiral arms about half of the mass of the interstellar medium is concentrated in molecular clouds, in which hydrogen occurs in molecular form (H2) and temperatures are as low as 10 kelvins. These clouds are inconspicuous optically and are detected principally by their carbon monoxide (CO) emissions in the millimetre wavelength range. Their densities in the regions studied by carbon monoxide emissions are typically 1,000 H2 molecules per cubic centimetre. At the other extreme is the gas between the clouds with a temperature of 10 million kelvins and a density of only 0.001 H+ ion per cubic centimetre. Such gas is produced by supernovas, the violent explosions of unstable stars. This article surveys the basic varieties of galactic nebulae distinguished by astronomers and their chemical composition and physical properties. Additional reading An excellent general reference work on all topics in astronomy, including all forms of nebulae, is Stephen P. Maran (ed.), The Astronomy and Astrophysics Encyclopedia (1992), with articles written for the technically trained person who is not familiar with astronomy. Works appropriate for the lay reader include Lyman Spitzer, Jr., Searching Between the Stars (1982), written by one of the most authoritative figures in the field; J.E. Dyson and D.A. Williams, Physics of the Interstellar Medium (1980); and numerous articles written at a semitechnical level in the journals Scientific American (monthly); Mercury (bimonthly), published by the Astronomical Society of the Pacific; Astronomy (monthly); and Sky and Telescope (monthly). Readers with a thorough knowledge of physics may wish to consult Lyman Spitzer, Jr., Physical Processes in the Interstellar Medium (1978), very comprehensive and very terse; Donald E. Osterbrock, Astrophysics of Gaseous Nebulae and Active Galactic Nuclei (1989); and Stuart R. Pottasch, Planetary Nebulae: a Study of Late Stages of Stellar Evolution (1984). John S. Mathis