variable star, star that varies, either periodically or irregularly, in the intensity of the light it emits. Other physical changes are usually correlated with the fluctuations in brightness, such as pulsations in size, ejection of matter, and changes in spectral type, color, or temperature. The class to which a variable star belongs is determined by a plot of its light curve, which is a graph of the star's apparent brightness versus time. The light curve reveals its maximum and minimum brightness and gives evidence for periodicity, if any exists. The approximately 30,000 known variable stars are grouped into three broad classes: the pulsating variables and the eruptive variables (both of which are intrinsic variables, because the variation is caused by a physical change within the star) and the eclipsing variables (which are extrinsic variables, because the variation is caused by two or more bodies eclipsing one another).

Intrinsic Variable Stars

Pulsating Variables

Pulsating variables account for more than half of the known variable stars. They are characterized by slight instabilities that cause the star alternately to expand and contract. This pulsation is accompanied by changes in absolute luminosity and temperature. The pulsating variables can be further divided into the following subclasses: short-term, long-term, semiregular, and irregular. Short-term variables have well-defined periods ranging from less than one day to more than 50 days.

Relatively rare among this subclass are the Cepheid variables; these yellow supergiant stars are historically important because, having periods roughly proportional to their absolute brightness, they provide a means of measuring galactic and extragalactic distances. A key research program of the Hubble Space Telescope is the measurement of Cepheid variables in distant galaxies in order to refine our concept of the size and age of the universe. Cepheid variables are classed as either population I Cepheids, which are found in the spiral arms of galaxies, or population II Cepheids, also known as W Virginis stars, which are found in star clusters (see also stellar populations). About 700 Cepheids of both types have been found in our galaxy.

A more common short-term variable is of the RR Lyrae group; about 6,000 of this type are known in our galaxy and are concentrated in globular clusters. They have periods of less than one day, and all have roughly the same intrinsic brightness. The latter feature, along with their wide distribution throughout the galaxy, makes them another useful distance indicator.

The long-term variables are the most numerous of all pulsating stars. They are red giant and supergiant stars with periods ranging from a few months to more than a year. The best known of these stars is Omicron Ceti, also known as Mira. Over a period of about 11 months, it brightens by about 7 magnitudes and then gradually fades. Semiregular variables are stars whose periodic variations are occasionally interrupted by sudden bursts of light. The best-known example is the red supergiant Betelgeuse, in Orion. Irregular variables show no periodicity in their variations in brightness. The amplitude of their fluctuations in brightness is in general smaller than the fluctuations of the long-term regular variables.

Eruptive Variables

The eruptive variables are highly unstable stars that suddenly and unpredictably increase in brightness. T Tauri stars, also known as nebular variables because they are young objects still embedded in nebulosity, are the least violent of these explosive stars. Novas and supernovas are much more dramatic. Novas are small, very hot stars that suddenly increase thousands of times in luminosity. Their decline in luminosity is much slower, taking months or even years. Most novas probably repeat their outbursts, the dwarf novas every few months, the recurring novas every few years or decades, and the standard novas over thousands of years. Supernovas, upon exploding, increase millions of times in brightness and are totally disrupted. More than 30 supernovas events are observed annually in distant galaxies. Three supernovas have been seen in our own galaxy, in 1054, 1572, and 1604; in 1987 a supernova erupted in a neighboring galaxy, the Large Magellanic Cloud.

Extrinsic Variable Stars

Eclipsing variables are not true (intrinsic) variables but rather are binary star systems, i.e., pairs of stars revolving around a common center of mass. The apparent brightness of an eclipsing variable fluctuates because the orbit of the pair is seen edgewise, so that first one star and then the other regularly blocks the light of its companion. Best known of this type is Algol (Beta Persei).

supergiant star: see red giant.

star-of-Bethlehem, in botany, low, spring-blooming bulbous plant (Ornithogalum umbellatum) of the family Liliaceae (lily family), native to the Mediterranean region but naturalized in North America and cultivated in gardens. The plant has rather stiff, grasslike leaves and a cluster of white star-shaped flowers marked on the back with green. It spreads easily and tends to become weedy. Although the fresh plant is poisonous, the bulbs have been prepared and used as food in the Old World. The flower has been associated with the star that guided the Wise Men to the manger (see Star of Bethlehem). The star-of-Bethlehem is classified in the division Magnoliophyta, class Liliopsida, order Liliales, family Liliaceae.

star cluster, a group of stars near each other in space and resembling each other in certain characteristics that suggest a common origin for the group. Stars in the same cluster move at the same rate and in the same direction. Two types of clusters can be distinguished—open clusters, also called galactic clusters because of their wide distribution in our galaxy (the Milky Way), and globular clusters. More than one thousand open clusters have been cataloged in the Milky Way, most of which are found in the spiral arms of the galaxy. Typically, an open cluster contains from a few dozen to a thousand loosely scattered stars and exists in a region rich in gas and dust. Among those which can be detected with the unaided eye are the Hyades cluster in the constellation Taurus, the Coma Berenices cluster, the Pleiades cluster, and the Praesepe cluster. Globular clusters are spherical aggregates of from thousands to hundreds of thousands of densely concentrated stars. Rather than lying on the galactic plane, these clusters are members of the outer halo, moving around the nucleus of our galaxy in highly inclined orbits. Because of their distribution around the galaxy, they provide an outline of its shape. About 150 globular clusters are known in the Milky Way galaxy, and others have been found in nearby galaxies. Visible to the unaided eye are Omega Centauri and 47 Tucanae, both in the southern skies, and M13 in the northern sky. Star clusters are cosmologically important as a first step to understanding the distance scale of the universe (see Hyades); and theoretical astronomers use observations of globular clusters to investigate the evolution and life span of stars. Because all the stars in a particular cluster are coeval (the same age), astronomers can infer that massive stars change more rapidly over time than less massive ones. X-ray sources have been detected recently in some globular clusters. Millisecond pulsars have also been found.

star anise: see under anise.

star, hot incandescent sphere of gas, held together by its own gravitation, and emitting light and other forms of electromagnetic radiation whose ultimate source is nuclear energy.

Properties of Stars

Stars differ widely in mass, size, temperature, and total energy output, or luminosity. The sun, a typical star, has a mass of about 2 × 10³³ grams, a radius of about 7 × 10¹⁰ cm, a surface temperature of about 6,000°C;, and a luminosity of about 4 × 10³³ erg/sec. More than 90% of all stars have masses between one tenth and 50 times that of the sun. Other stellar quantities vary over a much larger range. The most luminous stars (excluding supernovas) are about ten million times more powerful than the sun, while the least luminous are only one hundredth as powerful. Red giants, the largest stars, are fifteen-hundred times greater in size than the sun; if one were placed at the sun's position, it would stretch to halfway between Jupiter and Saturn. At the opposite extreme, white dwarfs are no larger than the earth, and neutron stars are only a few kilometers in radius.

The visible stars are divided into six classes according to apparent brightness; the brightest are first magnitude and the faintest are sixth magnitude. The stars differ in apparent brightness both because they lie at different distances from us and because they vary in actual or intrinsic brightness. Variable stars do not shine steadily but fluctuate in either a regular or irregular fashion. The supernova, or exploding star, is the most spectacular variable star; the eclipsing binary, where the two stars alternately hide and then reinforce each other's light, is not a true variable.

Light received from a star consists of a spectrum of wavelengths; the hotter the star, the shorter the wavelength at which the light is most intense. The color of a star is closely related to its surface temperature. Red stars have surface temperatures around 3,000°C; and blue-white stars have surface temperatures above 20,000°C; (see spectral class).

Stellar Structure and Stellar Evolution

The theory of stellar structure applies the laws of physics to calculation of the equilibrium configurations of stars. According to this theory, the mass and chemical composition of a star determine all its other characteristics. Because most stars are more than 90% hydrogen, variations in chemical composition are small and have a small effect. Variation in mass is the main factor; a doubling in mass increases the luminosity more than 10 times. For a star to be stable, the compressive force of gravitation must be exactly balanced by the tendency of the gas to expand. Thus, the size and temperature of a star are important, interrelated factors.

Despite the tremendous pressure generated by the massive layers above it, the central region, or core, of a star remains gaseous. This is possible because the core has a temperature of millions of degrees. At this temperature, nuclear energy is released by the fusion of hydrogen to form helium; the principle is the same as that of the hydrogen bomb. By the time nuclear energy reaches the surface of the star, it has been largely converted into visible light with a spectrum characteristic of a very hot body (see black body). The theory of stellar evolution states that a star must change as it consumes its hydrogen in the nuclear reactions that power it. Ultimately each star must die, rarely in a supernova explosion, when its capability for nuclear reactions is exhausted. The heavy atoms created in supernovas (see nucleosynthesis) are spewed out to become part of the interstellar matter from which new stars are continuously formed.

Location and Motion of Stars

The universe contains billions of galaxies, and each galaxy contains billions of stars. The stars visible to the unaided eye are all in our own galaxy, the Milky Way. Stars are not spread uniformly through a galaxy. They are frequently bunched together in star clusters of as many as 100,000 stars. Many stars that appear as single points of light in even the most powerful telescopes are actually systems of two or more stars orbiting one another, bound together by their mutual gravitational attraction; the binary stars are most common among these multiple star systems.

In ancient times, the stars were believed to be motionless; their fixed patterns in the sky were designated as the constellations. It is now known that the stars move through space, although their motion is too small to be detected during a human lifetime without exacting measurements. From the observed proper motion (change in apparent position on the celestial sphere), distance of the star from the earth, and radial velocity (motion along the line of sight), the true velocity of a star through space can be determined. See also brown dwarf.

shooting star, in astronomy: see meteor.

shooting star, in botany: see primrose.

serpent star: see brittlestar.

sea star, also called starfish, echinoderm of the class Asteroidae, common in tide pools. Sea stars vary in size from under 1/2 in. (1.3 cm) to over 3 ft (90 cm) in diameter. They are commonly dull shades of yellow or orange, but there are many brightly colored ones as well. There are about 2,000 species distributed throughout the world, mostly in shallow water along rocky coasts.

The body of most species consists of a central disk from which radiate a number of tapering arms—usually five, but up to 25 in some species. Some sea stars are pentagonal, the points of the disk not extending into arms. Each arm contains an extension of the body cavity and body organs. A network of calcareous plates located beneath the skin forms an external skeleton; the plates are joined by connective tissue and muscle, giving the apparently rigid sea star considerable flexibility. Calcareous spines, some of them movable, project from the skin.

Tube Feet

The underside of the body bears a mouth at the center and a groove running along each arm. The grooves contain rows of tiny, flexible appendages called tube feet. Sea stars move by means of the tube feet, which are operated by a hydraulic, or water-vascular, system unique to echinoderms. Seawater, circulated through the radiating canals of this system, enters and extends the tube feet. Each tube foot can be withdrawn by its attached muscles. The tube feet are equipped with suction cups, and the animal moves in any direction by gripping with some of its tube feet and pulling itself forward. A sea star that is turned upside down can right itself by turning an arm under and walking with the tube feet.

Sensory Tentacles

Each arm has a short sensory tentacle at its end that responds to chemicals and vibrations in the water, and a red photosensitive eyespot. A sea star often lifts the end of an arm to perceive light and movement.

Predation

Sea stars are carnivorous. Members of many species have protrusible stomachs and prey largely on bivalves, such as clams and oysters; they are extremely destructive to commercial oyster beds. The sea star wraps its arms around the bivalve, grips the shell with its tube feet, and opens it by sustained powerful suction. The shell needs to open only about 1/100 in. (0.25 mm). The sea star then extrudes its stomach through its mouth and inserts it inside the shell of the prey, where it digests and absorbs the soft inner tissues.

Reproduction

Sea stars shed their eggs and sperm into the water, and fertilization occurs externally, producing a swimming, bilaterally symmetrical larva. The larva settles and undergoes a sessile (attached) period while metamorphosing into the free-living, radially symmetrical adult form. A single female may produce over 2 million eggs in one spawn, but the eggs and larvae form part of the plankton on which many marine animals feed, and few survive.

Classification

The brittlestars, of a different echinoderm class, have long, slender, jointed arms and are found in deeper waters. Sea stars are classified in the phylum Echinodermata, class Asteroidae.

neutron star, extremely small, extremely dense star, about double the sun's mass but only a few kilometers in radius, in the final stage of stellar evolution. Astronomers Baade and Zwicky predicted the existence of neutron stars in 1933. In the central core of a neutron star there are no stable atoms or nuclei; only elementary particles can survive the extreme conditions of pressure and temperature. Surrounding the core is a fluid composed primarily of neutrons squeezed in close contact. The fluid is encased in a rigid crystalline crust a few hundred meters thick. The outer gaseous atmosphere is probably only a few centimeters thick. The neutron star resembles a single giant nucleus because the density everywhere except in the outer shell is as high as the density in the nuclei of ordinary matter. There is observational evidence of the existence of several classes of neutron stars: pulsars are periodic sources of radio frequency, X ray, or gamma ray radiation that fluctuate in intensity and are considered to be rotating neutron stars. A neutron star may also be the smaller of the two components in an X-ray binary star.

morning star: see evening star.

main-sequence star: see Hertzsprung-Russell diagram.

giant star: see red giant.

feather star, common name of a class of echinoderms that, as juveniles, are attached to the sea bottom by a stalk with rootlike branches; the mouth side faces upward. In the adult stage they break away from the stalk and move about freely. Feather stars have water-vascular (ambulacral) systems, similar to those in other echinoderms, that extend into the branched arms on the body, or crown. Some can swim by undulating movements of the arms. Feather stars creep about by means of projections at the base of the crown, called cirri, which can grasp bottom objects. They are marine animals, like all echinoderms, and are widely distributed. They are most common in relatively shallow, warm waters, but some live in cold water and a number of species occur in the ocean depths. Like other members of this class, feather stars may form extremely abundant local aggregations. Feather stars are classified in the phylum Echinodermata, class Crinoidea.

falling star: see meteor.

failed star: see brown dwarf.

evening star or morning star, planet that becomes visible in the western sky shortly after sunset or in the eastern sky shortly before sunrise. It can usually be seen in twilight, when it is too light for the true stars to be seen. Venus and Mercury are often seen as evening or morning stars. Other planets, especially Mars and Jupiter, may appear as evening or morning stars at certain points in their orbits.

eclipsing binary star: see binary star; variable star.

dwarf star: see white dwarf.

double star: see binary star.

circumpolar star, star whose diurnal circle lies completely above or completely below an observer's horizon. A star whose diurnal circle lies above the horizon never sets, even though it cannot be seen during the day. Designation of a star as circumpolar depends on the observer's latitude. At the equator no star is circumpolar. At the North or South Pole all stars are circumpolar, since only one half of the celestial sphere can ever be seen. For an observer at any other latitude a star whose declination is greater than 90° minus the observer's latitude will be circumpolar, appearing to circle the celestial pole and remaining always above the horizon. A constellation made up entirely of circumpolar stars is also called circumpolar. From most of the N United States (above lat. 40°N) the Big Dipper is circumpolar.

blazing star or button snakeroot, any plant of the genus Liatris, showy North American perennials of the family Asteraceae (aster family). The blossoms, rosy purple or white, are in somewhat feathery heads along a usually wandlike stalk. Medicinal use has been made of a few species by both Native Americans and settlers. Some are called gayfeather. Blazing star is classified in the division Magnoliophyta, class Magnoliopsida, order Asterales, family Asteraceae.

binary star or binary system, pair of stars that are held together by their mutual gravitational attraction and revolve about their common center of mass. In 1650 Riccioli made the first binary system discovery, that of the middle star in the Big Dipper's handle, Zeta Urase Majoris. True binary stars are distinct from optical doubles—pairs of stars that lie nearly along the same line of sight from the earth but are not physically associated. Binary stars are grouped into three classes. A visual binary is a pair of stars that can be seen by direct telescopic observation to be a distinct pair with shared motion. A spectroscopic binary cannot be seen as two separate stars, even with the most powerful telescopes, but spectral lines from the pair show a periodic Doppler effect that indicates mutual revolution. Some lines indicate motion toward the earth while others indicate motion away; later, as the stars revolve around in their orbit, this pattern reverses. An eclipsing binary has the plane of its orbit lying near the line of sight, and shows a periodic fluctuation in brightness as one star passes in front of the other. The more massive star (A) of a binary is called the primary, and the less massive (B) is called the secondary; e.g., Sirius A and Sirius B are the primary and secondary components of the Sirius system. It seems likely that more than two-thirds of the stars in our galaxy are binary or multiple (a system of more than two stars moving around their mutual center of mass), since many stars within 30 light-years of the sun are binary or multiple. The masses of the components of a spectroscopic binary can be determined from the observed motions and Newton's law of gravitation; binary stars are the only stars outside the solar system for which masses have been directly determined. Binary stars are thus important indicators from which the masses of all similar stars can be deduced. Measurements of the masses of some of the visual binary stars have been used to verify the mass-luminosity relation. Although most binary stars have distance between them, the components of W Ursae Majoris binaries are actually in contact with each other, their mutual gravity distorting their shapes into teardrops. There are binary systems in which one member is a pulsar: PSR 1913+16, for example, has an orbital period of 7 hr 45 min; in this case the other star is also a neutron star. The orbit period decreases as the system loses energy in the form of gravitational waves; used as a clock to measure the effect of the curvature of space-time on the binary's orbit, such a system confirms Einstein's theory of general relativity.

Wolf-Rayet star: see spectral class.

Star-spangled Banner, The, American national anthem, beginning, "O say can you see by the dawn's early light." The words were written by Francis Scott Key, a young Washington attorney who, during the War of 1812, sailed to the British fleet to obtain the release of a captured American. Key was detained by the British and witnessed from ship the bombardment of Fort McHenry during the night of Sept. 13-14, 1814. Defended under the command of Major George Armistead, the fort withstood the attack, and the sight of the American flag flying at dawn inspired Key's verses, which were written on the way ashore in the morning. After circulating as a handbill, the lyrics were published in a Baltimore newspaper on Sept. 20, 1814. The tune was taken from the English popular song "To Anacreon in Heaven." Although the army and the navy had for some years regarded "The Star-spangled Banner" as the national anthem, its designation as such first became official by executive order of President Wilson in 1916. This order was confirmed by act of Congress in 1931. The large flag that inspired the anthem, with 15 stars and stripes and originally 30-by-42-ft (9.1-by-12.8-m), has been in the collection of the Smithsonian Institution since 1912.

Star of Bethlehem, name given to the luminous celestial object rising in the sky that, as related in the Gospel of Matthew, led the Wise Men of the East to the manger in Bethlehem where Jesus was born. According to astronomers, the phenomenon may represent the conjunction of the planets Jupiter, Saturn, and Mars, or the appearance of a nova or a comet.

Star Wars: see Strategic Defense Initiative.

Star Chamber, ancient meeting place of the king of England's councilors in the palace of Westminster in London, so called because of stars painted on the ceiling. The court of the Star Chamber developed from the judicial proceedings traditionally carried out by the king and his council, and was entirely separate from the common-law courts of the day. In the 15th cent., under the Lancastrian and Yorkist kings, the role of the council as an equity and prerogative court increased, and it extended its jurisdiction over criminal matters. Faster and less rigid than the common-law courts, its scope was extended by the Tudors. Under Chancellor Wolsey's leadership (1515-29), the Court of Star Chamber became a political weapon, bringing actions against opponents to the decrees and edicts of Henry VIII. Wolsey also encouraged petitioners to use the Court of the Star Chamber as a court of original jurisdiction, not as a last resort after the common-law courts had failed. Depositions were taken from witnesses, but no jury was employed in the proceedings. Although its sentences included a wide variety of corporal punishments, including whipping, pillorying, and branding, those convicted were never sentenced to death. The court remained active through the reigns of James I and Charles I. The traditional hostility between equity and common law was aggravated by the use made of the Star Chamber by the Stuarts as a vehicle for exercising the royal prerogative, particularly over church matters, in defiance of Parliament. It was abolished by the Long Parliament in 1641. In its later period the court was so reviled that Star Chamber became a byword for unfair judicial proceedings. The court's harshness, however, has been exaggerated.

North Star: see Polaris.

Dog Star: see Sirius.

Barnard's star, star with the largest observed proper motion (rate of motion across the sky with respect to other stars); located in the constellation Ophiuchus. The star's large proper motion, 10.28″ per year (or half the moon's apparent diameter in a century), is due in part to the fact that it is the second-nearest star, being at a distance of 5.98 light-years. Barnard's star was discovered in 1916 by E. E. Barnard, an observer known also for his discoveries of 16 comets. It is a faint red dwarf star, apparent magnitude 9.5, of spectral class M5, lying near the bottom of the main sequence in the Hertzsprung-Russell diagram. Slight oscillations in its motion indicate that it has at least one unseen companion.

Any of a class of small, faint stars representing the end point of the evolution of stars without enough mass to become neutron stars or black holes. Named for the white colour of the first ones discovered, they actually occur in a variety of colours depending on their temperature. They are extremely dense, typically containing the mass of the Sun within the volume of the Earth. White dwarfs have exhausted all their nuclear fuel and cannot produce heat by nuclear fusion to counteract their own gravity, which compresses the electrons and nuclei of their atoms until they prevent further gravitational contraction. When a white dwarf's reservoir of thermal energy is exhausted (after several billion years), it stops radiating and becomes a cold, inert stellar remnant, sometimes called a black dwarf. White dwarf stars are predicted to have an upper mass limit, known as the Chandrasekhar limit (see Subrahmanyan Chandrasekhar), of about 1.4 times the Sun's mass. Dying stars that are more massive undergo a supernova explosion. As members of binary stars, white dwarf stars play an essential role in the outbursts of novas.

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Star whose observed brightness varies noticeably in intensity. Pulsating variables expand and contract in cycles, pulsating rhythmically in brightness and size. Explosive variables include novas and supernovas, which brighten rapidly due to sudden outbursts of radiant energy; the increased brightness lasts a short time, followed by relatively slow dimming. Eclipsing variable stars are variable only because light from one star is blocked by another in Earth's direction. Hundreds of thousands of variable stars are known. Seealso binary star; Cepheid variable; flare star; pulsar; T Tauri star.

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Star of very great natural luminosity and relatively enormous size, typically several magnitudes brighter and several times larger than a giant star. Like other classes of stars, they are distinguished in practice by examination of certain lines in their spectra (see spectroscopy). A supergiant may have a diameter several hundred times that of the Sun and a luminosity nearly a million times as great. Supergiants live probably only a few million years, an extremely short life for a star.

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Any massive celestial body of gas that shines by radiant energy generated inside it. The Milky Way Galaxy contains hundreds of billions of stars; only a very small fraction are visible to the unaided eye. The closest star to Earth is the Sun. The closest star to the Sun is about 4.2 light-years away; the most distant are in galaxies billions of light-years away. Single stars such as the Sun are the minority; most stars occur in pairs and multiple systems (see binary star). Stars also associate by their mutual gravity in larger assemblages called clusters (see globular cluster; open cluster). Constellations consist not of such groupings but of stars in the same direction as seen from Earth. Stars vary greatly in brightness (magnitude), colour, temperature, mass, size, chemical composition, and age. In nearly all, hydrogen is the most abundant element. Stars are classified by their spectra (see spectrum), from blue-white to red, as O, B, A, F, G, K, or M; the Sun is a spectral type G star. Generalizations on the nature and evolution of stars can be made from correlations between certain properties and from statistical results (see Hertzsprung-Russell diagram). A star forms when a portion of a dense interstellar cloud of hydrogen and dust grains collapses from its own gravity. As the cloud condenses, its density and internal temperature increase until it is hot enough to trigger nuclear fusion in its core (if not, it becomes a brown dwarf). After hydrogen is exhausted in the core from nuclear burning, the core shrinks and heats up while the star's outer layers expand significantly and cool, and the star becomes a red giant. The final stages of a star's evolution, when it no longer produces enough energy to counteract its own gravity, depend largely on its mass and whether it is a component of a close binary system (see black hole; neutron star; nova; pulsar; supernova; white dwarf star). Some stars other than the Sun are known to have one or more planets (see extrasolar planet). Seealso Cepheid variable; dwarf star; eclipsing variable star; flare star; giant star; Populations I and II; supergiant star; T Tauri star; variable star.

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Entry into Earth's atmosphere of multiple meteoroids (see meteor), traveling in parallel paths, usually spread over several hours or days. Most meteor showers come from matter released during passage of a comet through the inner solar system, and they recur annually as Earth crosses the comet's orbital path. Meteor showers are usually named for a constellation (e.g., Leonid for Leo) or star in their direction of origin. Most showers are visible as a few dozen meteors per hour, but occasionally Earth crosses an especially dense concentration of meteoroids, as in the great Leonid meteor shower of 1833, in which hundreds of thousands of meteors were seen in one night all over North America.

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or shooting star or falling star

Streak of light in the sky that results when a particle or small chunk of stony or metallic matter from space enters Earth's atmosphere and is vapourized by friction. The term is sometimes applied to the falling object itself, properly called a meteoroid. Most meteoroids, traveling at five times the speed of sound or more, burn up in the upper atmosphere, but a large one may survive its fiery plunge and reach the surface as a solid body (meteorite). Seealso meteor shower.

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or sea star

Any of 1,800 echinoderm species (class Asteroidea) that have regenerable arms surrounding an indistinct disk and that inhabit all oceans. Species range from 0.4 to 25 in. (1–65 cm) across, but most are 8–12 in. (20–30 cm) across. Their arms, usually five, are hollow and, like the disk, covered with short spines and pincerlike organs; on the lower side are tube feet, sometimes sucker-tipped, used for creeping or clinging to steep surfaces. Some species sweep organic particles into the mouth on the underside of the disk. Others either evert the stomach upon their prey for external digestion or swallow the prey whole.

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Reddish and heavy-spined starfish (Acanthaster planci) that has 12–19 arms and is often 18 in. (45 cm) across. It feeds on the polyps of coral. Beginning circa 1963, its population on Australia's Great Barrier Reef exploded. Destruction of coral reefs and islands was feared, and intensive efforts were made to kill it off. Since then other outbreaks have been recorded throughout the southern Pacific. The cause of the outbreaks is unknown, but several factors have been proposed, such as the decimation of the starfish's chief predator, the Pacific triton (a marine snail), by shell collectors. Other factors, including the runoff of nutrient-rich soil into reef waters as a result of shorefront development, have also been implicated. Population fluctuations could also be a feature of the starfish's natural ecology, and human influence may alter these cycles.

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Any of a class of extremely dense, compact stars thought to be composed mainly of neutrons with a thin outer atmosphere of primarily iron atoms and electrons and protons. Though typically about 12 mi (20 km) in diameter, they have a mass roughly twice the Sun's and thus extremely high densities (about 100 trillion times that of water). Neutron stars have very strong magnetic fields. A solid surface differentiates them from black holes. Below the surface, the pressure is much too high for individual atoms to exist; protons and electrons are compacted together into neutrons. The discovery of pulsars in 1967 provided the first evidence of the existence of neutron stars, predicted in the early 1930s and believed by most investigators to be formed in supernova explosions. Seealso white dwarf star.

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Star with a relatively large radius for its mass and temperature; this yields a large radiating area, so such stars are bright. Subclasses include supergiant stars, red giants (with low temperatures, but very bright), and subgiants (with slightly reduced radii and brightness). Some giants are hundreds of thousands of times brighter than the Sun. Giants and supergiants may have masses 10–30 times that of the Sun and volumes millions of times greater and are thus low-density stars.

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Star that varies in brightness repeatedly but sporadically, sometimes by more than one magnitude, within a few minutes. The cause is thought to be the eruption of flares like those observed on the Sun but much larger (see solar flare). Proxima Centauri, in Alpha Centauri, the closest star to the Sun, is a flare star.

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Any of the 550 living species of crinoid echinoderms (class Crinoidea) that lack the stalk that their relatives the sea lilies use to attach themselves to the ocean bottom. Most species have five feathery-fringed arms. Feather stars usually attach themselves to a surface or a floating object and feed on drifting microorganisms, trapping them in their sticky arm grooves. They live mainly on rocky bottoms in shallow water. Most abundant from the Indian Ocean to Japan, they are also found in the Atlantic.

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or eclipsing binary

Binary star in an orbit whose plane passes through or very near Earth. An observer on Earth sees one star pass periodically in front of the other and diminish its light through an eclipse. The star Algol, in the constellation Perseus, was the first such star recognized (1782); thousands are now known. By combining the brightness variations with spectroscopic information for both stars, astronomers can determine the mass and size of each star. Seealso variable star.

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Any star of average or low luminosity, mass, and size, including white dwarf stars and red dwarf stars. Dwarf stars include most main-sequence stars (see Hertzsprung-Russell diagram), including the Sun. Their colour can range from blue to red, corresponding to temperatures varying from over 17,500 °F (10,000 °C) to a few thousand degrees.

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Pair of stars in orbit around a common centre of gravity. Their relative sizes and brightnesses and the distance between them vary widely. Perhaps half of all stars in the Milky Way Galaxy are binaries or members of more complex multiple systems. Some binaries form a class of variable stars (see eclipsing variable star). Stars can be identified as binaries in various ways—visually by telescope, through spectroscopic observation, by changes in apparent brightness (when the dimmer star eclipses its companion), or by changes in the proper motion of the visible member (owing to the gravitational pull of the invisible companion).

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also called Star Wars

Proposed U.S. strategic defense system against nuclear attacks. Announced as a 20-year, $20 billion effort by Pres. Ronald Reagan in 1983, SDI was intended to defend the U.S. from a full-fledged Soviet attack by intercepting ICBMs in flight. The interception was to be effected by technology not yet developed, including space- and ground-based laser stations and air- and ground-based missiles. The space component of SDI led to its being derisively dubbed “Star Wars” after the popular film. Though the program was roundly criticized by opposition politicians and arms-control advocates as unworkable and as a dangerous violation of the Antiballistic Missile (ABM) Treaty of 1972, Congress granted initial funding for it. Early development efforts were largely unsuccessful, and with the fall of the Soviet Union in 1991 the concept lost urgency. During the Bush and Clinton administrations, ballistic missile defense was scaled back to focus on protecting the U.S. from limited attack by a “rogue” state or a single accidentally launched missile. In 2002 the U.S. withdrew from the ABM treaty to begin active testing of a limited antimissile program. Seealso antiballistic missile.

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or Dog Star

Brightest star in the night sky (apparent magnitude −1.44), a binary star about 8.6 light-years from the Sun in the constellation Canis Major. The bright component of the binary is a blue-white star 23 times as luminous as the Sun, about twice the size, and considerably hotter; its companion was the first white dwarf star discovered. Its name probably comes from a Greek word meaning “sparkling” or “scorching.” The ancient Egyptians used its predawn rising to predict the annual flooding of the Nile. The ancient Romans associated the rising of the Dog Star at dawn with the hottest part of the year, called the “dog days.”

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Star about six light-years away from the Sun, next nearest the Sun after the Alpha Centauri system, in the constellation Ophiuchus. Named for Edward Emerson Barnard (b. 1857—d. 1923), who discovered it in 1916, it has the largest proper motion of any known star. It is gradually nearing the solar system. The star attracted astronomers' attention in the 1960s when its proper motion was claimed to show periodic deviations attributed to the gravitational pull of two planets (see planets of other stars). The deviations were later proved to be artifacts of measurement.

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