The Law of Universal Gravitation
Since the gravitational force is experienced by all matter in the universe, from the largest galaxies down to the smallest particles, it
..... Click the link for more information. , and emitting light and other forms of electromagnetic radiation electromagnetic radiation, energy radiated in the form of a wave as a result of the motion of electric charges. A moving charge gives rise to a magnetic field, and if the motion is changing (accelerated), then the magnetic field varies and in turn produces an
..... Click the link for more information. whose ultimate source is nuclear energy nuclear energy, the energy stored in the nucleus of an atom and released through fission, fusion, or radioactivity . In these processes a small amount of mass is converted to energy according to the relationship E = mc2, where E
..... Click the link for more information. .
Properties of Stars
Stars differ widely in mass mass, in physics, the quantity of matter in a body regardless of its volume or of any forces acting on it. The term should not be confused with weight , which is the measure of the force of gravity (see gravitation ) acting on a body.
..... Click the link for more information. , size, temperature temperature, measure of the relative warmth or coolness of an object. Temperature is measured by means of a thermometer or other instrument having a scale calibrated in units called degrees. The size of a degree depends on the particular temperature scale being used.
..... Click the link for more information. , and total energy output, or luminosity luminosity, in astronomy, the rate at which energy of all types is radiated by an object in all directions. A star's luminosity depends on its size and its temperature, varying as the square of the radius and the fourth power of the absolute surface temperature.
..... Click the link for more information. . The sun sun, intensely hot, self-luminous body of gases at the center of the solar system . Its gravitational attraction maintains the planets, comets, and other bodies of the solar system in their orbits.
..... Click the link for more information. , 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 supernova, a massive star in the latter stages of stellar evolution that suddenly contracts and then explodes, increasing its energy output as much as a billionfold.
..... Click the link for more information. ) are about ten million times more powerful than the sun, while the least luminous are only one hundredth as powerful. Red giants red giant, star that is relatively cool but very luminous because of its great size. All normal stars are expected to pass eventually through a red-giant phase as a consequence of stellar evolution .
..... Click the link for more information. , 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 white dwarf, in astronomy, a type of star that is abnormally faint for its white-hot temperature (see mass-luminosity relation ). Typically, a white dwarf star has the mass of the sun and the radius of the earth but does not emit enough light or other radiation to be
..... Click the link for more information. are no larger than the earth, and neutron stars 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.
..... Click the link for more information. are only a few kilometers in radius.
The visible stars are divided into six classes according to apparent brightness; the brightest are first magnitude magnitude, in astronomy, measure of the brightness of a star or other celestial object. The stars cataloged by Ptolemy (2d cent. A.D.), all visible with the unaided eye, were ranked on a brightness scale such that the brightest stars were of 1st magnitude and the
..... Click the link for more information. 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 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
..... Click the link for more information. 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 spectrum, arrangement or display of light or other form of radiation separated according to wavelength, frequency, energy, or some other property. Beams of charged particles can be separated into a spectrum according to mass in a mass spectrometer (see mass
..... Click the link for more information. 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 spectral class, in astronomy, a classification of the stars by their spectrum and luminosity . In 1885, E. C. Pickering began the first extensive attempt to classify the stars spectroscopically.
..... Click the link for more information. ).
Stellar Structure and Stellar Evolution
The theory of stellar structure stellar structure, physical properties of a star and the processes taking place within it. Except for that of the sun, astronomers must draw their conclusions regarding stellar structure on the basis of light and other radiation from stars that are light-years away;
..... Click the link for more information. 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 black body, in physics, an ideal black substance that absorbs all and reflects none of the radiant energy falling on it. Lampblack, or powdered carbon, which reflects less than 2% of the radiation falling on it, approximates an ideal black body.
..... Click the link for more information. ). The theory of stellar evolution stellar evolution, life history of a star , beginning with its condensation out of the interstellar gas (see interstellar matter ) and ending, sometimes catastrophically, when the star has exhausted its nuclear fuel or can no longer adjust itself to a stable
..... Click the link for more information. 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 nucleosynthesis or nucleogenesis, in astronomy, production of all the chemical elements from the simplest element, hydrogen, by thermonuclear reactions within stars, supernovas, and in the big bang at the beginning of the universe (see nucleus ;
..... Click the link for more information. ) 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 galaxy, large aggregation of stars , gas, and dust, typically containing billions of stars. Recognition that galaxies are independent star systems outside the Milky Way came from a study of the Andromeda Galaxy (1926–29) by Edwin P.
..... Click the link for more information. contains billions of stars. The stars visible to the unaided eye are all in our own galaxy, the Milky Way Milky Way, the galaxy of which the sun and solar system are a part, seen as a broad band of light arching across the night sky from horizon to horizon; if not blocked by the horizon, it would be seen as a circle around the entire sky.
..... Click the link for more information. . Stars are not spread uniformly through a galaxy. They are frequently bunched together in star clusters 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.
..... Click the link for more information. 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 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.
..... Click the link for more information. 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 constellation, in common usage, group of stars that appear to form a configuration in the sky; properly speaking, a constellation is a definite region of the sky in which the configuration of stars is contained.
..... Click the link for more information. . 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 proper motion, in astronomy, apparent movement of a star on the celestial sphere , usually measured as seconds of arc per year; it is due both to the actual relative motions of the sun and the star through space. Proper motion reflects only transverse motion, i.e.
..... Click the link for more information. (change in apparent position on the celestial sphere celestial sphere, imaginary sphere of infinite radius with the earth at its center. It is used for describing the positions and motions of stars and other objects.
..... Click the link for more information. ), distance of the star from the earth, and radial velocity radial velocity, in astronomy, the speed with which a star moves toward or away from the sun. It is determined from the red or blue shift in the star's spectrum .
..... Click the link for more information. (motion along the line of sight), the true velocity of a star through space can be determined. See also brown dwarf brown dwarf, in astronomy, celestial body that is larger than a planet but does not have sufficient mass to convert hydrogen into helium via nuclear fusion as stars do.
..... Click the link for more information. .
Bibliography
See C. de Jager, The Brightest Stars (1980); G. O. Abell, Exploration of the Universe (5th ed. 1987); R. J. Taylor, The Stars: Their Structure and Evolution (1994); A. C. Phillips, The Physics of Stars (1994).
star
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). See also Cepheid variable; dwarf star; eclipsing variable star; flare star; giant star; Populations I and II; supergiant star; T Tauri star; variable star.
Star
The Xerox workstation that officially introduced the graphical user interface and desktop metaphor in 1981. It was the inspiration for Xerox's subsequent computers and for Apple's Lisa and Macintosh. All graphical user interfaces owe their roots to the Star. See Alto.
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| The Star User Interface |
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| If the graphical interface and simulated desktop in this picture seem amazingly similar to the Macintosh and Windows, there is a reason why. That is where they both came from. (Image courtesy of Palo Alto Research Center.) |
star
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