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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; this light enables them to observe only the stars' surfaces. Knowledge of the processes taking place in a star and of conditions within its interior must be inferred from the laws of physics and chemistry. A star is a nearly spherical body of incandescent gas, mostly hydrogen and helium. Because it is observed to be stable, astronomers can conclude that the inward pressure of gravitation holding the star together is balanced by the outward pressure due to the energy generated by the star, and that the rate at which energy is radiated away from the star's surface is equal to the rate at which it is produced in the interior. The most important properties of a star are its size, mass, luminosity, chemical composition,

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 configuration. Because a star's total energy reserve is finite, a star shining today cannot continue to produce its present luminosity steadily into the indefinite future, nor can it have done so from the indefinite past. Thus, stellar evolution is a necessary consequence of the physical theory of stellar structure , which requires that the luminosity, temperature, and size of a star must change as its chemical composition changes because of thermonuclear reactions.

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

Ursa Major and Ursa Minor [Lat.,=the great bear; the little bear], two conspicuous northern constellations. Known to many peoples from ancient times, these constellations have had various names; the configuration of the seven brightest stars has been called the Bear, Septentriones (the seven plowing oxen), the Plow, Charles's Wain, and the Wagon. Ursa Minor was once known as Cynosura (from the Greek for "dog's tail"). In the United States part of Ursa Major is called the Big Dipper (or the Drinking Gourd) and part of Ursa Minor, the Little Dipper. Four of the seven bright stars in the Big Dipper form the bowl and three the handle; five of these stars are of second magnitude. The middle star in the handle of the Big Dipper is Mizar (Zeta Ursae Majoris). A fainter star, Alcor, which appears to be near Mizar, was observed from ancient times. These two stars are sometimes called a double star, but since they do not revolve around a common center of gravity they are not tr

Polaris or North Star, star nearest the north celestial pole (see equatorial coordinate system ). It is in the constellation Ursa Minor (see Ursa Major and Ursa Minor ; Bayer designation Alpha Ursae Minoris) and marks the end of the handle of the Little Dipper. Polaris's location less than 1° from the pole (1992 position R.A. 2h23.3m, Dec. +89°14') makes it a very important navigational star even though it is only of second magnitude; it always marks due north from an observer. Polaris can be located by following the line upward from the two stars (the Pointers) at the right end of the bowl of the Big Dipper or, if the Big Dipper is not visible, by following the line through the left side of the square in Pegasus through the end star in Cassiopeia. The star is a Cepheid variable and oscillates in brightness roughly every four days. Because of the precession of the equinoxes , Polaris will not remain the polestar indefinitely; in 2300 B.C. the polestar was in the constellation

Azimuth, in astronomy, one coordinate in the altazimuth coordinate system . It is the angular distance of a body measured westward along the celestial horizon from the observer's south point.

Prime meridian, meridian that is designated zero degree (0°) longitude, from which all other longitudes are measured. By international convention, it passes through the original site of the Royal Observatory in Greenwich, England; for this reason, it is sometimes called the Greenwich meridian. Universal time , the standard basis for determining time throughout the world, is civil time measured at the prime meridian.